Bias Suppy Voltage Too High?

[Captn Dave]

Last night I noticed my 45s were both red-plating so I shut it down at once. Today I measured about 90 ma on both tubes. No typo... No measurement error. I don’t know how long they ran that hard, but they seemed to have survived somehow.

So today I started looking for a reason and didn’t find any smoking gun; or smoke of any kind. But I did notice that my bias supply voltage is -251 vdc. I looked back at my check-out notes and couldn’t find any record of that particular voltage. I do note that the drawing suggests -150vdc. Is that figure typical of your experience?

I’m using a Hammond 6K56VG, 540VCT. The B+ as measured across the bleed resistor R30 is 360VDC. Am I correct in thinking the bias is excessive at -251VDC? The raw bias at the diode is -395VDC.

If so, what might cause that? Low current obviously but why? I checked resistor values down stream and they check out ok. The resistance to ground as measured across the bleed resistor is about 53k which would suggest about 4.5 ma in the combined bias circuit.

My components match the schematic and the values check out. R6 measures at 10k. The voltage drop across R6 is 140vdc.

Any thoughts?

 

[w5jag]

High bias voltage ( max negative ) should cut the tubes off, low bias voltage ( less negative ) increases the current. (edit: I'm thinking with 45's and your power transformer, ball park bias would be -50 volts at the socket. )

Have you removed the power tubes from their sockets and measured the actual bias voltage at the tube socket, and then measured the adjustment range of that bias voltage?

There's not much in the circuit to go wrong; if the resistors are okay, that leaves the electrolytic caps, the mosfets, and the trim pots. If the problem affects both tubes at once, I would suspect the electrolytic caps in the bias supply as they are common to both channels.

edit: just saw this:

The B+ as measured across the bleed resistor R30 is 360VDC.

Shouldn't it be about 275 volts? Again, if the resistors are okay, I would suspect the electrolytic caps.

Maybe others will have some suggestions.