Bogen CH8 to 3-knob champ with 6L6 conversion filter capacitor questions

[Randall]

I think the bad node resistor was skewing things under load. It seems to be settled in now. I'll leave it to the owner/builder, a smart guy, to decide where to take it from here if he wants to adjust the cathode resistor/current.

[Dave H]

I was talking about with the cathode R changed to 931R and idling at 17W.

I should have guessed that I agree 17W and 42mA cathode current is low for 6L6 but it looks like Randell has fixed it. If the OT impedance is 7k like the marked taps on the chart below then I think he has it about right.

[g1]

Randall said it was running at over 100mA and burning out 6L6s.

Oh, ok. We are talking about different points in time.
I was talking about with the cathode R changed to 931R and idling at 17W.
I thought you were also speaking of the 17W iteration but you were talking about when he received the amp. Thanks for the clarification.

[Dave H]

Hot for a 6L6 single ended?

Randall said it was running at over 100mA and burning out 6L6s.

[Randall]

"17.1W is correct but there's a typo. It should read (440v - 39.8v)"

Yes, thank you. I fixed it.

I found a bad supply node resistor. It was a 10 ohm where a 1000 ohm should have been, and was distressed to the point it read 968 ohms in circuit, tricking me into thinking it was the 1K it was supposed to be. During unsoldering, one leg fell away. So now that it is corrected, and I have realized I forgot two things, A: this is class A operation so that whole 70% thing is not in play, and B: 6L6GC is 30 watts, not 25 watts. So now I am back to the original 470 ohm cathode resistor.

Here's where I'm at now.

Cathode resistor = 464 ohms
Plate voltage = 402.5 v
Cathode voltage = 31.6 v
6L6GC = 30 watts

This has this tube idling at 25.2.watts, and that is where I am going to leave it. Once again, trying to fix a problem in my own head only.

[g1]

What does the power supply look like now compared with the original schematic in post #1?
The original had a 500R pi filter, 5u filter caps and a 47k screen node dropper. If the pi filter is no more, filter caps 20u and the screen dropper 1k (as the AA764) then it will be running hot.

Hot for a 6L6 single ended?

[Dave H]

This amp ended up coming to me for help. It was running waaayyy hot and burning up 6L6s with a 470 ohm 5 watt cathode resistor. It would go over 100mA. I had to double the resistor to get it down to 17 watts. Someone check my methods, please? Plate voltage = 440. cathode voltage = 39.8v. Cathode resistance = 931R. So, 39.8v/931R = 42.7 mA. (400v - 39.8v) x .0427A = 17.1 watts, correct?

17.1W is correct but there's a typo. It should read (440v - 39.8v)

What does the power supply look like now compared with the original schematic in post #1?
The original had a 500R pi filter, 5u filter caps and a 47k screen node dropper. If the pi filter is no more, filter caps 20u and the screen dropper 1k (as the AA764) then it will be running hot.

[Chuck H]

What they said^^^^^^^^^^^^^^^^^^^^^

Also, the max dissipation @ voltage and for a given load is what you'd be aiming for WRT idle dissipation for class A I think. That is, just because the tube is rated at 30 watts max doesn't mean that a standing idle of 30W is ideal at 440Vp into the load provided. The idea here is center bias when conducting with a nod to a safe idle dissipation. I'm sorry I don't know the formulas, but it can be done pretty easily on a scope with a big rheostat as the cathode resistor. Chances are that at 17W dissipation idle the tube will be cutting off ahead of saturating. That means you're not getting maximum efficiency. But as Enzo pointed out, if it sounds good you're done. It's often the case in single ended guitar amps that a center bias during conduction can't be achieved without an idle current over max dissipation. I think this might be because of high plate voltages. High WRT class A operation anyway. And it's also not uncommon for single ended amps to be dissipating more at idle than when conducting signal. Which is just saying the same thing in a different way I guess.

[Enzo]

Less than 100% say.

If it sounds good now, why fight with it? results matter more than anything I can calculate on a napkin.

[Randall]

"17W would be considered cold for 6L6 single-ended I think."

I always get confused with the class A and dissipation. What would be a good target to shoot for with a 6L6GC?

[mozz]

Here's a bias calculator. You should be able to go 30watts diss with a6l6gc.
https://robrobinette.com/Tube_Bias_Calculator.htm

[g1]

I've had to go from 470R to 1K on the cathode of a single ended fender before. No big deal when considering how much the line voltage has gone up from 117V, and also that I wouldn't subject a modern tube to the hotter conditions they used to get away with.

edit: but that was 6V6, and that was to get it down to max. dissipation.
17W would be considered cold for 6L6 single-ended I think.

[Enzo]

Oh, it is single ended? You know that will be running class A, right?

[Randall]

It sounds about right. It's a single ended 6L6 Champ build, so no sets.

[Enzo]

High? Low? Who knows? Does it work, sound good? That is the issue.


Does a different set of 6L6 act the same?