Difference in tone between class A and AB ?

[Paul P]

If you were to take a push-pull class AB amp, say one with fixed bias,
and converted it to run class A all the time, what would be the difference
in tone ? I don't see many people doing this, so I'm wondering why.
It seems like a fairly simple matter of dropping B+ a bit and rebiasing
(supposing the bias supply can go low enough).

I presume amps are AB because they're louder that way but if volume
is not a consideration are there any advantages in going class A ?

Paul P

[Chuck H]

It's not as simple as that.

http://www.aikenamps.com/

Go to "tech info", then "advanced", then "the last word on class A".

Chuck

[Old Tele man]

...apples to oranges!

...Class-A PP to Class-AB PP comparison is 'apples-to-apples,' but not a comparison of SE-A to PP-AB!

[Paul P]

I've read Randall's articles several times and they're the reason I'm asking
about class A. He talks about everything except the tonal differences
between class A (push-pull) and class AB.

My reasoning has been the following : start with a Champ. This is definitely
class A. Put two together in push-pull and you get a 5E3 Deluxe. As long
as you're using a cathodyne inverter the 5E3 should also be pure class A
(though I don't see it referred to this way ?). Add fixed bias and a LTP inverter
and you get a later Deluxe or Deluxe Reverb and go into Class AB. The B+'s
are now quite a bit higher coming from a GZ34 and the bias voltage has gone
quite a bit more negative.

Now let's say we start with a AB763 Blackface Deluxe Reverb and want to
convert it to pure class A. We can change the rectifier to a 5Y3 to lower
B+, then raise the bias voltage (perhaps modifying the bias circuit to enable
a high enough voltage). Should it not be possible to get into pure class A
at this point ?

The main thing I'm not sure about is the LTP inverter. Does its gain prevent
going to class A ?

I'd like to get a general idea of what the difference in tone would be between
two more or less identical amps, one running class AB and the other
push-pull class A (preferrably, for the purpose of the discussion, with
fixed bias to remove the effects on tone from using cathode bias).

Paul P

[Steve Conner]

Class-A means that both tubes are always conducting. To achieve this in a push-pull amp, it has to be biased extremely hot.

This means that in your test with two "identical" amps, the Class-A unit would need its B+ lowered by a lot to save burning out the power tubes. This would decrease its power output. So one major change in tone would be caused by the reduction of clean headroom.

Another major change would be the lack of output stage compression. This is caused by sagging plate and screen supplies, and self-biasing of the power tubes when they're driven into grid current. Neither of these effects is possible without straying into Class-AB operation. Supplies don't sag in Class A because the output stage draws the same current at all times. And if the tubes were overdriven to the point of self-bias, they'd just bias themselves back into Class-AB on loud notes.

And IMO you wouldn't want to eliminate either of them, as they're responsible for a lot of the excitement in cranked tone.

In practice, no push-pull guitar amp ever made was Class-A, even the Vox AC30, they are all just hot biased Class-AB.

[MWJB]

The PI does not have any bearing on whether an amp is class A or AB.

There's no reason why your fixed bias LTP amp couldn't be class A if it was designed to be.

An SE amp tends to have a crunchier harmonic character than a PP amp (whatever factors that you can account for being equal), when being driven. Though in reality in recording sessions amps of differing designs can be dialled in to such a point that distinctions can be very difficult to pinpoint.

A 5E3 being 2 champs is rather simplistic, perhaps for practice/in a studio situation, where volume is less of an issue than tone, they could cover a lot of similar ground. Generally though, playing them in the room, they have very different characters. SE vs PP is one of many factors that makes them different. 5E3s were designed as AB, they may or may not end up being biased to the point that they always conduct plate current - which is one outlook on what is Class A (though not Randall's).

I think for your purposes it would be more useful to experiment with what you like the sound of (cathode vs fixed bias, LTP vs cathodyne PI, SE vs PP, high plate currents vs moderate plate currents - a fixed/cathode bias switch would be a good starting point)...these are more accessible factors and perhaps more tangible to the masses than whether an amp specifically meets a set of theoretical perameters such class (which reading Randall's thouhts on the subject, very few amps do).

Typically yes, AB amps appear louder as they have better dynamics, higher plate voltages. Class A amps require lower plate voltages to allow the higher currents required for class A operation. Note that this is a rather simplistic breakdown, I don't expect to be taken to task on Class A definitions/grid conduction etc., ... you should perhaps accept the fact that a real comparison beween the 2 ideals may not be practically possible as you would really have to build 2 compromise amps to properly compare the differences and neither is likely to be a shining example of it's type.

In short, think more about the character of tone that you are looking for and use the differences that you have already highlighted (current, method of bias, PP, SE etc.) to achieve it. These will either fuel your desire & experience to come to your own conclusions...or you may well decide that life is too short and just build amps that sound nice irrespective of class :-).

[Paul P]

Thanks for the replies. I'm going to start building my amp this weekend
so I'll soon be able to try all these different things out. In the meantime...

I fail to see much difference between two Champs and a 5E3 Deluxe.

They have the same plate voltage or just about (an AA764 Champ
schematic shows 360v and a 5E3 is around what, 350v ?). They are
both cathode biased with about 500 ohms per tube cathode resistors.
They both have 220K grid leak resistors.

Apart from the Champ being single-ended and the Deluxe being push-pull
the differences are that the Champ uses negative feedback while the
5E3 doesn't and the 5E3 seems to use a lower voltage on the screens.
The 5E3 has 1500 grid stopper resistors but I don't think that changes
anything.

How then can the Champ be pure class A and not the 5E3 ?

Paul P

[Steve Conner]

Same reason as the Champ is rated at 3 watts and the 5E3 at 18, which is more than two Champs. The extra wattage comes from the Class-B region of operation, which isn't accessible to a SE output stage. A 5E3 probably runs in Class-A up to "Two-Champs-worth" of output power.

[Paul P]

Ok, but how does the 5E3 get into class B ? Is it because the signal
voltage swing going into the power tubes is (can be) greater than in
a Champ ? The front end gain doesn't look to be all that different
between the two.

Paul P

[MWJB]

"I fail to see much difference between two Champs and a 5E3 Deluxe." Aha, that's the problem, you hear the differences, not "see" them :-)

"They are both cathode biased with about 500 ohms per tube cathode resistors.
They both have 220K grid leak resistors." this description covers countless amps made around thru the 50's & 60's.

5E3 has different volume control wiring, 2 different sounding channels, it's <20W PP as opposed to 2x4-5W SE, no NFB...etc. As I said, for recording, they can cover similar turf but if you want to do a small coffee bar/medium bar gig take a Deluxe.

Push-Pull AA764 is closer to a tolex T&B Princeton circuit-wise.

Champ is Class A because it is single ended. It can't be anything else.

5E3s come in a relatively wide range of plate voltages (up to 410v plate to ground, at 50mA) and currents depending on year/who built it. Some are no doubt biased closer to class A, but that's not how they were designed (wall voltages have risen since then).

As Steve said, most PP amps drop into AB if you push them hard enough.

[MWJB]

The front ends are irrelevant (up to the PI anyway), it's more related to bias conditions and the function of PP. The Champ has no second tube at the other "end" of the OT to take over when one half is driven into cut off, at more moderate bias. You could bias up a 5E3 to be closer to class A (e.g. RCA tube manual suggests this might be 55mA at 400v to ground for 6V6, so 435-ish at the plate?)...your amp is going to get very hot in this state, possibly fade during a long set, have poorer dynamics, tube life will be diminished and it will make less power than if you back off the current. Much of this simply means more warranty repairs & less headroom than the competition for a manufacturer.