MusicMan 1650RD troubles

[Zouto]

Hi, I got a severly humming MusicMan 1650 RD for repair. Filter caps and drivers were replaced already with the right substitutes, fuses are ok. Checking the voltages I found out, that the plate voltages on the 6L6s were far too low, 317 Volts, voltages on G2 (306 Volts) and G1 (+ 31Volts) are ok. I checked filter caps, resistors and diodes, swapped 6L6s for no bettering. The OT's primary resistances are ok, plates to CT: 52 and 54 Ohms.
Any ideas, what to check next?
Thanks for help, hints and advice
Zouto

[drewl]

Is that hi or low power mode?
plate voltage should be double on high power.
what ac is the power trans putting out?

[Zouto]

Is that hi or low power mode?
plate voltage should be double on high power.
what ac is the power trans putting out?

The MM1650RD has no Low Power mode, just an ON-OFF switch. I have to unsolder the wires to check the AC Voltages of the PT.

[52 Bill]

Are both plate voltages exactly the same or is one lower than the other?

In any case check the two 470pf caps on the tube sockets.

[Zouto]

Are both plate voltages exactly the same or is one lower than the other?

In any case check the two 470pf caps on the tube sockets.

Meanwhile the plate voltages lowered to 307 and 308 Volts.
Good idea, I'll check and swap both 470pf caps.

[g1]

Are all the other voltages (heaters, +/-16V) ok?

[Zouto]

Are all the other voltages (heaters, +/-16V) ok?

Yes, heaters (6.8Volts AC) and +/-16V supplies are ok. Although no fuses blow, could it nevertheless be some short in the PT? Unsoldering the OT's CT did not change the Voltage on it's solder lug on the PCB.

[g1]

That is a full wave voltage multiplier (doubler) circuit. So the 2 diodes and the 2 caps must be good, and it must be wired up correctly (with attention to center tap). It sounds like the doubler is not working right. New caps can be bad, or there could be bad connections. Diodes can open under voltage.

[Zouto]

Culprit found: one of the brand new 100/350V Spague Atom caps was defective. It looked good, but I got max.38Volts instead of 595Volts at it's positive terminal. Replaced both with 100/350V F&Ts, amp is running fine now, plate voltage is 610V.