25 June 2012

Guitar Amp: How it works? - Part 5

Elementary, my dear Watson

After seen the several blocks of that Fender's Champ schema, we can guess following things:

  1. The amp has 3 tubes
  2. V1 is the only pre-amp tube. Tube is type 12AX7. First triode is the first gain stage and second triode is the second gain stage and Driver. So, this tube will wear at the same speed than the power tube.
  3. V2 is the single Power or Output tube. Tube is type 6V6GT so, the power of this amp could be around 5W to 7W, a low wattage amp that will not work for a gig but, that should work really good for home practicing and studio recording.
  4. V3 is the tube rectifier. Tube is type 5Y3GT. This means that this amp will have that sag compression that tube rectifiers give.
  5. The amp is designed to work with a single speaker with impedance of 4 Ohms.
  6. The amp has a single-ended Class A, cathode-biased topology so, it will not need to re-bias the output tube after substitution.
Because of the few preamp stages, this amp would maintain a great clean headroom and, when entering in distortion, it should deliver a saggy overdrive kind of distortion, never going so hard. This must be understand between the volume range of this amps so, it will break-up at very low volumes, in fact, because the total available power is very low.
The output type (6V6GT) will provide strong basses and a thick distortion (instead of crunchy or creamy), the sound can be easily missed in the mix (not cutting the mix enough).
This amp is probably well suited for Vintage-alike sounds and, possible for Blues Territory, every time that very fast riffs aren't needed (the tube rectifier creates some kind of slight delay in amp's responsiveness).
Because of its Class A design, we can expect a warm and rich tone.

Let me find some youtube demo of this amp around and let see how close we were with our deductions.

This little amp is a clear example that we don't need so complex designs to achieve outstanding sounds and that, usually, less is more. The low wattage of this amp allows us to crank it in any place, at home, at studio so, you can always get the loved harmonics that tubes generate, achieving a sound full of nuances.

Some Reflexions

For a guitarist, it makes no sense to go till the last electronics detail of an amp but, to understand the basics is a good thing to understand how to achieve the best from our amps.

We discussed that the signal is really being created by the last output tube, which signal is being modulated by its previous gain tube, which signal is being modulated but his previous gain stage and so goes on, until we reach the first stage, where the signal generated at this stage is being modulated by our guitar' signal (remember that key-copying machine that we talked about). That makes me think that the amp has more relevance than the guitar itself, since the real tone is generated within the amp, just copying the nuances from our guitar' signal.

Sometimes, we are expending way more money in a new guitars than in an amp that can satisfy our needs.
By, example, we can go for that Gibson '59 LP or Fender Custom Shop or PRS private Stock or any other kind of guitar with a prohibitive price and, use one of the cheapest amps in the planet.
This is very human but, definitively wrong. Sometimes, you can achieve a better result pairing a middle-priced guitar with an outstanding sounding +amp. The really important thing here is to have a comfortable guitar, with a set of pickups that can cover the sounds you are after. Let the amp to extract all the juice from your guitar.

Rightsizing the amp has a lot of benefits. Choose an amp that you can crank in your usual environment.
If you are usually using your guitar and amp at home, don't go for a 50W or 100W monster , because you will never get the best from that amp. Those need to go really loud to sing as the should.
You will early discover that even 5W in a Tube Amp is really loud for a flat and, way louder than those "dormitory levels" that we sometimes want (to no disturb rest of family and neighbors).

When going to most of small Recording Studios, you will find that high powered amps are usually an issue. The sound tech will start to claim about the loudness of that amp, asking you to roll off volume and, therefore, making you to loose the mojo coming from that cranked amp.

Same is usually happening in small venues, where the sound is all passed thru a Mix board and, where the overall loudness can be even lower than your amp's loudness when cranked.

I know all this is really easy to say and difficult to follow. I was also warned about all this but, you know, we all have the picture inlayed in our brain, those Marshall Full Stacks that our rock heroes used in their concerts are a heavy load for our preferences. If we buy something smaller, we think we aren't reaching the heaven but, it's the same case as to use a Porsche 911 to drive at 40 Km/h speed. OK, you are getting all the attention and, maybe you will attract some girl but, at the end, you are loosing all the fun that the car can bring to you.

Usually, an amp of 15W to 30W can cover the most of ground. It can be used at home (with some attenuator, for sure), in Studio environment and even being miked into a PA in a venue or, do an small venue without mics. Think that: you can make that amp to sound louder just with a speaker swapping.

Most of people confuses amp's power with loudness.
The output power of the amp is the power that the amp has available to move a speaker or set of speakers but, what at the end determines the overall loudness is the speaker itself.
One amp can sound weaker or louder depending on the speaker's efficiency.
By example, with same power handling capabilities and impedance, one speaker can be delivering 95 dB, while other can deliver 105 dB of SPL (Sound Preassure Level).
A difference of 10 dB in speaker's loudness seems few?. Let analyze that.

The difference between a 100W amp and a 50W (maintaining everything else equal) is just +3dB of volume increase. From 25W to 100W we have a +6 dB of volume increase and, from 12.5W to 100W we have a +9dB of volume increase. Do you get the picture?.
To choose an alternative speaker with +3dB can have the same effect as doubling the power output of your amp (despite of other nuances that a big amp can bring to the sound).
This is a well known case with the Vox AC30, a 30W amp that usually sounds way louder than some 50W and 100W amps. The trick is that the AC30 was using the highly efficient Celestion Alnico Blue while most of Fender's were using a less efficient Jensen speaker, by example.
Sometimes a low power amp can sound as biga as a high powered amp, depending on the set of speakers so, don't be fooled with amp's power. This never says the whole truth.

