Hermetico Guitar: February 2013

27 February 2013

Amps: tubes, tubes and more tubes - Part 4

Introduction

During part 1, we talked about how a tube works, discussed about NOS and New Production tubes gap and, named the few plants that are currently making tubes for guitar and bass amps.

In part 2, we talked about some of the technicals characteristics that make a tube different to other, among with which of those characteristics are more interesting for us, guitarists, when searching a tube and, we talked about what tube resellers do for us.

In part 3, we were discussing about pre-amp and power tubes more often used in guitar amps, in a very generic way, without going into detail of each tube. We talked also about some "typical" pre-amp positions and which kind of tubes can better suit those positions.

In part 4, we are going into the detail of each tube. When, available, I will add some comments from Myles Rose and / or some comments from The Tube Store and, if I've personally tested those tubes, my own comments.

Some overall considerations

The changes achieved in each tube swap are very different, depending on the amp's design and, rest of tubes that remain unchanged in the amp so, a tube that works fine in amp X, in position N, doesn't mean that will work fine in amp Y, either in position N.

Tube production is somewhat inconsistent and, not all the tubes of the same batch of the same Maker/model have exactly the same measurable technical characteristics. When you check several tubes of same maker / model, you can get an overall idea but, there can be a notable gap between the "worst" tube and the "best" tube within the same maker / model.
I am talking about this because, this can be of help to you to understand how some people seems to like tube X while others hate it and, viceversa. Sometimes, the tube model that you hated in a previous amp is the one that will work better in your next amp.

So there are not bad tubes or good tubes, all them are bad and good, just depending on the amp, position and musical style. The only bad tube is the one that goes far away of its technical specification. If they are between a reasonable deviation margin (maybe, a tolerance factor of 10%) they will do their job.

Each tube (model/maker) has its own sound and they can behave in different ways if they are being pushed hard or soft. Some tubes can sound cool or hollow while the volume and gain controls are in low settings and just stunning at higher gain and volume levels. Others can sound really good while clean and with low gain and volume settings but, really ugly when pushed hard and forced into distortion.

Tube makers are constantly changing their tubes, in the aim of enhance them and, this can bring you very different results when comparing tubes that you bought (and were produced) along different years.

As we already discussed, the added value of a tube store that performs a battery of tests is that they are selecting the tubes with the best performance in the several technical characteristics so, they are choosing the good apples for you.

Light technical refresh on certain tube characteristics

As we already discussed, some characteristics are measurable with technical devices, while others are just subjectives and depend on the guitarist itself. Let refresh a bit some of the characteristics we are interested on mostly.

Gain

Gain defines de amplification power of the tube. A gain of 100 means that if the tube receives 1 mV at its input, it will deliver 100 mV at its output.

The 12AX7 tube has a gain of 100, the 5751 has a gain of 70, the 12AT7 of 60, the 12AY7 of 45 and the 12AU7 of 19.
We can understand the gain as a way to make a Zoom in the input voltage, to get a magnified image in the output. If we compare it to a typo that we are zooming in in our PC's monitor, you will easily understand that we can zoom in up to certain amount but, once we reach some zooming percentage (gain) the image can overflow the size of the monitor (distortion) and we can loose the complete typo (fidelity), maybe just seeing the diagonal stroke of a "N" instead of the whole "N".

Output

The output current is the number of electrons that are being injected in the circuit. Complex designs, with lot of alternative signal paths, several channels, several modes by channels, as can be usually seen in high gain amps, like some Mesa Boogies, Diezel, Soldano or Rivera (to name a few) are in the need of a strong flow of electrons to feed so much electronics components.

The output of a 12AX7 is 1.2 mA, the 5751 is 1.0 mA, 12AY7 is 3.0 mA and 12AT7 is 10 mA (barely ten times a 12AX7 !).
Therefore, to substitute a type of tube of family 12A with other kind of tube has its impact. Tubes with high current draw can bleed some of the current needed for the rest of tubes.

By example, it's is closer to swap a 12AX7 with a 5751 (just a difference of -0.2 mA) than with a 12AT7 (a difference of +9 mA).

To follow with our example of our monitor, we can think on that current as like the strenght of the light that lights the notebook screen. Very low and we couldn't barely see the image. Very high and, the image will look burnt.

Transconductancy

It's somewhat the speed with which the tube can convert the input signal into the resulting output signal. It has high importance when the tube should quickly respond to speedy riffs, by example. This value is very responsible of the note-by-note definition of the tube.
Transconductancy of a 12AX7 is 1600, while for a 5751 is about 5000.

Following with our example of PC and monitors, the transconductancy could be seen as the refresh ratio of our screen. In a game, a low ratio will generate wakes or "ghosts", when the image changes quickly.

Linearity

Is the ability of a tube to behave consistently along the full range of its gain. Some tubes work really good while in clean but, when they start to distort, some frequencies are cancelled or reinforced or, the definition note-by-note is lost or they add some unwanted sonic artifacts.
The more even a tube behaves along the full gain range, the more linear it is.

Current production tubes

Current production tubes have big deviation in all those (among others) parameters.
Practically, all current production tubes have a gain factor under 100.
Only in the 9th generation of Sughuang tubes (commercialized by Groove Tubes or Ruby Tubes), and Sovtek's models 12AX7-WB and 12AX7-WC go close, with a gain of 92.
Rest deliver different values, between the 82 of the Mullard re-issue and the 87 of the Electro-Harmonix.
This means a clear drop in the amplification possibilities of our amp but, sometimes, it can be a desired thing, to increase the headroom.

Only the Chinese 9th generation offers a 100% of the specification transconductancy value of 1600, followed by the Mullard re-issue (around a 94%, 1504). But we should take into account that NOS Mullards had even less TC value (around 1450). The ones with a lower TC value are the Sovtek's models 12AX7-WA and 12AX7-WB (80%, 1280) what can explain their lack of definition note-by-note when they are pushed hard.

Very few tubes are offerring an output of 1.2 mA. Just the Mullard re-issue and the Sovtek 12AX7-WC.
The JJ is a tube on-steroids, delivering a 12% over specifications (around 1.34 mA) and, they can drop to around an 80% (0,99 mA), as in the case of the EH 12AX7.

As we can see, these basis parameters are very different for each maker / model of tube and, even between different production batches. All that makes more difficult to score a the right tubes for a certain amp.

But, still didn't talked about sound and reliability.

Reliability

The reliability is an objective aspect, that can be measured. The tolerance to several working regimes, the time they last, the sturdiness... Not all the tubes will stand same amp designs neither same power levels.
Sound

Without any doubt, this is the subjective aspect that more interest to us. What for some people can be a delicious sound can be ugly for others. So you are the only judge here.
We usually valuate tubes by their sound, reliability and price, always looking to the optimal relationship between those parameters.

By example, it was really sad to discover that the tube that had the "best" sound to my taste, wasn't able to stand the stress of my amp. That was the case of the Gold Lion EL84, that cost about double other EL84 tubes and lasted barely 3 hours.

It's also very interesting to experience how a tube that sounds ugly in an amp can be the right medicine in other amp, and viceversa.

Some New Production 12AX7 tubes

As mentioned in the Introduction, I will describe some of the New Production 12AX7 tubes. When available, I will include information about some of their technical characteristics or, some comments coming from Myles Rose, or some comments coming from The Tube Store or, my own comments if I've experienced such a tube.

TAD 7025 Grey Anode

Personal comments:
This tube is being produced by Sino in China, tested and labeled by TAD.
This is probably one of the brightest tubes I've ever tested and, under high gain can be even harsh and piercing.
The low end is poor, even bonny, with edgy and crunchy basses, without body.
Mids and trebles are well represented, specially while clean but, the tube can sound very harsh and piercing in already bright amps.
When it breaks, the distortion is creamy and very compressed, loosing lot of definition note-by-note but, you achieve a good Trash sound, with nice palm-mutes and chugs.
This tube has one the of the lowest noise levels and very low microphonics issues, allowing you to work with the full range of volume and gain in all channels, with few feedback or a very controllable one, even at high volume / gain levels.
Tested in Koch Studiotone, in V1 and V2, give good results except for a clear lack of bottom end and certain indefintion when the gain is pushed hard.
I didn't liked it in the rest of my amps.

TAD 7025-S (Mullard type)

The Tube Store comments:
Great sound. It resembles the NOS Telefunken 12AX7 tones. Good gain. Low noise. No microphonics. Excelent en Blackface and similar amps.

Personal comments:
Probably, one of the best current new production 12AX7 tubes. A tube that works very linearly, from very low volume / gain settings to high gain / volume settings. Delivers a rich harmonical content, with a warm body. Breaks up in a nice crunch distortion type and stands very defined the whole range of gain.
Can darken a bit your amp if your amp is naturally dark or tame a bit the bright sound of your amp.
Excellent for V1 and, eventually for V2 also. It's version Highgrade is adequate for V1.
Worked really good in all amps.

