• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

With a good MOSFET follower, can you hear a change in the audio from class A to class A2?

Haven't written here for a while, but feel the urge to come with the only correct answer to your question: it depends.

Now seriously, what do you want to achieve?
A EL84 is normally driven in A, can't really take A2
A 6L6 is normally driven in A, but can go in A2.
A 6L6 playing in A2 all the time will probably not be in a very linear operation point...
Transmitting triodes used in audio (like 833) will need to be played in A2 all the time, some people call that class A3 (when there is grid current during the whole cycle).
 
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Just wondering if it's better to go all A2, or some class A + A2 ?
Usually one implements A+A2, i.e. going into A2 when the input signal is high, a bit like AB class in a push-pull.
Pure class A2 I guess would be using a transmitting tube that needs control grid current to work, something like a 811A.

In my opinion, I would never go into A2 for anything related to Hi-Fi. The only reason to use A2 is to squeeze more power from the output tubes.
 
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I did some tests with the RL15a wired as triode, G1 driven by source followers. I increased the driving signal while measuring the output signal with ARTA, the fundamental and distortions went up linearly, I could not see where the transition between A and A(B)2 took place there. At the end it was putting out some 20W with very little distortion (and no overall feedback). OTOH, a pair of 6W6GTs driven from the same source followers clearly showed of distortion at certain point, measuring it was indeed the transition to class 2. Conclusion: I will make a class A(B)2 amp with the RL15a, but not with the 6W6GTs.
 
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My SF driven (metal plate) 300B SE capable working in A2.
801a PSE amp also.
All of them very good amplifier, due to mainly the power tube driving mode. SF has very low output impedance, so even few ten mA grid current peak doesn't cause any problem.
Many tubes don't tolerate slipping grid voltage to positive, the distortion rises quickly.
 
Magz is right :) What tube am I playing in A2 at home all the time... 811A I think? To have that one delivering some power with "normal OPTs" and <400V B+ it will require grid current in idle mode (the 833C at 2000V and 160mA will have about -40V at G1?) .
 
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I have some experience with RF 100TH and OT100 tubes, for both it is advantageous to work in class A2 because you can choose a static point of operation where the anodic voltage is 650-750v but the most important thing is that the driver can support the inevitable grid current, For OT100 I chose Ug=-4V Ua=650v Ia=85mA so the operation in class A is insignificant however the harmonic distribution is inferior to a strict amplifier in class A
 
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Quite simply = No

But depending on the frontend of the amplifier
if it even justifies more bias with mosfet.

If the amplifier is plain old .001 to .004 %
at 1 kHz
But at 10 to 20K it climbs to the usual plain old .01%

Then NO
boiling transistors does nothing.

With " enhancement" type mosfets
Crossover distortion will still be higher at high frequency.

And high bias will help fix that.
Not more than 100ma to 140ma per device is needed to get the lower crossover
distortion at high frequency.

Otherwise at 1 kHz it dont do dingle wingle.

If your saying to yourself im gonna run 100ma bias per device
to have the best high frequency crossover distortion = yes good.

If the amplifier is at .1 % distortion at high frequency already.
then higher bias is a udder waste of time.
Because it dont do dingle wingle .1 % is .1%
and no amount of bias will change that.

If you had a amplifier that actually did .001 % at 20 kHz
I would be rather impressed. and if high bias made that happen
then it would be easily justified. if not needed to reach those numbers.
And as said most the magic would be the frontend to even touch that low
of distortion. And output topology far far above something with the bias just cranked up