"Class A" nonsense

[Merlinb]

I think many here seem to forget that it's more that an amp OUTPUT STAGE can be "capable" of operating in a certain "Class" rather than BEING a certain class amp.
Any Fender Champ 6V6 will operate in Class A mode when the guitar is mildly strummed, but if you were to push the output STAGE so hard that it's cut off almost 1/2 the time, then it's running nearly Class B.

The fact that a class A amp can be driven to cut-off does not change its class- it remains class A. If you were to bias a single-ended valve at/near cut-off, it is still class A, but with exceedingly high or infinite input sensitivity (it becomes a half-wave rectifier). A single valve is incapable of delivering any more power once it clips, that is why a single device is always class A by defninition (although some textbooks don't seem to have written their definitions very clearly).

[Steve Conner]

Well, in RF applications, a single-ended amp biased beyond cutoff is called Class-C. The tube only amplifies the positive half-cycles, and the negative ones are filled in by the resonance of the plate tank circuit.

It's highly non-linear (the tube is basically a switch, either cut off or fully on) so is only suitable for FM, or AM with high-level modulation of the plate voltage.

Personally I like to stretch the analogy to audio, and say that a heavily overdriven Champ is running in Class-C too. :-)

In Merlin's example you would find that the supply current draw increased with drive, therefore it isn't Class-A. Unless he were to advertise a rated power of 2 milliwatts or whatever it can make before cutoff.

[Enzo]

hasserl

I'm not aware of any, even the mighty 5150 has some level of undistorted output. It may generate distortion in the preamp fairly early, but in that case use the clean channel.

I admit to a little hyperbole there, yes. Especially since the powr amp only is what gets rated class A for this discussion. But in an amp like the 5150, ther is no clean channel. the schematic calls it a clean channel, but it is only clean by comparison to the other chanels. PV calls it a "rhythm" channel.

[Robert M. Martinelli]

tekhed7 wrote : I think many here seem to forget that it's more that an amp OUTPUT STAGE can be "capable" of operating in a certain "Class" rather than BEING a certain class amp. ( end quote ).

Standards exist for a precise purpose; the standards say that the class of operation has to be determined by the amp's behavior at maximum output power ( thus specifying a precise and well defined measuring point ); according to this statement, it has no sense saying that a certain amp can change its class of operation at different volumes ( output power levels ) IMHO. Once the measuring point has been set, the results have to be referred to that point alone.

Should every manufacturer be left free to set his own standard, the resulting "anarchy" would lead to an even greater confusion ( look what's happened already with the "PMPO" rating, used shamelessly by some "consumer electronics" manufacturers )

I respect every other opinion on the matter, however I'm going to keep on sticking to the standards.

( and, just for the record, I absolutely don't care about the class of operation of an amp - if it sounds good, it's good.... )

Cheers

Bob

[Merlinb]

Well, in RF applications, a single-ended amp biased beyond cutoff is called Class-C. The tube only amplifies the positive half-cycles, and the negative ones are filled in by the resonance of the plate tank circuit.

It's highly non-linear (the tube is basically a switch, either cut off or fully on) so is only suitable for FM, or AM with high-level modulation of the plate voltage.

Personally I like to stretch the analogy to audio, and say that a heavily overdriven Champ is running in Class-C too. :-)

Class C requires a tuned load since the 'output signal' is the modulating signal- that's what you're ultimately trying to get out. Just making something a half-wave rectifier doesn't make it class C.

In Merlin's example you would find that the supply current draw increased with drive, therefore it isn't Class-A.

Class has nothing to do with current draw from the power supply, only with current flowing in the device itself. A single ended valve is non-linear and will always increase its average current draw from the power supply, somewhat. This doesn't change its class either. Again we come back to the simple question which answers all this: if the device clips, do we continue to get output signal? Class A = no
Class B = yes
Class C = yes

[Steve Conner]

Well then, by your own admission the overbiased single-ended circuit is Class-B or C. The negative grid excursions will clip early, but the positive excursions will keep on developing more and more power. The supply current will increase, and so will the current in the device and the (extremely distorted) audio output.

This is not just an abstract point: I've seen people build single-ended guitar amps like this. It's what ends up happening if you use a PT that's too high voltage for the job. The plate and screen voltages end up too high, so you have to apply a lot of negative grid bias to stop the tube from overdissipating, and this makes it clip very asymmetrically. The result is a supply current that increases sharply under drive. Would you still consider a bodge job like this to be Class-A? I'd call it Class-AC.

Don't even get me started on Class-OMG and Class-WTF.

[Merlinb]

Well then, by your own admission the overbiased single-ended circuit is Class-B or C. The negative grid excursions will clip early, but the positive excursions will keep on developing more and more power.