Shughuang 12AX7-C

Output: 93 TC: 100 Gain: 92

Myles Rose comments:
Only the 9h generation of this Chinese valve achieves the best in all departments. Warm and lineal , the darker one. Very good for Blues, Rock and Jazz.

My personal comments:
Those tubes were commercialized by Groove Tubes and Ruby Tubes, exclusively and, I hadn't experience with them but, according to the description, I bet this is exactly the same valve as the TAD 7025-S that The Tube Amp Doctor is commercializing.

TAD 7025-WA

My personal comments:
I wasn't expecting great things of a WA tube type but, this tube is really good. Tone is quite similar to the tone of the 7025-S, maybe a bit darker.
Excellent harmonical content, very lineal, warm, detailed. Darker than the Mullard reissue or the Tung Sol reissue. More headroom and when it breaks, is more creamy than crunchy.
Its WA specification makes it specially good to provide more headroom to your amp and, to minimize noise and microphonics issues.
I was greately surprised by this tube.

Mullard 12AX7 reissue

Output: 100 TC: 96 Gain: 82

The Tube Store comments:
Plain EQ'd. Sounds a bit hi-fi compared with Tung Sol. Medium Gain. Good noise level. Better for audio applications.

Myles Rose comments: Good in V1 and V2 for Fender Blackface and Marshalls. Not so bright as the JJ ECC83S. Dr.Z uses it in PI / Driver positions.

My personal comments:
This is probably the current new production tube that I personally like more. It has the better definition of all tested and behaves very linearly. When it breaks, it does it with a nice crunch. This tube shines the harder is being pushed, then is when the full harmonical content is released. It has a very good noise level and it didn't showed microphonics issues in none of the amps I've tried it. It stands defined even under the harder gain. The drawback with this tube is if you want to run your amp at quiet volumes. Since it has a low gain ratio, it stands clean very much and this forces you to raise the gain and volume knobs. If you can really crank your amp, don't miss testing this tube. If you cannot crank your amp, you better go for other options.

Svetlana 12AX7 reissue

Don't confuse this tube with the Winged S series of the authentic Svetlana factory. These are tubes produced by New Sensor, that are the only company having the rights to use the Svetlana name in USA and Canada.

The Tube Store comments:
Softer than the EH and less gain than the Tung Sol. Very open and bright without going piercing or harsh. Good noise level. No microphonics issues. If you like the JJ ECC83S but you will prefer it a bit less strident, this is your tube.

My personal comments:
You can think on this tube as in a silkier version of the JJ ECC83S. Shares lot of the JJ character but, trebles are silkier, I would say even angelical in clean. The drawback is that it has noticiable less output and gain than the JJ what makes you to push the gain and volume a bit more so, it's not recommendable if you need to play at low volumes. When pushed hard, the distortion is more creamy than crunchy and has a lot of punch that sounds perfect to me for metal works.

NOS JAN/Philips 5751

Even being a NOS tube, there is stock enough to be found in tube stores with a price very similar to new production tubes.

The Tube Store comments:
One of the best available NOS tubes. Quite well same gain as a 12AX7, very low noise and, good sonical representation for every kind of application. It shines in single-ended simple designs.

My personal comments:
This tube has a very hi-fi sound, even more hi-fi than the Mullard re-issue and, an outstanding definition note-by-note (transconductancy is about 4 times the one of a regular 12AX7). Provides a lot of headroom, because it breaks really late. If your amp should be run quiet, this is not a good tube for V1, because you will need to dial up the gain and volume to get some musical sound. If you can crank your amp and you need lot of headroom, this is your tube.
Steve Ray Vaughan was using one of them in V1 in all his amps. I've checked it in V1 and it sounds so hi-fi that I wouldn't recommend it as a tone shaper (if you cannot crank the amp!).
I've also checked it in PI positions and, it's a very good tube in that positions but, once more, it delays the break up spot of the power tubes what force you to raise gain and volume. Probably, the cleanest, defined and tridimensional sounding of the tubes I've tested but, the sound can be a bit cold or sterline at low levels.

Ei 7025-Y

Since Ei closed, this is now a NOS tube.

Output: 46 TC: ? Gain: 90

Myles Rose comments:
The brightest of all tubes. Good in V1 and V2 for Fender Tweeds.

My personal comment:
No experience

Tung Sol 12AX7 reissue

The Tube Store comments:
British sound with the output of a Chinese tube. Enhanced basses and trebles

My personal comments:
This tube has a lot of steroids, a warmth pad and the deepest basses of all tubes, among with the treblier trebles.
Maybe, I hadn't the best tubes (since they weren't tested by the Tube Store) so, I had several tubes that brook after a couple of days, in the Vox Night Train (V1).
Not so powerfull as the JJ ECC83S is, powerfuller than the rest of tubes. Basses kick hard really early and, that helps to bring some life to the sound when your amp should run quiet and, maybe this is one of the reasons why so many people loves this tube.
Compared with some others, I've found them as going confussing when pushed hard. The excess of basses makes very confussing the body of the sound and the highs can go really harsh or piercing. Probably best for cleans at low volumes.

EH 12AX7

Output: 83 TC: 91 Gain: 87

The Tube Store comments:
Very balanced sound. Accetable noise level and excelent against microphonic issues. Very versatile in any position.

Myles Rose comments:
Very lineal. Not so lineal or warm as the Chinese 9th generation but equaly versatile. Brighter than the Chinese one but darker than the LPS. Quiet, reliable. Very good to push the front-ends of hi gain amps.

My personal comments:
This tube is a real workhorse. Not so expensive than other tubes and that always do a good job, in any position. Maybe, not the best in any single aspect but good enough in all them. It's a handly tube to stock. When no other tube seems to work, the EH will do its magic.
Tends to be a bit trebbly so, watch out if your amp is already bright. It has a very good fuzzy crunch behaviour when pushed hard and maintains a reasonable definition. It's one of the tubes with an earlier break up and, this allows you to work with it at lower gain / volume settings.
It isn't my first election but, I am quite sure to test it in every amp and, results are very variable depending on the amp. In some amps is quite perfect while in other amps, other tubes work better.
In the Koch Studiotone, they pushed the amp very well, in V1 and V2 positons.
I love it in V1, in my Fender Princeton Reverb reissue.
A bit bonny and harsh in Marshall DSL.

Sovtek 12AX7-WA

Output: 91 TC: 80 Gain: 88

The Tube Store comments:
Acceptable sound, without character. Prone to pops.

Myles Rose comments:
Reliable and quiet. Typical default tube, with some lack of life.

My personal comments:
Usually a tube that comes stock in many new production amps, because it's quiet and reliable but, the truth is that sonically is a tad sterile and, practically any other tube is sonically an enhancement over this tube.
It lacks definition when pushed hard and, the distortion is creamy but confussing and, overall, it lacks some dimensionality.
If you swap one of those, you can even use those in cathode-follower positions or other not-tone-shaper positions.

Sovtek 12AX7-WB

Output: 91 TC: 80 Gain: 92

My personal comments:
In all aspects, equal to the WA version but, with slightly more gain.

Sovtek 12AX7-WC

Output: 100 TC: 89 Gain: 92

Myles Roses comments:
Very close to 12AX7 specifications. Good tube

My personal comments:
I've found this tube lifeless and somewhat sterile in tone-shapping positions but, it can work nicely in other positions, specially in cathode-follower positions or as PI or Driver.
Contrary to what Myles Roses says and to what New Sensor ads in its page, this tube didn't worked well, in no position. As PI, when trying to bias the Marshall, I've noticed that one of the power tubes was giving crazy readings and, suspected that this would be due to clear unbalanced triodes. I swaped this tube with an LPS and end of issues. Also, I've removed the WC from V3 and thrown there a JJ ECC83S that worked way better.
I personally don't recommend this tube for no position.

Gold Lion ECC83 / B759

The Tube Store comments:
Very smooth and gentle to the ear. Wide and full mids, controlled basses and trebles. Excellent to tame the bright channel of a JCM800. Creamy distortion.

My personal comments:
After seen how the expensive Gold Lion EL84s and 6V6GT failed and lasted so short in two different amps, I didn't wanted to test more Gold Lion tubes anymore.
I don't mind if they are the best New Production ones but, they are sometimes expensiver than some NOS tubes so... what's the point?.
No experience.

NOS JAN/Philips 12AX7-WA

The Tube Store comments:
One of the best available NOS tubes. Good gain, good noise level and low microphonics. Wide mids. Can result boomy or harsh if you overdo the EQ.

My personal comments:
No experience.

EH 12AY7 / 6072A

Output: ? TC: ? Gain: 44

Myles Roses comments:
Good in V1 for Tweeds. In Marshall DSLs, in position V2 give the old Plexi sound.

