06 February 2013

Amps: tubes, tubes and tubes - Part 2


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

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.

1 comment:

  1. Great series of blog posts about tubes. Thanks for sharing!


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