(10-19-2014, 04:26 AM)Liquids Wrote: I've seen my power tubes pulse like that. Not red plating, but some other 'glow' -- when playing pretty loud and on transient 'attack' in the rare times I've been able to crank it. Keep an eye on it!
Are your power tubes matched? If it's only two (and assuming it's an inner or outer pair, not 2 L or 2 R tubes), you might want to track down if they are indeed seeing the proper screen resistance, or if they're not really matched.
In this amp, it's easy to measure bias. If you are safe and know what you are doing, you can measure the voltage across the plate resistors. It's not easy math unless you made them ~10R or 1R, but the voltage across 5R only adds one extra step. So you figure out which resistor to which tube's plate, and measure the voltage carefully on either side of each plate resistor at idle.
Rule out 5% differences given that the resistor's tolerances are within that range--if you didn't replace these resistors, measure each resistors value when the amp has no voltage on it, so you know the spread of resistance since time/use/heat may have caused the values to drift over the years. I think if you see a wide spread (as opposed to a reasonable variation within a range) you'll spot it. BE CAREFUL when doing this. We're talking 500v+ DC on your probes and this measurement is done powered up. Be sure you are practicing all safety precautions.
If you want to be safer and easier on your tubes, you can tweak the bias resistor that is 75k (R49). It's been a while since I did this and I am a little foggy right now, but increasing that resistor to 82k, 91k, maybe 100k should increase bias voltage, causing idle current to go down...a little may go a long way. Hopefully I'm not getting it backwards at the moment.
I bought a quad set from Tube Depot.
I used 10R for the Plate Resistors, replaced both the Plate and Screen with new resistors.
Are you talking about measuring each side of the Plate Resistor with the Probes? Or the Positive probe to the PR and the Negative probe to ground?
And are you saying measure while the amp is On Standy By or in Operating mode?
I measured the DC when the amp on stand by - black probe to ground and positive to the tube pin side of each plate resistor, I got -.454DC.
I tried briefly with standby off - in operating mode and got 535V DC, I only measured on spot.