My personal comments:
Well, the issue with this tube is that the gain drops and, that makes you to push the volume of your amp, what goes against my interests. Probably, a nice trick if you can crank your amp but, not a good idea if you want to play it at quiet volumes.
I honestly prefer to "plexirize" my Marshall with the help of the Wampler Plexi Drive.

TAD 12AY7

My personal comments:
Good sound. Same applications as the EH 12AY7 but better sounding, IMHO.
Same drawbacks.

Sovtek 12AX7-LPS

Output: 83 TC: ? Gain: 87

The Tube Store comments:
Soft and very EQ balanced. Prone to microfonics issues in combos. Good gain and low noise level.

Myles Rose comments:
Excellent and detailed sound. More bright than WA, ECC83S or EH but less than 7025s.
Prone to microphonics in combos.

My personal comments:
Indeed, this is the only Sovtek preamp tube that has an usable tone. Very detailed and clean.
I don't like it so much as a tone shaper (V1 or V2), because I find other tubes sounding with more character in those positions but, it works really good in other positions.
You cannot use in a cathode-follower position (typically, V3) but, it's the best tube for PI positions of all those I've tested. This tube shines like a PI tube, in all amps I've tested it.
In the Reverb Driver of the Fender Princenton Reverb reissue helped to open the reverberation sound and the overall sound of the amp, that was a tad dark.
Is still my natural choose for PI positions, even than one of the LPS lasted really few in a high gain little combo (Koch Studiotone). I am stocking this tube, with balanced triodes.

JJ ECC83S

Output: 112 TC:? Gain: 84

The Tube Store comments:
Very evenly EQ'd. Not so harmonically rich as other. Good gain, low noise and acceptable microphonics.
If Philips or Mullard seem to you as excesive rich, try the JJ. Excellent price tag.

Myles Rose comments:
Early Marshall and Vox sound. Strong response in mids. A bit tamed brights (but no so much as in the WA tubes). It can sound dark or bright, depending on the amp. Ideal to push the complex front-ends of high gain amps: Riviera, Diezel, Bogner...
My personal comments:
I had very mixed experiences with this tube. Probably, this tube is the one that can be more inconsistent, from tube to tube and then, it makes lot of sense to purchase VERIFIED and matched tubes from an specialized tube reseller, as TAD, GT, etc.
Some of the tubes I bought bulk (standard test), had a failing triode or where very undetailed sounding.
But, the TAD 12AX7-Cz tubes that I've tried (selected JJ ECC83S tubes) are really good in all the senses, tone wise, gain, detail, dimensionality and character.
They are cheap enough so, I recommend you to pay the extra bucks to your tube dealer to select the best tubes.
Even that I don't like them in the Vox Night Train or the Fender Princeton or in the Koch Studiotone in V1 or V2, I liked them in the VHT Special 6 Ultra and, they are the best tube for my Marshall 1923C 85th Anniversary combo, in V1, V2 and V3. That amp seems to be designed around this tube!.

TAD 12AX7A-C (Sino)

My personal comments:
I don't like this tube in any single way. Sonically poorly detailed, goes even worst when pushed hard, delivering a very confussing sound. Lot of gain. When it breaks, its creamy but blured, more adequate for modern high gain than for classic Blues or Rock.
But I liked it very much as PI, it makes the amp very responsive to speedy riffs.
It could be used also in the critical cathode-follower positions so, don't throw it to the trash can, it still have some use

24 February 2013

Amps: retubing Marshall 1923C 85th Anniversary - the BIAS nightmare

Introduction

I was deeply testing lots of pre-amp tubes combinations, as I've commented in my previous blog entries.
Once the pre-amp configuration was clear to me, there was the time to try to swap the stock power tubes with a matched pair of Svetlana Winged C EL34s. Tone wise, I think stock tubes were already Svetlana, since I am not finding a tone change, neither a dimensionality change or more or less harmonics or a change in power or a change in the distortion character.
But, I can say this after a real nightmare trying to biasing myself this amp.

As the Marshall 1923C 85th Anniversary is based in the well known JCM2000 DSL50 platform, I started to search for information about how to bias this amp and, http://www.eurotubes.com has a video that explains how to do it yourself and it seemed really easy.

Well, the operation is easy but... you cannot get good results if you don't take into account some other important things when biasing your new tubes. This entry will go in detail about the things that I had to take into account before I've reached a good biasing of my new tubes.

Being my very first biasing, I was very uncomfortable with the overall process. Now, I would like to save this experience in this blog entry to be sure I can recall everything for the next time.
Since I've bought a matched quartet, next pair of tube will not need re-biasing (hallelujah!) but, I will needed it in some time.

Mixed and confusing information

I've started to search Internet and, basically Marshall forums (http://www.marshallamps.com/forums) and, found a lot of apparently contradictory information and, lots of different biasing values recommended by very different people.

What's biasing?

The first thing we need to understand is that biasing a tube is to provide the right amount of current for the tube while it's in IDLE status and, that means that it isn't receiving any signal (our guitar signal) so, be sure to completely roll off gain and volumes when biasing your amp.
In Idle status, the tube is alive, and there is some flow of electrons that we want to avoid to reach the plate, by negativizing higher the grid (the pin that receives our guitar signal).

The plate (anode) can dissipate a certain amount of power (in form of heat) before starting to deteriorate and, this amount of power is being named Maximum Plate Dissipation Power, and it changes for each tube type and each amp class (class A, class A/B...). This is the energy transformed into heat that the tube is releasing or, how hot can go tube components before they start to disintegrate themselves, while in idle status.

The Marshall DSL50 is a class A/B amp loading EL34 power tubes. The Maximum Plate Dissipation Power for an EL34 in class A/B is 25 Watts.

It is recommended to bias the tube to achieve a Plate Dissipation Power less or equal to a 70% of its Maximum Plate Dissipation Power, for class A/B amps. And a 90% is the maximum for Class A amps.
That means that when biasing, we can get a maximum of (25 W * 70% ) 17.5 W of Plate Dissipation Power and, beyond that value, our tube will start to worn really fast.
But, most of people will tell you that a 70% means that the tube works really HOT and, that the recommended ratio is around 60%, being 50% a value that is often seen as COLD.
So, our biasing should achieve a Plate Dissipation Power between the 60% (15 W) and 70% (17.5 W).

The formula is as follows:

Plate Dissipation Power / Plate Voltage = biasing current

So, if our plate voltage was 475 V, our biasing values would be between

(60%) 15 W / 475 V = 0.0316 A = 31.6 mA
(70%) 17.5 W / 475 V = 0.0368 A = 36.8 mA

But, if our plate voltage was 450 V, our biasing values would be between

(60%) 15 W / 450 V = 0.0333 A = 33.3 mA
(70%) 17.5 W / 450 V = 0.0388 A = 38.8 mA

So, we can see that the biasing values are HIGHLY DEPENDENT on the Plate Voltage and, this is just when issues begin.

Plate and Cathode method

If you want to read to someone that knows something about this topic, please, read this thread in Marshall forums: http://www.marshallforum.com/workbench/10890-biasing-70-vs-90-vs-plate-current-vs-cathode-current.html?referrerid=0.

To make it shorter, from the beginning amps where biasing using the Plate measuring method. This is a very dangerous operation (since voltage inside amps are higher than 300V and can go around 500V) and, it's how amp techs did it all the life and, still do. This is the accurate way of biasing an amp and, why it's highly recommended to let a qualified tech to do it.

The second method was issued by some amplifier makers, as Marshall, to provide an easier way for the common guitarist to do its own biasing. This is an "easy" way, not so dangerous and more inaccurate but, the only way we, the guitarists without proper electronics equipment and knowledge, can use (if the amp brings us such a possibility).

The rules about the Plate Dissipation Power still apply to both methods but, we are measuring different things in each method.

In the Plate measuring way, we are accurately measuring the Plate voltage and, therefore, the exact value that allows an accurate biasing procedure.
In the Cathode measuring way, we are summing together two values: the Plate and the Grid currents so, we need to adapt our equations above to do it in the right way.

The maximum power dissipation of the Grid is 8 W and, since we are in class A/B, same 70% rule applies for this component of the tube. So, we have to consider both dissipation powers in the equation, when measuring via cathode. That is:

Maximum Dissipation Power = Max. Plate Dissipation Power + Max. Grid Dissipation Power
33 W = 25 W (Plate) + 8W (Grid)

70% of dissipation will be now: 33 W * 70% = 23.1 W
60% of dissipation will be now: 33 W * 60% = 19.8 W

And applying those values to the equation mentioned above, we will have these values for 475V:

(60%) 19.8 W / 475 V = 0.0416 A = 41.6 mA
(70%) 23.1 W / 475 V = 0.0486 A = 48.6 mA

And, for 450V:

(60%) 19.8 W / 450 V = 0.0440 A = 44.0 mA
(70%) 23.1 W / 450 V = 0.0513 A = 51.3 mA

So, we see that the biasing values are VERY DIFFERENT in the Cathode method than in the Plate method, for same Plate Voltage.

