Peavey Classic B Series - Plate Current

[Helmholtz]

When speaker impedance is unknown, measure the DCR and multiply by 1.2. Guitar speaker impedance typically is around 20% larger than DCR.

[Helmholtz]

I am getting more like 100mA and 54mA

The difference is not good. Check grid signals for max clean amplitude and symmetry with NFB disconnected

How do you measure cathode current?

[tdlunsfo]

Speaker DCR is 4 ohm. Each speaker DCR measures ~13.5ohms. I don't typically measure cathode current and use plate current. I have two inline bias probes that I use for plate current. I do have another bias probe that will measure cathode current if needed. I'll get on the signal request. I do think I have the 4 ohm and 16 ohm tap swapped on my watt meter so my readings above are off... of course this doesn't help me on the tube plate current drifting apart.

[Helmholtz]

Connecting any leads to power tube plates may induce oscillation, This typically only happens at one side of the OT, while the other side (opposite phase) causes no problems.
Oscillaton stops with NFB disconnected.

[tdlunsfo]

Ok, just to go back and clarify some readings.... I went back to measuring AC volts on the output into a 4 ohm dummy resistor. I get around 13Vrms at onset of distortion and this equates to about 42w. For reference, I have the MV maxed and distortion starts at around 2.5 on the Normal volume. If I push past 2.5 I can get up to 16V rms and this is when the tube plate current really starts to spread as mentioned previous. With the NFB removed, things seem to have improved. Distortion is still ~13Vrms and I still get a spread but not as much as before with 56/44ma at 16Vrms.

[Helmholtz]

13Vpp equals 4.6Vrms and means 5.3W into a 4 Ohm load.

(BTW, electrical units like Ohm, Ampere, Volt, Watt etc. are always abbreviated with a capital/upper case letter according to international standards).

[g1]

Maybe you mean 13V RMS instead of P-P ? That would give your 42W.
Same as you said, I've never monitored current at full power. Maybe that is introducing something odd. If currents had a big mismatch you should see something funny on the scope. If you disconnect the probes, do you get a symmetrical looking sinewave at full power?

[The Dude]

What happens to bias voltage on grids? Perhaps a leaky PI coupling cap?

[Helmholtz]

No reason for cap leakage to change with signal.

But yes, grid signals should be monitored. Already asked twice.
With a grid bias of -55V output clipping should not show below a grid signal of 110Vpp.

[tdlunsfo]

Maybe you mean 13V RMS instead of P-P ? That would give your 42W.
Same as you said, I've never monitored current at full power. Maybe that is introducing something odd. If currents had a big mismatch you should see something funny on the scope. If you disconnect the probes, do you get a symmetrical looking sinewave at full power?

This^ and my mistake as I have a true RMS meter.

[tdlunsfo]

No teason for cap leakage to change with signal.

But yes, grid signals should be monitored. Already asked twice.
With a grid bias of -55V output clipping should not show below a grid signal of 110Vpp.

I'm getting to it but it will have to be later.. kids are home now.

[tdlunsfo]

(BTW, electrical units like Ohm, Ampere, Volt, Watt etc. are always abbreviated with a capital/upper case letter according to international standards).

Ok... copy.

[tdlunsfo]

What happens to bias voltage on grids? Perhaps a leaky PI coupling cap?

I monitored this and mentioned it in my first post. Bias voltage moves a few volts with heavy load but stays the same between tubes.

[tdlunsfo]

Grids at onset of grid distortion.

[Helmholtz]

This looks like ~37Vpp. Is the scope calibrated?
Best verify with your DMM, should read 13.1Vrms.

If so, the grid drive is too low for full output and you should check PI DC voltages.

BTW, what is your signal frequency and what is the bandwidth of your meter?
I typically recommend a 400Hz signal as many meters don't measure accurately above 400Hz.