We didn't talk about speakers (because it's a world in itself) but, speakers are so important as the rest of parts in an amp. Speakers are more or less efficient in some frequency ranges and, they can have their own equalization of the sound generated in the amp. Speakers add their own distortion and have their own break up spot, also.
So, to pair the exactly wanted speaker with the exactly wanted amp is some kind of art and, a good (but expensive) experimentation way.
We can be expending a lot of money swapping tubes, while maybe just a change of speaker can do the trick.

Tube swapping as well as speaker swapping, are expensive experiences.
I would like to see in the future an instrument' shop having an area were you can test YOUR amp with different tube makers/models and speakers, to choose the most interesting ones for your needs.

My last reflexion is about the willing of buying an amp with the expectation of to change its sound later.
In the same way that you will be never able to make a Gibson LP to sound like a Fender Stratocaster, and viceversa, you will be never able to make a low-gain amp to sound as a high-gain monster.
We saw that the foundational tone is being sculpted within the amp and, it depends on several topologies, the different biasing values, tube types, etc.
We can change some details of the overall foundational tone. We can achieve a crunchier, or creamier, or thicker distortion by swapping some tubes (each tube can deliver a different distortion grain) or, we can wider the clean headroom or make the amp to break up earlier (working on the first stage and PI). We can make the amp to sound weaker or louder, swapping the speaker, or make it to enhance a bit more certain frequential range but, we cannot change the basis foundational tone.
A vintage-alike amp, that break-up just in slight distortion will never have that thick wall of sounds that a high gain monster with several cascading gain stages will produce. The opposite is also true.

Sometimes, it is better to run a good amp, with a very simple design and outstanding clean tone and, to use pedal effects to achieve different levels of gain and distortion sounds. For sure, the best distortion that you can always achieve is the one produced in the own amp but, at least, you can emulate most of the goods of a high gain monster by using a high gain distortion pedal in front an outstanding clean sounding amp.
David Gilmour achieves all his broad palette of sounds running a bunch of pedals in front of its outstanding clear sounding Hiwatt amps.

A cheaper experimentation path is just to understand what the pre-amp and power amp does to your sound. If you have gain and volume controls, theoretically you can get a better control of your pre-amp and power sections separately.
Lower the gain (to avoid the pre-amp to distort) and maximize the volume (to make the power amp to distort). Play with several volume levels (usually, interesting things start around 7 of a 10 dial). Increase bit a bit the gain and hear what happens. Then go opposite.
Lower the volume knob until you clearly hear your instrument and roll on the gain knob (usually, interesting things occur around 7 of a 10 dial). Hear the sound and then, begin to increase the volume knob, while hearing the changes. This should let you to achieve the most preferred sound from your amp, with the right amount of gain and volume.

If you have a tone stack, take into account the following: usually the tone control closer to the input of the amp is the one that has more relevance about the overall tone of the amp, moving the basis frequential range up or down, the rest of tone knobs just tame a bit that foundational tone.
So, imagine that we have Bass, Mid and Treble, in that order. Put your Mid and Treble all down and, start rolling on the Bass knob until you get a nice presence of basses, without going boomy or muddy. Then, start to roll on the Mid knob, until you get a nice presence of middle frequencies, without going honky or strident. Then start to roll on the Treble knob until you get a good definition of trebles, without being piercing.
Finally, tweak a bit more all knobs to find the best spot, because all the tone stack works together, as an unit.

If you have a Presence knob, start with it after you worked the tone stack. Presence knob should increase the high-end frequencies and give some air to the sound but, overdoing it you will obtain piercing trebles that can fatigue the ear in short time.

If you have a Contour control (some kind of set of basic equalization values), start with all tone control in middle position and roll on the Contour knob, until you get the basis equalization setting you want to work on, then start with the tone stack and presence knobs as mentioned above.

Take into account that everything can change as soon as you swap your guitar. Maybe what was good for an LP will not work for a Strato so, you could be forced to re-dial your settings very often.
Also, take into account that tone controls are intended to adapt the amp' sound to the environment. Maybe, in your practice room there is more resonance of certain range of frequencies but, once you move your amp to the Studio, the room characteristics change as well and, this usually needs you to rework the amp' settings.

The more you go experienced with amp's controls and how they change with every type of guitar and room, the easier it will be to readjust the controls.
In my honest opinion, it worths the spent time to play with all the controls at least once for each amp, carefully hearing the sound (not how good and exciting is your playing) and understanding how it changes with every knob's twist.

Finally, have into account that tubes need to run for between half to one hour to be in their best performance level so, play with those adjustments just after warming up the amp.
Also, take into account that a new speaker needs some time to stabilize and deliver its best. Sometimes, we are talking about 6 months to 1 year so, be patient with your new amp.
There are some technics to speed up the break-up time of the speaker (as described in Celestion' site), where we can achieve the 90% of the speaker's characteristics in a single session of about 1 hour but, for the 100% we will need that long time.

Well there is a lot more than can be discussed about amps and, a bunch of different implementation details for each amp design but, I am not skilled for it and, I am not in the need to go so far in electronics.
I hope that I made few mistakes while describing amp's characteristics but, please, if you find something that I should correct, let me know. No offense. I am not having the only truth and, maybe I misunderstood something when going deeper into electronics stuff.
What I really wish is that this info can be enough to make you aware of the basics about what is happening inside your amp and that can push your curiosity to know more about this imprescindible piece of gear for every electric guitarist.


  1. very good friend, is the first time I see something so well explained, it is very useful, now I understand! finally!
    when you can explain how a reverberation chamber and its circuit.
    thanks congratulations and greetings

    1. Thanks a lot for your feedback.
      I am happy to see that I was of help to someone else.


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