Measuring Plate Voltage?

It seems a very dangerous operation, according to the warnings of most of people very done to amp works and, something I wanted to avoid, since I am not confident on handling voltages of such a high value.
The operation seems to be related to measure the output of the power transformer and, it's not being described in detail, to avoid people to try something like that without the needed knowledge and expertise handling lethal voltages.

So, ok, reading the different threads around biasing the DLS50, I've also found the official biasing procedure by Marshall here: http://www.drtube.com/schematics/marshall/JCM2000%20Bias%20Instructions.gif, which is recommending to bias to 45 mV (or 45 mA, since the bias reading is being done thru a 1 Ohm resistor and, therefore V = A).

While lots of manuals are recommending maximum 38 mV for EL34, this Marshall's value seems to be out of scope but, is it?.
Not really!. If we take into account that the biasing method is cathode reading and, we also consider grid dissipation, a "good value" would be 41.6 mA (@475 V) and, maximum would be 48.6 mA (@475 V) so, this value falls in between both.

Plate voltage varies depending on the source voltage (your mains voltage) but, it also varies depending on the tolerance values of all the components of the amp in the path, as well as it depends on the Power Transformer capabilities.
As per comments of most techs, the usual Plate Voltages are around 470V, but, there are variations between 450V and 500V.

In the ideal world, as the amp was designed, for your mains theoretical value (220V in Europe), the Plate Voltage would take the value of 475V.
In this scenario, the recommended biasing value of 45 mV would determine the following power dissipation:

power dissipation / plate voltage = bias value => power dissipation = bias value * plate voltage
power dissipation (45 mV @ 475V) = 0.045 A * 475 V = 21.38 W

That, represents a 64.77% of the maximum dissipation value so, between the safe 60% and the maximum 70%. It's a reasonable value, indeed.

I will explain later how I've got my amp biased without directly measuring the Plate voltage but, using an alternative approach, measuring the mains voltage. But, first, let me talk about my first wrong attempts. That's how I've learned and reached to that approach.

First attempts (Wrong)

Well, thanks to the probes and the measuring device that comes with the TAD BiasMaster, the operation is easy (if you have the right values) and safe. You haven't to remove the chassis and, you haven't to use your multimeter probes in the narrow space of the three pins used for biasing the power tubes.

My first try went really bad. In first place, the current values measured by the BiasMaster were varying very much between a second and the following one, the amp seemed to be in a constant instability.
That was my first error. I made the biasing WITHOUT turning off volume and gain controls and, therefore, the tubes weren't at its idle status. So take note of this: turn off your volume and gain pots before biasing!. Remember, we want to measure the tube on its idle status. The tone stack was left to their noon positions, also, in following attempts.

Second try, with the volume and gain controls off, I had similar situation than before and, I've seen that the left tube was giving very crazy readings, sometimes even zero. I realized that the biasing I did in the first attempt (that was for 37 mV) was giving me a reading of about 44 mV, probably because the controls were open during the first attempt.

Anyway, this issue with the left tube and the highly oscillating values reading in both tubes made me think if I had some issue with my mains current.
I've used my multimeter (set to 600V AC) and plugged its probes in a wall socket to measure the voltage and, in the space of 1 minute or 2, I've got readings ranging from 215V to 235V, with the numbers oscillating really quickly.

So, I thought: "there is no way I can consistently bias the amp in this situation, because I don't have stable current values". I've decided to leave the biasing for one day later, expecting a more stable voltage source.

Next day, voltage was way more stable, oscillating between 234V and 235V. Very high!. Way over the theoretical 220V that I should have!. And then, my thought was: "if the ratio of disipation varies with the plate voltage and the plate voltage can vary so much, mabye 45 mV is too much for my actually voltage. What can I do?".

So, I've decided to follow an indirect approach, to determine the biasing value depending only on the mains voltage.

The right attempt

Table of biasing values

Seeing that my mains voltages were over the common value, I wanted to make a table calculating the Dissipation Percentages for each Mains Voltage, given a certain biasing value.
My goal was to keep the dissipation percentage just a bit over 60% in the full range of measured voltages (from 215V to 235V) and, a maximum 65% at the top notch voltage value of 240V (which I've seen a few days, some time ago).

The idea was, to have a table of different biasing values to apply depending on the mains voltage I was receiving at time of biasing my amp. Look at this table (click on the image to see it full size):

On top, the data I was using for calculations, the EL34 and grid dissipation values and, the percentages that correspond to maximum dissipation (70%), Marshall value (65%) and the safe value (60%).

The table below left calculates the BIAS values for each mains voltage, based in the theoretical plate voltage. To do that, I've supposed a "lab" plate voltage of 475V for a "lab" stable mains value of 220V so, I can establish a direct relationship between the expected plate voltage depending on the mains voltage

The columns BIAS, correspond to the bias values for a 70% of dissipation (column Max) a 65% of dissipation (column Marshall) and a 60% of dissipation (column Safe).
Rows highlighted in green stand for the standard values that the company should be delivering in my mains wall sockets (between 220V and 230V).

Rows highlighted in yellow (including the green ones), are the values measured in my mains.
240 is the maximum Voltage my power conditioner will support before switching off the unit.

At the end, there are three rows. The first one calculates an average value taking into account the whole range of voltages. The second one just the values highlighted in green and, the third one, the values highlighted in yellow (including green).

This would give me an idea about which are the right values for each disipation ratio depending on the mains voltage. The average values should give me the rigth value to cover all voltage variations while remaining under the target disipation percentage.
You can see that the average values are very close in the three cases and, it seems that 44 mV should be a safe value in my crazy electrical environment, instead of the 45 mV recommended by Marshall.
But, I've read that at 60%, the amp has better dynamics and punch and, the tubes are under way less stress (and last longer). That average 40 mV (safe 60%) are very consistent with the values that Eurotubes is recommending in it's video.

The second table calculates the dissipation percentage for each "plate voltage" (theoretical value calculated based on the mains input), given a certain biasing value. So, we can see how the plate disipation ratio varies with mains voltage variation. Since my readings were of 234V to 235V, I wanted to achieve the 60% on this spot and, avoid going over the 65% at maximum mains input (240V).

If you look to both tables, which rows are related, you will see that the "theoretical" 220V (in dark green), corresponds to a "theoretical" plate current of 475V and that, with a biasing value of 41 mA, this means a dissipation of 59.02% (see last column of table 2).

At the measured voltage, during biasing, 234V, the related Plate Current should be around 505V and, the dissipation corresponding to a biasing value of 41 mA would be of a 62.77%, what is a very comfortable value.

You can see that, biasing at 41 mA, the maximum percentage doesn't reaches the 65%. At 240V, the dissipation would be of a 64.38%, still safe.

I would probably go up in the biasing value, maybe up to the 42.5 mA that some people recommends (instead of the "hot" 45 mA that Marshall recommends) but, I wanted to check first if this would give me a good sound, first.

The damn Phase Inverter tube

Once again, the mixed information drove me to buy tubes for PI position without matching triodes, because some people stands that is even better to the sound to have unbalanced power tubes. So every tube I've bought wasn't checked for balanced triodes. My fault.
I've read (after my purchases), that this is damn wrong. The amp maker is already designing the power stage slightly unbalanced, to give the right sound in the notch positions (when a tube goes down and the other goes up in a pull/push design) and, therefore, we need a very balanced PI tube to allow those power tubes to remain just as unbalanced as they were designed, to get the best tone.

Some time ago, I've read the reports of Myles Rose about all the tests he performed about several batches of tubes of each maker and model. The Sovtek 12AX7-WC was identified as a good tube, with values very close to 12AX7 specification so, I've decided to source a few. Later, I've read in the New Sensor's page, that the 12AX7-WC is highly recommended for Cathode-Follower positions, as well as an awesome PI tube.

Even than my preferred PI tube is the 12AX7-LPS, since I had some reliability issues in one of my amps (the Koch Studiotone Combo), I decided to throw there a Sovtek WC, instead of the LPS.
Both tubes were bought to TAD and, they come with the basic test (oh... it seems to work!) but, without checking for matching triodes (highly recommended for PI tubes).

So, my suspect was that I was taking that crazy readings for the left power tube because I had a PI tube with a bad triode. So, for my next biasing try, I swaped that WC tube (that never suit my bill) with an LPS.

Eureka!.

Clearly, the WC tube had very unbalanced triodes (contrary to what Sovtek says, and contrary to what Myles Rose found in his tests). Once the LPS was in place, the readings for both tubes were very consistent and, it was a child's game to bias the tubes to my target value (41mV, 41mA).

I also removed the WC in V3 (cathode-follower) and plugged there a TAD 12AX7-Cz (a verified JJ ECC83S), since I wasn't confident of the WC tube type anymore. I think I am gonna buy tubes only with balanced triodes, seen how inconsistent were the readings with that tube.

