What causes too high plate voltage?

[slidincharlie (Carlo P)]

Hi everybody,
I have a new (old) amp that I'm trying to troubleshoot.
Actually I spend much less time on amps than I used to do... say a few days per year. This makes everything harder. Furthermore the preamp circuit in this amp here is a real bird nest, so messy and full of parts that many of them cannot be reached with a DMM probe.
The problem:
V2, that is the 12AX7 tube that drives channel #2, shows 304 Vdc on pin 1 instead of 200 Vdc, and 3.7 Vdc on pin 3 instead of 1.6 Vdc.
V1 has also higher voltages than expected from the schematic, but less dramatically.
Now generally speaking what may cause a voltage rise in a tube? What should I check, provided that I succeed in probing some more or less hidden component?
FYI the two 25u/25V caps on V2 cathods are both old, while I have already replaced the single 25u/25V cap on V1.
TIA,
Carlo

[drewl]

For a preamp tube usually the cathode resistor drifting higher in resistance or the tube is not conducting.
No filament voltage can cause it also.

As far as all the voltages being high, that's because the line voltage presently is 10 to 15 vac higher than it was in the 60's causing all the voltages in the amp to be higher.

[Jazz P Bass]

Isn't that half of V2 the Tremolo circuit?
Is the Trem On?

[slidincharlie (Carlo P)]

For a preamp tube usually the cathode resistor drifting higher in resistance or the tube is not conducting.
No filament voltage can cause it also.
As far as all the voltages being high, that's because the line voltage presently is 10 to 15 vac higher than it was in the 60's causing all the voltages in the amp to be higher.

Ok, I'll check the cathode resistors and the filament voltage. I tried two tubes in the V2 position to no avail. Thanks.

Isn't that half of V2 the Tremolo circuit?
Is the Trem On?

The tremolo was off at the moment of voltage readings.

[52 Bill]

The tremolo was off at the moment of voltage readings.

Turn on the trem and see what happens to the voltages. The trem pedal turns off the oscillator by raising the cathode resistor value until the tube no longer runs. That would cause the voltages to be too high.

Carlo, it's nice to see you back here!

[g1]

While you have the trem. turned on to check the voltages, it is usually best to have the depth and speed turned right down so you will get stable readings.
Maybe it would be best to start by describing what kind of sound problems you are having with the amp.

[slidincharlie (Carlo P)]

Turn on the trem and see what happens to the voltages. The trem pedal turns off the oscillator by raising the cathode resistor value until the tube no longer runs. That would cause the voltages to be too high.
Carlo, it's nice to see you back here!

Hi Bill!
You guys were right. With the trem on the voltage on pin 1 goes down to about 220 Vdc. Cathode voltage (pin 3) is still high (2.7 Vdc) although lower than with the trem off.
The cathode resistor is on specs. While I'm at it I'm replacing both 25u/25V caps on this tube. Geez this chassis IS a nightmare!

(...) Maybe it would be best to start by describing what kind of sound problems you are having with the amp.

You're right of course. Actually this amp is the typical big PITA: low volume, various hisses and hums, noisy trem, strong hissy noise when I turn the 'TREBLE/BASS' switch on (treble side), etc... I have done some general overhaul like a caps job, a three-prong power cord, a few new resistors to replace drifted ones and the like, including a new EF86 tube to replace a noisy one. I'm just trying to proceed slowly to see if I can get a 'normal sounding' amp out of it. Everything is made harder by the messy chassis. BTW, the preamp and the power amp sections stay in two different chassis's connected by a multi-wire cord.