Measuring Plate Voltage

[ultra-gain]

Those amps are intentionally biased to the cold side. The distortion comes from the preamp. FWIW, when you bias an amp, it is with no signal and volumes down. You are setting idle current. Don't try to adjust bias while playing.

I had read that. I had also read that you could bias by ear within the margins of the operating range. I also garnered from the above and some other research that the bias adjustment on the XXX is narrow enough to not quickly destroy the tubes. So I just went for it.

[Enzo]

You can adjust the bias in this amp by ear, but in that case you are not using any measurements. You didn't hurt anything.

[loudthud]

Is this one of those Peavey amps where all the power tube heaters are wired in series? Pulling one tube cuts off all the others.

[Enzo]

No, this is the XXX, all four 6L6 heaters wired in parallel across 6v supply.

[ultra-gain]

For the sake of someone else like me searching for info. And if some of you who know more want to comment on my readings that would be appreciated if you ha anything helpful to add.

With one 6L6 removed from the circuit I measured;

pin3 to ground = 560VDC
pin3 to pin8 = 504VDC

I'll update this when I get my plate current probe.

[g1]

One thing I recall seeing posted here was that JB said the resonance control (for 5150 but probably applies here too) depends on cold bias to work properly.
I would guess the bias range is limited to an area that allows for proper resonance function. But if you are biasing by ear, it would be interesting to take note of how it affects the resonance circuit. (especially for lower frequencies, I would think)

pin3 to ground = 560VDC
pin3 to pin8 = 504VDC

Pin8 should be grounded so those readings should be the same. Double check them. You may have to dig in more with your probes to get a good reading.

[ultra-gain]

thanks for the reply g1. I wonder if I made a mistake with regards to my "ground" on the first measurement. I just assumed the negative terminal on the "bias test" points(for reading negative bias voltage) was connected to ground. I bet ya I'm wrong and my 560VDC is an error.

I ordered a plate voltage probe from ebay so eventually I'll have this down when combined withe current probe.

I'll have to listen to the resonance control while tweaking the bias. I think it will be hard to tell. Here is why. The bias cold volume goes down significantly. So while it sounds like the bass range comes up drastically when biased hot its hard to tell if I'm not just hearing the overall volume go up, because its significant. Guess I could record a couple licks and normalize it; two of the resonance control positions and the two extremes of the bias settings. That would be entertaining.

[Enzo]

Remember, bias is a negative voltage. Note that your ground versus pin 8 difference is exactly the amount of bias voltage. I suspect the red probe terminal is ground and the black is bias. Easily verified whichever.

Use the bias test points for measuring bias voltage only, use a real ground for everything else.