Testing the amp, biased at 41 mA

Yes, yes, yes!.

The amp started to sound really good and, today, I've enjoyed playing for first time in the last month.
I don't know what will happen when my mains voltage will drop up to the low 215 V I've measured, maybe it will go very cold, I dunno.

If this is the case, I will rise the bias maybe up to 42.5 mA and I will test again and, maybe, I end biasing it to 45 mA, as per Marshall recommendation but, I don't want to go so high with a mains power that behaves so crazy.
I would probably go straight to 45 mA if I had a Power Stabilizer, that delivered a constant flow of 220 V AC, independently of the mains voltage.

But, look at this table, calculating the disipation percentage for a biasing value of 45 mA:

At 45 mA, the Plate Dissipation percentage can go over the maximum 70%, for mains inputs over 237 V and, I am actually very close (around 235 V !!!).

The safest 44 mV that I've calculated as the average value (see table 1), will bring me a safer solution, since no mains voltage up to 240 V would cause a dissipation over the 70%. See next table:

And, probably, the safest value that will give me a good tone along the whole range of voltages would be 42.5 mV (as read in some forum, casually).

You can see that, this will ensure me a good value (60.06%) in the lower side of the table (215V mains / 466V plate) and that, at the actual mains value (234V) I am getting more or less the same dissipation ratio (65.07%) as if I was applying Marshall's suggestion for "standard" values.
Maximum dissipation ratio would be of a 66.74% for the highest mains voltage (240V).

So, I would test a biasing value of 42.5 mV, just to check that I am achieving a better tone during the whole range of mains voltages, if the currently set 41 mV seems weak when the voltage drops to 220 V to 215 V.

I am fully conscious that this is NOT an accurate method, but it's an approach that seemed to work in this particular case so, I wanted to log it and to share with all you. Anyway, I am under the recommended Marshall's value so, I still have some room!.

I think, I could take same approach with other amps if I would know which is the "standard" plate voltage expected at "standard" mains power, and, what's the maximum dissipation power of plate and grid of each related tube (6V6, 6L6, EL34, EL84, etc).

What I can certify is that a bias of 41 mV, works awesome at my actual mains voltage (234-235 V) and delivers a great tone. That means a disipation value between 62.77% and 63.04%.
Let see what happens when the mains voltage drops seriously down!.

Final Update 25/02/2013

I've re-biased it to 42.5 A (or 42.5 V) and, I am getting a consistent sound along the whole range of mains voltages. Today, voltage was oscilating around 224 to 226 V, what is a very good value and, everything sounded well. If the voltage goes up, I will never reach the 70% of Maximum Disipation Power so, I am safe and, if it goes to 235 V as yesterday, I will be in at 63% (still below Marshall's value). Today, I was around a 62%.

So, I am expecting the amp going a bit hotter when the mains voltage rises (but not so hot to achieve a bad tone) and a bit colder when voltage rises (but not so cold to achieve a sterile tone).
I think is a good compromise value for my crazy mains power.

I am very happy now that my amp was bringed back to life.

10 February 2013

Amps: re-biasing the Marshall 1923C 85th Anniversary

Introduction

I had some suspects that power tubes were a tad worn, due to the use, because the sound can be weak and thin, some days. A few years ago, I've bought a quartet of matched Svetlana Winged C EL34 and, not being sure which EL34 are mounted stock in my amp, I've thought this is the time for a change.

Also, thinking on that some or other day I should BIAS those new Winged C, I bought a TAD BiasMaster with 4 octal probes. That makes the Biasing procedure easier and safer. I had some surprises, during the process.

Fighting with BIAS

Well, the 1923 is an amp based on the JCM2000 DSL50 platform and, therefore, it shares same biasing spots and techniques. To access the tubes you need, at least, to remove the upper metallic grid on the back of the combo. This allows you to swap pre-amp tubes with ease but, if you need to put those BiasMaster's octal probes, you need to remove also the wooden panel that separated the both metallic grills.

I had to tilt the combo to clearly see the guide slot in the center of the power tubes socket, to be able to insert the octal probes in the right position. To do it, I've firstly removed the old stock tubes.
Once the probes are inserted in power tube sockets, I've connected each of the two probes to the BiasMaster device and, switched it ON.

The amp was switched on, in Standby mode for a while and then, switched on the amp, leaving the tubes to get their correct operating conditions for about 5 mins, to have stabilized readings.

I've got no readings for tube 1 and, went scared!.
Switched off everything, waited for tubes to get cold, removed tubes and probes.
I've thougth about several possibilities:

One of the new tubes isn't working
One of the probes isn't working
One of the ports for probes in the device isn't working

And, I did the silliest thing, I've set apart the new tubes and, wanted to check the stock tubes (since I knew they were working), I've swapped the pair of probes that I was using, getting the other double-set and, I've changed the ports from 1 and 2 to 3 and 4.

After warming up the amp, I had no reading in tube 3 (previously 1), again.
I had a look to the tubes and, the left one had no light. OMG. Maybe, not properly inserted in the probe.
I've removed everything, once more and, carefully re-inserted the probes and, each tube in its probe and, re-started the test.

Wow!. Both stock tubes were very unbalanced and biased around 30 mA, what is a very cool setting.
Even that Marshall recommends 42.5 mA to Bias the EL34, everybody else says that this is really hot and, that tubes will last no long that way. It seems that the correct values are between 37 and 38 mA and that, beyond those values there is no clear tone improvement.
I've decided myself to go for 38 mA, since I've also seen the video of Eurotubes setting a DSL50 to that levels.

Even that the biasing procedure, with those probes at least, is easy, it goes a bit tricky.
Each of the two trim pots affects the bias of one of the tubes, the left one the left tube, the right one the right tube but, both biasings are highly dependent.
To raise the current level, you move the trim pot to left hand and, to lower the current you move the trim pot to right hand. But, the funny thing is that, when you rise the current in tube 1, the current in tube 2 goes down and, viceversa so, you have to check every little turn in each trim pot until you can compensate both bias values. Gets a little while but, doesn't go more complicated than that.

So, seen that my stock tubes had a wrong bias, I wanted to give a try to stock tubes, once re-biased, before replacing them with new tubes, just to check if the sound was improved in some way.

Testing re-biased stock tubes

I would lie if it say that it was a big improvement on tone. Not really. Maybe, a solid body, while as it was the sound went thin from time to time and, clearly richer harmonically content, as a bottom warm path. I had to dial down the gain to get a clearer sound.

Throwing new Winged C EL34s

Once I've succeed rebiasing the old stock tubes, was time to see if a new brand Winged C EL34s will make some difference. Maybe not, since stock tubes seem to be in a good condition but, why don't check it before closing the amp?.

This time, I wanted to go slightly lower in the bias and, biased both tubes to around 37 mA (a very usual biasing value for EL34s). I've leaved the controls as they were in the previous test.

Ok. This clearly lowered the dirtiness of the sound in clean channel.
The overall sound is nice. I've found the Delay more notable now. The signal is strong enough without distortion but, I am probably missing some 3D projection of the sound . Maybe this can probably be because can run the amp at lower volumes with better results now.
I have the sensation that piercing highs gone away.
When I roll on the volume, the bite of the EL34 is there. Without going piercing the mid-highs explode with attitude.

Not sure which was the stock tube. Physically seems the same as the Winged C but, the tube isn't labeled the same way and has the logo Marshall, instead. Sonically, I don't hear a big gap or difference between the one and the other. The new one is quietest and has less highs or at least, they are mellower, not piercing at all. Maybe they are both the same tube but, age and use modified its natural tone.

Wondering if...

I am wondering if, now that the power tubes are correctly biased, Mullard reissues in V1 and V2 will make a difference, respect of my previous tests.
I know, I know... I should stop making tests someday.

09 February 2013

Amps: tubes, tubes and more tubes - Part 3

08 February 2013

Amps: Retubing Marshall 1923C Combo - Part 4

PI tubes test

During my previous tests it was very clear that this amp loved JJ ECC83S tubes and, even that I liked how it sounded with a Sovtek 12AX7-LPS in the PI position (V4), I wanted to check if it made any difference to use the JJ ECC83S instead.

Well, the JJ is darkening a tad the sound, seems to have more body than the LPS but, the amp went more noisy and was somewhat distorted, even clean.
The LPS brings an opener sound but lacks some body that the JJ had. The LPS was always more defined in speedy riffs. Difficult election.

I guess the issue is that none of those tubes were checked for triode balance and, probably, the LPS has a better balanced pair of triodes, while the JJ should be a bit unbalanced, what introduces crossdistortion in the power stage.

I think, I have to source some triode-balanced tubes to test the PI with better criteria. In the meanwhile, the LPS I've got works better than the JJ I've got.

Video

To me, the best test is to try the amp making some music (or noise) and, therefore, I did a video during my tests with some backing tracks behind.
I've covered "Shine on you crazy diamonds - Part 1" by Pink Floyd (not the whole song!) in first place, to check the goodness of EQ, settings of pedals and, overall loudness. Then, I was improvising over three blues backing tracks, always in the classic / clean channel and, adding Wampler effects during the test.

