"Class A" nonsense

[Robert M. Martinelli]

OK... HOLY F!SH F#$K ALREADY.

Chuck

Hi Chuck,
sorry if I go "off topic" for a moment, but I take the chance to learn something new ( or to improve my English ) every time I got the chance......can you explain to me the meaning of the above phrase? Is it a "slang" expression?
Thanks
Cheers
Bob

[Merlinb]

No, it doesn't. In the case of this cold-biased SE amp, the output signal gets more and more distorted with increasing drive after the negative peaks hit cutoff, but the output power carries on increasing until the positive peaks hit grid current.

You're still misunderstandng me. I'm talking about what the output signal does while the device is actually clipping, on a single half-cycle, not over several cycles.
In class A while the device is in cut-off the output signal clips and stops increasing- it can't do anything else. We can only get change in output signal while the device is conducting, i.e., 360 degrees of a cycle.

Just to make it clearer, in a class AB amp when one device goes into cut-off, the output signal does not clip, because there is another device that takes over and 'fills in the gap' as it were. We can get a continual change in output signal for less than 360 degrees current conduction in any one device.

[Steve Conner]

I'm not misunderstanding. I'm trying to make the point that output power with half a cycle missing is still output power, because "RMS" audio power is measured as a time-averaged quantity that has little to do with the actual instantaneous power at any moment.

You're certainly correct that the instantaneous output power wouldn't increase any more on those half-cycles where the tube was in cutoff. But the power on the positive half-cycles would keep increasing until it was limited by grid current.

Therefore, the supply current would increase, the mean audio power read on a wattmeter would increase, and the signal through a speaker would get subjectively louder. Until in the limit, we'd have a chopper driving the speaker with a square wave. Very distorted, but also very loud, theoretically 100% efficient, and quite musically valid.

If you pulled one tube out of a 50W marshall, gapped the OPT to stop it saturating, and cranked it right up, you'd still get at least 25 watts.

[Merlinb]

If you pulled one tube out of a 50W marshall, gapped the OPT to stop it saturating, and cranked it right up, you'd still get at least 25 watts.

Agreed- the average output power will increase when a cold-biased valve is driven hard, but that doesn't change it from being anything other than class A. Efficiency is still limited to a theoretical maximum of 50%, and that is only achieved through 360 degree conduction of the device. (And in any case, output power is not a measure of class of operation, conduction is)

[Steve Conner]

Well, I still disagree with that. Drive it hard enough and you're in Class-C. The efficiency is more than 50% and the conduction angle is less than 180 degrees.

Valve plasma globe conversion

[pdf64]

To drive it hard enough to do that though, you've taken it past the onset of clipping and therefore (as I understand it) outside the scope of audio amp operation class. As you demonstrate, driven beyond clipping, audio amp operation class is meaningless, or at least requires another definition.

[Merlinb]

Well, I still disagree with that. Drive it hard enough and you're in Class-C. The efficiency is more than 50% and the conduction angle is less than 180 degrees.

Valve plasma globe conversion

Relative to the input signal conduction is less than 360 degrees, yes, but relative to the output signal we still have 360 degree conduction (i.e., when the device stops conducting we stop getting output signal too).
As PDF64 points out, at such high distortion levels it is no longer meaningful to relate output signal to input signal in terms of audio class, we need to stick to current conduction.

[Chuck H]

This post wasn't directed at you Bob (or anyone in particular). I just tagged to the end of the thread. Lots of fun peripheral debates going on here, and they should continue as part of this thread, but I (for one) do think the particular characteristics that qualify an amp to be called class A (or not) has been settled.

So if the AC30 is supposed to be a 30 watt amp (as the name and popular conception imply) then it is not a class A amp. Niether is the AC15 I guess. These would both be AB1 amps with a hot bias.

Of course theres another debate to that. If these amps reach cutoff AFTER their true RMS output has been reached, could they still be considered class A??? regardless of their titles or any assumptions, perhaps this should be the criteria.

Uh oh, now I've started doing it?!?

