Fender Champ Power Output Rating

[bobloblaws]

push pull amps are like making toast, you press the lever down and the toaster starts to heat up, the toast is done and it pops out and the toaster cools off,
well, a class A amp like the Champ is like having the toaster on all the time, maybe not as hot, but always on, the tube does not like this, it will protest after a while, so we have to limit the plate dissipation of the tube a bit so it does not fail.

That is an interesting analogy. But in the example I gave, my amp was biased so the 6V6 was dissipating about 10.3 watts which is about 85% of the max 12 watts. That is more than the conventional 70% for push pull amps. Robinette's bias calculator (https://robrobinette.com/Tube_Bias_Calculator.htm) indicates that 90% is safe for cathode bias amps and 100% is safe for class A fixed bias. The general thrust of the other replies appears to be that it is a question of efficiency and not less dissapation that results in the lower output. Or am I confused by your use of the term "plate dissapation".

[cjenrick]

at 10 watts Class A i am afraid you are going to be buying a lot of tubes,

[eschertron]

The latest Champ, the reissue 600, runs with 360vdc on the plate and -22vdc bias, for ~338vdc anode-cathode potential. Schem here: Champion 600

Using an online tool, I read 15.5W total dissipation. This includes plate and screen; if screen is 20% then plate is ~12.4W and the screen is the other 3.1W. To answer the OP, this 90% continuous plate dissipation has nothing to do with audio output power. The output power using the schem's indicated 7vac is about 5W. Cranking the amp will get significantly more.

[bobloblaws]

Using an online tool, I read 15.5W total dissipation.

I get similar (15.9W) with Robinettes calculator.

...if screen is 20% then plate is ~12.4W and the screen is the other 3.1W.To answer the OP, this 90% continuous plate dissipation has nothing to do with audio output power.

I understand now that I had originally confused tube dissipation with audio output power, but what value constitutes 90%, and of what? Max dissipation for that tube is 12W, no?

The output power using the schem's indicated 7vac is about 5W.

I'm not trying to be nit-picky, I just want to understand your points accurately. Did you mean 3.35vac? That is what I see on that schematic at the output, TP13. There is a reference to 7vac at TP5.

The output power using the schem's indicated 7vac is about 5W. Cranking the amp will get significantly more.

My understanding was that when the output spec is listed at 5W it refers to the max output. Or does it refer to distortion free output?

[bobloblaws]

at 10 watts Class A i am afraid you are going to be buying a lot of tubes,

The online calculators seem to indicate that 10W plate dissipation is well within the acceptable range for a 6V6. If I'm missing something I'd really like to know what it is.

[eschertron]

I understand now that I had originally confused tube dissipation with audio output power, but what value constitutes 90%, and of what? Max dissipation for that tube is 12W, no?

The number I'm using is 14W, although I have seen ONE source that calls out 12W. Not sure the reason for the discrepancy. Maybe the GT designation indicates 14W?

I'm not trying to be nit-picky, I just want to understand your points accurately. Did you mean 3.35vac? That is what I see on that schematic at the output, TP13. There is a reference to 7vac at TP5.

My understanding was that when the output spec is listed at 5W it refers to the max output. Or does it refer to distortion free output?

Yes, TP5 is where I'm getting the number. In the online calculator I used, 7vac (or about 20vpp) at the grid of the power tube resulted in ~5W output power. I didn't check, but I assume this is about the limit of clean power. Or it 'just happens' to be the voltage at the grid with the design input signal. With the 6V6 slammed, you'll get 7 or 8W of snarling distortion.

edit: I think it's up to the maker to assign a distortion figure to their rated outputs. For Fender, it's generally a clean signal. For some others, not so much - presumably because the user won't play the amp clean. Why advertise an amp as an 80W amp when it can be advertised as 150W?