Note:
There is no audio post-processing using Pro-Tools or any other DAW. TSound and image were taken with a Zoom Q3HD, with the mic set to Low Sensibility, to avoid peaks and, therefore, compression.
In the video editor, I've used a compressor to raise the loudness (NOT TO MAXIMIZE), while preserving full dynamics and, being sure that the sound corresponded to what I heard live while playing.
After the compressor, just a brickwall limiter, to avoid overs when the sound will be converted to MP3.
No corrective EQ, delay or whatever else audio effect added. I could make it to sound nicer but, I always work in the same way, I want to be sure that the sound in Youtube goes so close as possible to the sound I am hearing in the room while playing.
Notice that I am not using any Noise Gate, not in the pedalboard and neither in the video editing (as a plugin). I don't like to fake the sound, What you hear in the video is what you would hear in the room.

Update 09/02/2013 - Swapping V3 and V4

Reading my notes about the Sovtek 12AX7-WC and, the information that New Sensor gives in its FAQs page I wanted to give this tube a new oportunity, after that V1 and V2 were clearly determined for JJ ECC83S tubes.

As per Myles Rose comments, the WC is a very close to specifications tube and, as per New Sensor comments, WA, WB and WC tubes work really nice in cathode-follower positions and, the WC specially good in PI positions, since it comes with well matched triodes. So Iwanted to test a couple of 12AX7-WCs fin V3 (Cathode-follower) and V4 (PI). I don't like none of those for tone positions.

So, the pre-amp was configured as:

V1: TAD 12AX7-Cz (tested JJ ECC83S)
V2: TAD 12AX7-Cz (tested JJ ECC83S)
V3: Sovtek 12AX7-WC
V4: Sovtek 12AX7-WC

Results are really good. The amp remains quiet, more or less same sensations than with the last configuration (TAD 12AX7-Cz in V3 and Sovtek 12AX7-LPS in V4). Maybe, slightly darker and, maybe, a bit less tridimensional but good enough. Since my LPSs didn't lasted for long in PI positions, I think this solution can make a very affordable set of pre-amp tubes with great results.

I am happy to see that I've got one more tube type available for PI positions (the Sovtek 12AX7-WC), among with a good NOS JAN/Philips 5751 (but raising the headroom), the Sovtek 12AX7-LPS and the Mullard 12AX7 reissue.

I am also happy to have more options for that cathode-follower positions (The Sovtek 12AX7-WA, -WB and -WC) among the Chinese 12AX7A-C and JJ ECC83S.

The series WA, WB and WC are designed for very low noise and microphonics so, its a good thing, indeed.

06 February 2013

Amps: tubes, tubes and tubes - Part 2

Introduction

In part 1, we described the basis components that make a tube and, a very light description on how they work. We also talk about the few Plants that are delivering tubes nowadays and, something about NOS tubes. We will continue adding some more information in this part.

Some interesting characteristics

For sure, the only way to know if a tube will sound good in a certain amp and in a particular position is just to test it there. There is no short path but, at least, we can control several technical characteristics that allow us to check if the tube is more or less close to the ideal specifications.

Tube names, as 12AX7, 7025, 5751, 12AT7, 12AU7, ECC83, ECC81, ECC82, EL84, EL34, 6L6, 6V6, KT66, KT77 or KT88 (to name a few) are specification names. Under each name, there are a bunch of technical characteristics that, when measuring the tube, should get a value between the range specified for each tube category.

There are a lot of characteristics under each specification but, maybe, the ones that we are more interested on are: Gain, Output and Transconductancy. But, first, let bring some clarity about the specification names above and, very specially those related to the family of 12Ax7 tubes.

As we said, there were two main spots were tubes were produced during the Golden Age: USA and Europe. That time, European devices mounted European tubes, while American devices mounted American tubes but, they made tubes with same characteristics in both sides of the ocean but, followed different naming systems.

So Europe called their tubes beginning with an E, like the ECC81, ECC82, ECC83, EL84 and EL34, by example, while USA used some other naming system.

Specifications for power tubes are very different. So EL84, EL34, 6L6, 6V6, KT66, KT77, KT88 are really very different tubes, each one of the rest.

But, under the called family 12A of tubes, there are tubes that have an American Specification and one European Specification but, that are absolutely equivalent. So.

EEC83 = 12AX7
ECC81 = 12AT7
ECC82 = 12AU7
etc

Also, there are other tubes, that doesn't follow that 12A naming that are considered as part of the family, like the 7025 or the 5751. In fact, a 7025 is a 12AX7 that follows specifications to the highest grade, having the lowest microphonics. The 5751 is another kind of tube and, as different as a 12AT7 can be respect of a 12AX7.

Each of those tubes have characteristic values that are different of the rest of tubes and, that's because they were designed to accomplish some particular design goals.

By example, a 12AX7 (or ECC83) should have a Gain of 100, should drain a specific current value, and should deliver a certain output value, among having a certain transconductancy value.
The 12AT7 (or ECC81) should have a gain of 60, and drain about 10 times the current that a 12AX7 drain, and have a higher tranconductancy value.
The 5751 must have a gain of 70, drains slightly more than the 12AX7 and has a better transconductancy.

It is said that all those tubes under the "12A family" are direct substitutes, one of the other but, that's a partial truth. For sure, you can swap any of those with any other but, not without collateral effects.

By example, while the 12AX7 should drain around 1.0 mA, a 12AT7 drains around 10 times more current and, that means, that can stole some of the current that the rest of 12AX7 tubes need. So, if a particular tube position in a particular amp was designed for a certain type of tube, you should go as close to that specification as possible. By example, instead of a 12AT7, you could consider a 5751, that has a current drain closer to a 12AX7 and, a gain (70) closer to that 12AT7 (60).

Why people swaps a 12AX7 with other types?.
When you want more headroom, you can try to lower the gain of the pre-amp stage. In that way, the first tube doesn't delivers so much output to the following tube, that delivers less output to the following one... that delivers less output to the Power tubes so, all the tubes remain under their own break up levels (when they start to distort). That's why SRV was using a 5751 in V1 (instead of a 12AT7). A 12AY7 or a 12AU7 will change the overall gain, even more.

But, as we said, each tube has its own signature sound and, the way they distort, when breaking up, is very different, depending on the model. Some deliver crunchy distortion, while others deliver creamy distortion so, swapping the tube affects the tone also, not just the headroom.

To lower a bit the overall volume of the power stage, you can think also into use a lower gain tube in your PI position. Many people uses a 12AT7 in that position, to help to lower the overall volume of the amp but, once more, maybe a 5751 is a better election, because it's closer to a 12AX7 in specifications.

Anyway, you can always experience yourself the impact that each tube type has in a certain position of your amp. No risk to the amp but, a clear effect over your tone.

Well, we saw what gain means. A tube with higher gain will break up earlier and, therefore, it will add it's own distortion to the sound and, will push harder the following tubes in the gain cascade. That could be very interesting to achieve early distortion at lower levels. A lower gain tube will give you a late distortion and, will allow you to raise the volume higher before you get some distortion.

The output is the density of electrons that the the tube is delivering at its output. The more electrons, the easier will be to push other cascaded components inside the amp. Modern High Gain amps have very complex pre-amp sections that require lots of current and, therefore, tubes with high output levels help to feed all those components.

Transconductancy can be seen as the "velocity" to react from one signal to the following one and, has a lot to see with the feel of immediateness of the amp. Some amps seem to react more slowly to your picking and, you find yourself playing slower riffs, since there is some kind of "delay" between the instant when you pick your note and the instant when you hear it. Sometimes, this can be only clearly noticed when you are trying really speed riffs and, this can make a clear difference for an Shredder and, not being of a great importance for a Classic Blues player, by example. For sure, the sag of a tube rectifier adds more delay to the response time than a solid state rectifier. A tube rectifier will be probably preferred by a bluesman.

What tube stores / resellers do?

Something you couldn't probably do. They measure characteristics and, choose those tubes with good characteristics (closer to specifications), relabeling those tubes and, charging a fee for their selection work.
Think that they are buying a big amount of tubes, testing them and rejecting those that are under certain specification level. For sure, they have their own acceptable deviation range. Let say, they get tubes that are below or above a 15% respect of specifications for each characteristic that they measure.

They are usually measuring, gain, output, plate current and transconductancy. Those numbers give a good information about the technical quality of the tube and, a bit about its sonical characteristics.
High gain means early break-up or distortion, higher transconductancy means higher immediateness, higher output means higher possibility to drive complex front-ends.
Usually, they use a numbering (color or mix of both) system that classify their measurements under a certain range, but they usually don't share what that classification system means: What means a tube has the number 6?. Maybe, a gain between 60-69?. Who knows!.

