Cathode Biased vs. Real Class A


Some get pissed if you call an amp Class A, when its really just cathode biased. Can you guy's explain to me in general terms (and then technical if you wish) exactly what cathode biased means, and why it has become erroneously known as "Class A". I have a TopHat Club Royale 2x12 that is of Vox heritage, so I'm interested...


TGP Tech Wrangler
Staff member
Randall Aiken has taken the time to put together some articles here. He knows the stuff and explains the stuff better than I ever could. :)


Originally posted by Scumbag
Here's another explanation in non technical terms.


["Limited" class A amps with push/pull output transformers are driven into class A by running the plate current well over design specs so the tubes stay on considerably longer. They could be biased into pure class A but their life would be measured in minutes or less.]

[At So Flo, we believe in "Truth in Advertising"...]

I wonder whether all advertising truths are actually half-truths. :confused:

It's certainly possible to build a push/pull class A amplifier that runs the tubes within spec. All you have to do is lower the plate voltage to control the dissipation. I wonder whether these guys think that the only way you can get a class A push/pull amp is to start with an A/B design and run the bias really, really hot. :rolleyes: Geez, this is basic electronics...


OTM - there are plenty of circuits that use cathode bias and no bypass cap. are you talking about just push-pull or just a certain class of tubes or such? maybe i misunderstand what you are saying....


John Phillips

Push-pull Class ABs need the bypass cap.

Push-pull Class As don't need it. The reason they don't need the cap is because, since the waveform is equally and oppositely amplified at all times by both tubes, the AC current in the resistor (assuming it's a common one, not separate for each tube) cancels out, so you don't need the cap to bypass it around the resistor.

Single-ended stages (power or otherwise) which are all Class A, don't 'need' it, but it increases the gain at frequencies above the roll-off caused by the cap - so a larger cap value gives more bass response. All single-ended power stages I can think of use one.

Yes, I am one of those guys who gets annoyed by the false Class A thing. It's misleading, usually used in advertising BS, and confuses people who don't know how amps work by mixing up different and unrelated circuit characteristics that have different effects on the tone.

Class A, cathode bias, and no-negative-feedback - and sounding good - (all of which seem to be "associated" in some companies' minds and hype) actually have nothing whatever to do with each other and you can in fact have an amp with any combination of them, for or against.

I don't quite know how they started to get confused, but I think it's important to keep on about it, or otherwise it may become 'accepted' - if it's not too late already.

Rule of Thumb: if it says it's Class A, it isn't - with very few exceptions. Anything derived from the Vox AC30, or producing more than about 20W from four EL84s, definitely isn't.

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