Cathode Biased vs. Real Class A

[aarondavis]

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...

[aleclee]

Randall Aiken has taken the time to put together some articles here (http://www.aikenamps.com/TI_Aiken_adv.htm). He knows the stuff and explains the stuff better than I ever could. :)

[Old Tele man]

in simple terms: Class-A operation means the signal is being amplified for a complete 360-degrees of input signal. If the signal EVER stops being amplified, even for a moment, it is (technically) no longer Class-A, but rather is considered to be Class-AB operation...albeit a very "hot" Class-AB (ie: "almost" Class-A)

Cathode-bias is just a the electrical condition where the controlling-voltage (ie: bias) is applied to the tubes' cathode rather than to it's control grid. This cathode-bias voltage is "generated" by the tubes' idle-current as it flows through a suitably-sized "cathode" resisitor.

NOTE: cathode bias can NOT be used on Class-B operation tubes, it can ONLY be used on Class-A and Class-AB circuits that are "close-to" Class-A operation, and then only with an added "bypass" capacitor across the cathode resistor(s) to filter out and stabilize cathode current pulses.

With all push-pull circuits, there could be just one common resistor shared by the two tubes, or two separate resistors (but must have bypass capacitors each). The "closer" the Class-AB operating point is to Class-B operation, the more necessity for the bypass capacitor(s).

[Southbay Ampworks]

Here's another explanation in non technical terms.
http://www.soflotubeamps.com/soflo/smartale.htm

[TieDyedDevil]

Originally posted by Scumbag
Here's another explanation in non technical terms.
http://www.soflotubeamps.com/soflo/smartale.htm


Interesting:

["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...

[baald]

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....

baald

[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.

[Old Tele man]

re: "...are you talking about just push-pull or just a certain class of tubes?"

ANS: just push-pull circuits...not referring to preamps, voltage amps, phase-inverters, etc. (ie: single-ended).

....John Phillips' description is correct.