I have the suspect (just a suspect) that those tubes that doesn't pass the filter are sold as bulk tubes. This suspect is based in the fact that, when I am buying a relabeled tube, I am having no issues but, when buying several tubes of the same Maker / model (but not relabeled) to the same store, I am having issues (one triode fails, more noise, less gain, less output...). And, this, in a set of 10 tubes seems very suspicious.

So, my recommendation is: buy their relabeled tubes, because they are the best they have. Yes, you are paying them a fee but, at least you know they will get you the best ones.

The other thing is to clarify with the seller which tube is which. By example, TAD is using 12AX7-A to relabel Chinese tubes, 12AX7-Cz is the name that gets the JJ ECC83S, 7025 are Just 12AX7-A that go closer in technical specifications (and really bad in sound). Groove Tubes call 12AX7-R1, 12AX7-R2 to some Russian tubes, which ones? Sovtek, EH?. I knew it and I don't recall it. Watford Valves is relabeling those tubes under the brand Harma, by example. Sometimes, something in the model name can bring you some information about which tube is is reality but, I think you have the possibility to ask and, they should answer you back.

One curious case is Eurotubes, they verify their tubes without relabeling and, they are mainly selling JJ tubes.

Which is the best tube?

This is a typical question that everybody (including myself) formulates one day. And, it has no answer.
There is not such a best tube for each amp, each position and each player.
When retubing an amp, you can go two ways: to retube the amp with the tube maker/model that were already there (if the tone was ok to you) or to start the tube dance.

If you start the tube dance, you will visit one or more forum, will ask for the best tubes for your amp and, you will get LOTS of contradictory information. What seems to work for some, it's the worst for others.
What's happening there?.

Even being the same amp, every player giving you information, can be playing with a very different equipment (guitar, pedals), with a very different style and, giving a very different use to the amp (bedroom, studio, small gigs, big gigs) so, to get the information about which is the best tube set for them, without knowing all those details can confuse you more than help. Be sure to understand which equipment, style and amp use is behind every recommendation and, get the one that better describes your needs.
Also, some tubes can work very good in clean and really ugly under distortion, and viceversa.

My own experience swapping tubes in several amps says that I cannot discard any tube (before I try it!) and, that the results that I've got for an amp can be totally the opposite I can get in other amp. By example, a Sovtek EL84 had an even tone and smooth distortion but, lacked dimensionality in a Vox Night Train and in a Koch Studiotone but, bringed back to life, with a magical tone, the Pro Junior of a friend of mine.
One more example, the JJ ECC83S sounded with less definition than most of other tubes in a Vox Night Train but, in my Marshall 1923C is the tube that better works at all levels.

The real thing is that you never know in which amp and in which position a certain tube will work the best.
So, I recommend you to take your own notes about your experiences so you can review them later.

Anyway, there is some rule that seems to be true. Vintage amps, with vintage designs love more NOS tubes, while modern designed amps, seem to like more modern tubes.
Probably, after that gap between NOS and Production Tubes, amp designers changed some components to adapt their design to the available new production tubes so, those amps are somewhat, correcting the gap in sound between NOS and new production tubes.

More to come... stay tuned.

05 February 2013

Amps: tubes, tubes and more tubes - Part 1

Introduction

Once Upon a Time there was a guitarist unsatisfied with the tone of his POD X3 Live, that previously substituted a Marshall Valvestate of 30W that never liked and, that one (bad or good) day, decided to try a tube amp. The sound of that tube amp cursed the guitarist and He remained prisoner in Tubeland for the rest of his life. The End?.

After several years testing several amps and trying lot of tubes (or valves), I think it's time to make some reflexions around that cursed bottle.

Without any doubt, nothing can be better than the sensation of the air being pushed by the kick of hot tubes. It doesn't matter how good the solid state amp is, the wonderful that that modeling software works, that wonderful multi-effect pedalboard than can modeling any kind of mythic amp: nothing can sound better than a tube amplifier!.

By example, Amplitube 3 achieves a very nice effect, getting very close to those amp models that it's modeling and, they sound very close to a miked amp recorded in some studio but, there some things that no software can emulate, in any way.

Firstly, the air that amp's cabs move are way more than the air that studio monitors will do.
Loudness (or our sensation of volume) isn't the same, when the air pushed by the amp's cab evolves your body and directly hits you.

Secondly, the feedback. When the gain of an amp is pushed hard, there is some feedback with your guitar that, you could even control and use as an additional effect. Our position respect to the speakers, as well as the kind of guitar and pickups used, make this possible. A controlled feedback was one of the best weapons used by our beloved Jimi Hendrix, by example. Well used, can be an expression tool as interesting as a good Wah.

Alright!. There is some unexplainable magic that can be only found in tube amps, that cannot be modeled by any software, even the most sophisticated but, for this magic to take real place, the tubes of such an amp should offer their best to the amp.

There are a bunch of tube types and models, for pre-amp stages and power stages but, the most spread tube type is, maybe, the 12AX7 (American Specification) or ECC83 (European Specification). Together with the 12AX7, it's often seen some 12AT7 tube and, less often some others.
As power tubes, the most commonly seen are EL84, EL34, 6V6 and 6L6.

What a tube is?

A tube is an old design based around the concept of a triode and, enclosed in a bottle that resembles the old fashioned light bulbs.
The principle behind it is very easy to understand. A triode has 3 elements: the cathode, the anode (often named Plate) and the grid.

The cathode is a metallic plate (or just a filament) that looses electrons when hotted hard. To heat the cathode, the tube uses one more component, named filament (or heater) and, that follows the same principle that that filament that we can find in any typical light bulb. Its highly resistive material transduces the electrical energy into a big amount of heat (it goes so hot that produces light!). The filament is in close contact with the cathode, to transfer the dissipated heat to the cathode. Some cathodes are just filaments.

When the cathode starts to warm enough, it starts to loose some electrons (negative). Those electrons travel inside the tube looking for some positive pole (that has a lack of electrons) and, the anode (or plate) is just that, the receiver of those electrons.

When the cathode is hot enough is when it looses the maximum number of electrons by unit of time (its saturation state). Imagine then, a constant thick beam of electrons traveling from the cathode to the plate, while the tube continues receiving electricity.

This would give us just two states: the tube is off (no electrons) or the tube is on (full flow of electrons), despite of the initial state, when the tube is warming up and going from zero electrons to its max capacity (saturation).

This is how a Tube Diode worked (and still works) but, one more component was designed to regulate that flow, as if it was a faucet, that you can open more or less to regulate the exact flow of water you need at each instant.

This third component is some kind of filament wrapped around some posts, and that follows a spiral pattern and, that stands between the plate (outer and bigger component) and the cathode (inner and smaller component). For how it looks like, it was named the grid.

Usually, the signal that comes from our guitar and enters the amp is directly governing that grid so, the fluctuations in our sound (transduced to electrical flow) are used through the grid to control the beam of electrons that flow from the cathode to the anode. The trick is that the grid can go from zero to values more negative than the cathode and, since negative and negative repels, the emitted electrons go back to the cathode, without reaching the plate. The more negative the grid goes, the less electrons are catched by the anode and the more electrons are rejected back to the cathode.

The interesting thing of a tube is that fluctuations "sensed" in the grid produce proportional fluctuations in the anode, but of greater dimensions. So, if the values seen in the grid move between -5 and +5 V, maybe we can see variations between -100 and +100 V in the anode. This proportion between the input voltage and the output voltage is being called the Gain of the tube.

What means that a tube has a gain of 100?. Well, that means that the output voltage will be 100 times greater than the input voltage, at any instant. So Gain = output voltage / input voltage and, therefore: output voltage = nominal gain * input voltage.

Imagine a pickup that delivers 400 mV (0.4 V) maximum and, that this value reaches the tube, if the tube has a gain factor of 100, the output voltage corresponding to that input voltage will be 0.4 * 100 = 40V, but since the output of a pickup fluctuates between positive and negative values, the input voltages will be comprised between -0.4V and +0.4V and, the output voltages, between -40V and +40V.

We see that the total variation of the input voltage is of 0.8 V (summing up the negative and positive halves), while the output voltages will vary in 80V. Since the total variation of the output voltage is clearly greater than the variation of the input voltage, we say that there is a voltage amplification.

So, if a tube has a gain of 60 (like a 12AT7, by example), same input values mentioned above will generate the following outputs: -24 V to +24 V (a total variation of 48V).
tube with a gain of 40 (like a 12AY7) will produce an output between -16V and +16V (a total variation of 32V), etc.

Ok. We got the picture so... why a 12AX7 has 8 pins instead of 3 (one for each component)?.
Because a 12AX7 has TWO triodes packed in the same bottle. So 3 x 2 = 6 pins. And, the other two?. Are used as the positive and negative poles for the filament that heats the cathode (usually named Heater).