Chuck

[Old Tele man]

...pulling one output tube from a push-pull circuit reduces the output power to one-FOURTH, not simply to one-HALF.

...just repeat the "jingle" "...one-half of one-half is one-fourth!" until you've memorized it and THEN you'll understand.

[pgambon]

Any amp with two output tubes (or more) that claims to be Class A is suspect. It seems usually they aren't.

[mooreamps]

I never said ; nor was I implying class B was usable for audio. Class B grounded grid is used as a power tube bias for HF transmitter ; like the output tube of an AN/FRC-39 HF transmitter.


Secondly, any pair of output tubes running in push/pull, can be biased as Class A ; as per the example in my last post ; and no it is not what the input signal is doing.


-g

[tekhed7]

I'm beginning to think the original title of the thread is very apropo.
Audio amps- esp. guitar amps- see lots of dynamic range. This translates into a stage operating in 1 class or another, depending on the signal, biasing, available voltage, etc., at any 1 point.
My own feeling is that defining the class of an entire amp is silly as there are usually many stages involved. (I know, I know. Standards for the HiFi industry were set.... in the early 70s, I believe? I had issues with them then... & I do now.)
Some stages might run Class A while others (i.e: the output stage) might "run AB".
(For tubed push-pull amps, one can further classify as AB1 & AB2.)

I think it's interesting to note that an amp like the good ol' Peavey CS-800(C series)...
that likes to blow it's outputs occasionally?... It has it's outputs running Class C! For those not familiar with the beast, one pair of TO-3 "drivers" are what provide the sound at very low levels, while the 10 main TO-3 outputs only kick in when the waveform develops more than 0.6 volts across the 5.6 ohm emitter/base power resistors.
So what class should we call this output stage?.... AB/C ? LOL

BTW, I was incorrect in previously posting that one would need to buy an acoustic guitar to avoid all crossover distortion. I hear they now have 'Class A' amplifiers that have no crossover problems at all, as the "valves" are always on! Wow! (Tongue planted firmly in cheek

[Steve Conner]

I'm beginning to think the original title of the thread is very apropo.
Audio amps- esp. guitar amps- see lots of dynamic range. This translates into a stage operating in 1 class or another, depending on the signal, biasing, available voltage, etc., at any 1 point.

Yes. This is why I labour this point about output stages operating in Class-B, Class-C and so on. The amp designer didn't intend it this way. In other words, this part of the dynamic range is actually outside of what it was designed to do.

But it turns out to be musically useful. This is why we like tube amps over transistor ones, because of this extra "headroom" that isn't really headroom at all. When you crank a Marshall or whatever up to 11, the tubes are conducting less than 180 degrees because of self-biasing from grid current, so the efficiency is more than the 66% theoretical maximum of a Class-B amp, the power output is more than 50 watts, and the operating conditions are generally so like a Class-C RF amp that I don't know what else to call it.

Of course you can call it a Class-AB amp because it was designed to be one, but I maintain that at the instant of being cranked, it is actually operating de facto in Class-C.

For those not familiar with the beast, one pair of TO-3 "drivers" are what provide the sound at very low levels, while the 10 main TO-3 outputs only kick in when the waveform develops more than 0.6 volts across the 5.6 ohm emitter/base power resistors.
So what class should we call this output stage?....

That would be Class-BA, as in Boat Anchor Douglas Self developed a taxonomy for classifying output stages like these. He would probably say that the drivers operate in Class-B (Peavey have that non-adjustable bias system with just a couple of diodes, that ends up running things a bit cold) and the main outputs in Class-C, and then since the drivers and finals combine their outputs in parallel, he'd call it Class B.C or something.

[mooreamps]

I suppose they can call it anything they like. All those old RCA standards were never written with "guitar" amplifiers in mind.


-g

[Diablo]

I was in the local Guitar Center today showing off my Tweed Bassman clone, and a fellow stopped me dead in my tracks. He asked me: "Is the amp true class-A". I paused and smiled and thought of this thread. I told him I wasn't really sure, but it probably wasn't, and I'm not sure that any are......