I think you misunderstand me. When the valve hits cut-off the output signal clips and stops increasing, that's the point I make.
Also, if you drive the positive input peaks to the point of grid-current clipping, again the output signal clips and cannot increase further. The output signal exactly follows the current conduction of the device, for the whole cycle. No change in current = no change in output sugnal.

The result is a supply current that increases sharply under drive. Would you still consider a bodge job like this to be Class-A?

Yes of course. As I said earlier, it simply becomes a single ended, Class A amp which has higher input sensitivity than if it were 'centre biased'.

[Steve Conner]

I think you misunderstand me. When the valve hits cut-off the output signal clips and stops increasing, that's the point I make.

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.

[jmaf]

Hi jmaf,
Maybe due to my bad English, I expressed myself in a wrong manner....I meant that in a class A amp the current draw does not change ( only ideally, it changes a bit in reality ) between "idle" and "working" conditions.

In class "AB1" the current swing is way more significant and always very evident.

My apologies, I'm working hard to improve my English, but it seems I'm miserably failing.....

Cheers

Bob

Robert, I lived 1 year in Roma and I can tell you, if you see me try some italian, you'll deserve more than apologies for the assassination of your native language.

From my perspective, as a portuguese speaker, your english is perfect, I'm here to learn from the more senior members and your posts are extremely didactic.

I just asked because I have this mental model of an output transformer and I tried to visualize a constant current draw in class A. I understand your point perfectly now, thanks. Alas, this whole discussion taught me a lot, I guess that's why I lurk on this forum most of the time I can.

[tekhed7]

With all due respect, a Class A amp might not be designed to put out Any real power. It may just be a voltage amp. This whole idea of an Audio amplifier as a Whole being of a certain class is a manufacturer's creation, as far as I'm concerned... standards be damned.
Again, an amp STAGE can only be truely defined to be operating "in a certain class" by virtue of the device's measured operating parameters AT THE TIME OF MEASUREMENT.
As has been pointed out, it came out of the radio world where input signals are far more consistent & lack the wild dynamics of [processed] guitar signals.
Ya got your Class A your AB1, AB2, Class B & C. Anything else... (Class D, G, H...) is all from the imagination of the audio world's sales dept's, & as such is defined & redefined over the decades.
I'm sticking with the ARRL handbook definitions, thankyou very much. But, still, this thread has been very educational.

[balijukka]

Standards exist for a precise purpose; the standards say that the class of operation has to be determined by the amp's behavior [U]at maximum output power[/U

Can you please show us this standard. Who? Where? When?

jukka

[Old Tele man]

Class A = 360° conduction
Class B = 180° conduction
Class C < 180° conduction

...Class AB is anything between Class A (360°) and Class B (180°); suffix "1" denotes no grid current, suffix "2' denotes grid current.

[Robert M. Martinelli]

Can you please show us this standard. Who? Where? When?

jukka

Hi jukka,

EIA, to name one ( cited already by Old Tele Man ), BTW you're missing two "W"s, namely What and Why.

I've tried to explain them already in my previous posts, so I'm not going to try to describe them once more, FWIW I've been re-reading the whole thread and my posts, and I don't feel my statements can be considered "wrong". I just tried to point out that, if we don't want the discussion to go on forever, we first have to agree to take the measurements/state the class of operation at one point, or to refer to some standard, otherwise the whole discussion is pointless IMHO.

Cheers

Bob

[Robert M. Martinelli]

Class A = 360° conduction
Class B = 180° conduction
Class C < 180° conduction

...Class AB is anything between Class A (360°) and Class B (180°); suffix "1" denotes no grid current, suffix "2' denotes grid current.

Hi OTM,

As usual, just perfect - concise and straight to the point - I needed almost one page to say the same thing - but I totally agree.
Cheers
Bob

[Chuck H]

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

What bias class an amplifier is operating in is a seperate issue from it's indicated bias class of operation because an amp only has to correctly operate within it's indicated bias class at it's indicated output power. (period)

There seem to be a few posters that think an amp that ends up in class AB1 (or even B or C for single ended amps) should not be called class A, and that's just not true. If the amp does function properly in class A up to it's rated output then it's a class A amp. The adjustable input level (or at least pre distortion level) control on a guitar amp is essential. To attenuate this control such that the highest possible input signal could not drive the amp beyond class A would render an amp useless for many instruments and playing conditions that provide a lower input signal. If the amp ends up in AB1, B or C at a higher wattage than is indicated by the MFG then the amp is being operated improperly and it should be expected that it will not perform to it's indicated specs. Not that it's a bad thing to do this, or even unintended by the MFG. But it is secondary to the bias class of the amp.

Chuck