All those components are packed together inside a Crystal bottle that needs to be void of air, to allow a better flow of electrons (in the void). When closing the bottle, it always remains some air inside. The final trick is to burn this remaining air, with the help of some chemical component. The Getter is that kind of disk that you can see in the top of the valve (inside) and, that contained the chemical product used to burn the air that was remaining inside the bottle.

Power tubes are, usually Pentodoes and, mount a single triode with two added control grids (suppressor and screen), that were designed to solve some issues that were detected in triode designs (as capacitancy and other issues).
1 triode + 2 grids + 2 poles for the heater = 7 pins, but Pentodes usually have 8 to 9 pins. I don't remember for what are the others. Probably, the octal design has a common negative for each additional grid, while in the 9-pins design, each additional grid has its own negative pin.

The tube: that delicate thing

Every material used for each component, including the flash, affects the way a tube works and, any unwanted impurities that mixes components can affect the way a tube works. And, how a tube works determines how our amp will sound, at the end.

The quite well artisan production of a tube makes all them different. Tube specifications fall into a very variable range (depending on the Maker and Model) and can have a big impact in our tone. By example, a tube that theoretically had to have a gain of 100 (a 12AX7), and have just an slightly lower gain of 96 is missing some of the total voltage variation that the amp designer expected in his design.
In my opinion, voltage is very related to dynamics, the wider the voltage variation, the more dynamic the sound is, so we are loosing some dynamics, in that case.

Myles Rose has reviewed several batches of several makers and models, taking notes of their technical values measured for each specification and, found some models more consistent than others and, some variations around a 20% below specifications. That could mean to have a tube delivering a gain of 80 instead of the expected 100.

The tube, during its short or long life, its suffering a real torture. The structure of the filament and rest of elements deteriorates in a faster or slower way, depending on multiple factors: from the materials of each component, to the way tubes were biased in the amp, to were they are placed and, how hard are subjected to mechanical stress.

The three components of each triode are, usually mounted on some structures named spacers, that dilate with the heat and enter in contact with the flash, absorbing some of those vibrations and, avoiding that those are being actualy transferred to the triodes.

You never know which will be the life of each tube. Even same maker / models can last very differently in the same amp and same spot. Some spots are harder.
By example, power tubes often last half the life of a pre-amp tube but, the PI or Driver tube (part of the pre-amp) lasts more or less the same as the power tubes, because those are the most stressed tubes in the amp.

If the tubes are inside a combo, the speaker is electromagnetically affecting the tube and, worst than that, the speaker is kicking waves of air that hit tubes hard, what can make that some parts go lose, introducing microphonics issues.

Well, you see, a tube is some kind of art and, every single one, even being very similar to others of the same batch, is somewhat unique. The bad thing is that we don't even know how long it will last, even being specifications-correct.

That kind of rawness and lack of perfect linearity is probably why tubes have some magic that transistors haven't. Each tube creates its own functional noises, its own set of harmonics and, grain and type of distortion. They have a "signature" sound, indeed.

NOS and New Production tubes

If you still didn't heard or read the term NOS, you will, for sure.
NOS means New Old Stock and, refers to those tubes that were produced during the Golden Age of tubes and that were remaining in their original boxes, laying on bins of certain stores (or military surplus depots).

It's paradoxical that the new production tubes cannot reach the same consistent specification values of tubes that were produced 60 years before. Everything seems to be more evolute nowadays, in terms of tools and production techniques so, what's the issue?.

We need some perspective to analyze it. During the '50s and '60s, tubes were everywhere, in TV sets, radios and many many other electronics devices.
When the transistor appeared, because of it`s small size, reliability, cost and more linear behavior, it started to replace the tubes in each existing design. In fact, a transistor is able to amplify our guitar signal with higher fidelity than any tube. A high fidelity means that it doesn't introduces any kind of distortion and, that the amplified sound is some sort of a resized clone of the original sound.

The big business that was to produce tubes, was starting to be impossible to maintain, due to the lack of demmand. Therefore, the big factories that were producing tubes in USA and Western Europe closed.
Only some factories remained in the old Sovietic Union., since they weren't affected by the offer-demand law of capitalism.

Audio is a very curious world. The perfect work of a transistor made the sound more mechanical and less attracting to the human ear. It seems that the magic was in that several distortion forms that tubes were introducing in the sound what made them really attractive to our ears, sounding more musical.

Musicians wanted to go back to tubes and, Audiophiles wanted also to see their loved tubes pushing their expensive high fidelity sound systems. So, they started to get some tubes from those Sovietic factories or Military depots. But, those tubes were sonically very inferior to tubes that were produced in Western Europe or USA. Maybe, they were absolutely correct for Military applications but, they all sounded sterile for musical applications.

Those musicians that still had those vintage amps that delivered the most of tones we can hear as recorded today, started to be in the need of replacing their old tubes and, went totally disappointed with the sound of those Rusian tubes and, started to search for NOS tubes everywhere.
That raised the prices of NOS tubes and, lot of people made a lot of money with all this.

NOS tubes (at least, the ones aftersought) already disappeared from the scene and, a new kind of tubes was sold as NOS. Many people started to rip old gear, like old disk players, radios and, any other kind of electronics device powered with tubes (as organs) to get one tube of that historic golden period and, they started to sell them as NOS, when they were not new anymore (they were already used in that device).
Those made still more money than the first ones and, the bad thing is that lot of people wasted their money in a tube that was already worn, or not delivering the best tone, or that lasted short.

There was a big gap of time were the only available New Production tubes were sonically very inferior to NOS tubes but, fortunately, some companies decided to invest in the remaining factories and, enhance the tubes to reduce the gap against NOS tubes. This was the case of Electro-Harmonix, that invested in New Sensor (Russia) or Ruby Tubes, that invested in a Chinese factory.

Nowadays, they are some New Production tubes that sound quite well the same as NOS tubes or, at least so good that you don't feel in the need to go for a NOS tube and pay the high difference in price against the small difference in sound. Reliability and Production Consistence has been enhanced, also, while prices are way lower so, we are again in a new Golden Age or... a Silver Age?.

Who produces tubes?

If you are in the market for a tube, you will see lots of brands and lots of models within each brand but... are there so much makers?.
The truth is NOT. Tube Makers (that produce tubes for musical amps) are really very few, we are talking about New Sensor (Russia), JJ (Slovakia) and Sino/Shughuang (China) and, eventually, EI (Serbia) and Svetlana (Russia).

It seems as if each Amp Maker had its own Tube Production Plant, since the tubes that came in their amps have their named labeled (Marshall, Mesa, Engl, Koch, ....). All those tubes come from one or more of the few factories mentioned above.

There are lot of tube stores that sell their premium tubes with their name (TAD, Groove Tubes, Watford Valves, etc...), don't they have their own production plant?. NOT, they don't. They just purchase batches of tubes, test them and relabel the nice ones under their own brand name.

To increase the confusion, New Sensor bought the name of very well established tube producers of the golden age and, it's producing tubes under several brand names: Sovtek, Mullard, Genelec, Electro-Harmonix and Svetlana, are some of those brand names.
New Sensor is an American company that produces their tubes in russian Reflector's factories. One of that plants, named Xpo-pul is producing tubes under the name Svetlana but aren't the tubes really manufactured by JSC Svetlana, in Saint Petersburg. Since New Sensor bought the brand name, only New Sensor can sell tubes under the Svetlana name in USA and Canada. But, JSC Svetlana is still producing the real deal, the winged S and winged C tubes and, its selling those tubes without the brand name but, with the winged S (preamp) or winged C (power) logo. The Winged C EL34 is the most appreciated tube for Marshall amps!.
JJ was Tesla-JJ in the past and, is producing now all their tubes under a single brand: JJ.
Ei produced tubes under its own brand. It seems that factory closed in 2007 and, according to those who experienced that tubes they were the best sounding ones but, didn't lasted long sounding good and, they presented a high ratio in microphonics issues.
Shunghuang is producing no-branded and rebranded tubes. It's the factory that can produce the tubes you want, according to your specifications (usually called STR, like the TAD EL84-STR, by example) and labels them with your name
As any Chinese factory, can produce from real crap to the highest quality product and, it all depends on how much do you want to pay them. They have a dedicated high fidelity brand of tubes, with very high price tags and, they produce the cheapest tubes at same time.
Anyway, chinesse tubes are going everytime better and are really affordable. The 9th generation of chinesse tubes is really good, if we rely in Myles Rose.

There was one more factory in China, I think that drived by Ruby Tubes but, that company brook and, the related Chinese factory isn't producing those tubes anymore. Since they aren't producing Ruby Tubes anymore, those have become NOS tubes!!!. And, since they produced one of the finest 12AX7 tubes, they are aftersought, nowadays.

Paradoxical world, tubes world!.

Some videos

I think this couple of videos about a tour for the Ei Factory can be of your interest.

This one is a complete manual process of a high audio tube type. Since components are bigger, it's easier to understand what was written above.

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