Originally Posted by BloozeGit
Hey JohnnyCrash, thanks for that detailed reply!
Regarding the screen current calculations, are you referring to the 5k resistor between the first two filter caps? I'm guessing that's an expedient way of estimating the screen circuit resistance, since the 22k resistor is relatively large. Does this assumption also ignore the 390V on the other end of the resistor?
My main concern was that yesterday I tried diming it to see how it'd hold up. It got really harsh then went soft and buzzy for a while, at which point one of the power tubes started glowing red. That prompted me to start looking at voltages...
The OT primary is also spec-ed at 7000 ohms as opposed to the usual 8000 ohms called for in the datasheet, would that also be cause for concern?
Obviously, glowing red would mean they're red-plating. At these voltages, I'm surprised they're doing this, since I run mine with 405 volt plates and 22v at the cathode when in SS mode and my new ElectroHarmonix 6V6's don't seem to mind.
What brand 6V6's are you using? What wattage is your cathode resistor rated for?
Still, 390 doesn't seem tot much, but since you're red-plating you may want to consider changing the cathode resistor to bias the 6V6's a little colder. Try a 300 ohm, or even larger - then recalculate calculate wattage as I've done above. Try to get them down to about 12watts.
Also, check the amp for any miswiring or mistakes before making any changes.
I'd avoid dropping the overall B+ for now, and focus on looking for mistakes and changing your cathode bias resistor value.