Peavey Classic B Series - Plate Current

[Helmholtz]

Using the previous settings the screens are 366Vdc

Ok, if the screen voltage drops by around 100V, the PI supply must follow.

Question is, why is the screen voltage that low.

What value does the 400R/5W screen dropper measure?

[tdlunsfo]

464R in circuit

[Helmholtz]

464R in circuit

I'd replace it with a 390R/10W/5% flameproof resistor. Should increase clean output somewhat.

Still wondering about the large voltage drop across this resistor even at idle.
Schematic shows a 10V drop while you measured 30V. Sure the 40µ screen filter is not leaky?

[tdlunsfo]

Caps have been replaced but anything is possible. I'll do some experimenting tomorrow.

[tdlunsfo]

ESR on the screen electrolytic is 1.7 with a peak ESR tester. Not the best, but passes based on Peak's chart (however accurate that is). I subbed in a 390R (measured 387) and readings are as follows:

At onset of distortion at Output - 12.5Vac, 475Vdc screen 485Vdc plate. ~39W

At heavy distortion (60mA on left tube) - 16.2Vac, 378Vdc screen, 436Vdc plate. ~65W

I do notice that these new tubes don't spread as far as the used tubes. At heavy distortion the left tube is 60mA and the right is 56mA. I feel like we are down to minor variation between matched tubes or biasing resistors on each side.

I'm starting to think that 39W clean is all this amp is designed for.

[Helmholtz]

ESR on the screen electrolytic is 1.7 with a peak ESR tester. Not the best, but passes based on Peak's chart (however accurate that is).

ESR has nothing to do with electrical leakage.
Leakage increases when an ecap was stored or not used (i.e. not charged) for a longer period. So may show with a "new" cap.

I do notice that these new tubes don't spread as far as the used tubes. At heavy distortion the left tube is 60mA and the right is 56mA. I feel like we are down to minor variation between matched tubes or biasing resistors on each side.

Agree that the difference between 60mA and 56mA is insignificant.

Generally DC measurements with large signal present are somewhat questionable.
It requires the meter to do fast averaging of the AC content.
Meter manuals don't specify that kind of measurement and readings may differ from meter to meter.

Did you crosscheck with your cathode probe?

Is the 68k dropper for the PI supply good?

[g1]

I'm starting to think that 39W clean is all this amp is designed for.

From what I could gather, you are getting PI clipping rather than hitting full power. The power tube grids should not clip til around 110V p-p, yet your PI waveforms showed clipping at a much lower level.

[Helmholtz]

From what I could gather, you are getting PI clipping rather than hitting full power.

Yes, but signal could be distorted and limited before the PI (scoping would tell).
I like to test power amps with feeding signal directly to the PI input.
Requires a generator amplitude of a some volts, though.

What is the nominal power of this amp?

[g1]

What is the nominal power of this amp?

Not sure which exact spec. you want, but the amp is rated for "50W RMS @ 5% THD into 4 ohms".

[tdlunsfo]

Sorry I had to knock out some other work today. One thing I noticed is that when I hit single notes with the normal volume cranked, you get a momentary volume drop regardless of master volume settings. I'll get back on it tomorrow and I think I will start by replacing a couple of these filter caps just to rule that out.

[tdlunsfo]

Did you crosscheck with your cathode probe?

Is the 68k dropper for the PI supply good?

68K is a brand new metal film and I have not checked the cathode current.
I replaced the Screen and PI filter caps but no change. Still 39W at onset of clipping.

[Helmholtz]

Is the signal at the power amp input clean when the output starts to distort?

[tdlunsfo]

Just to clarify, power amp input is PI input? I've set this aside for now but will get a scope shot of this tomorrow.

[Helmholtz]

Just to clarify, power amp input is PI input?

Yes.