Higher Voltage measurement on Base Tube Amp

[jmaroto]

Hi maybe I'm about to ask a very stupid question. I'm almost done on my way to repair a Peavey Classic 30, and already all the boards are fixed.
When doing my preliminary voltage test I found the next results.

Plate Should be 410DC : have 411DC
Screen Should be 405DC : have 407DC
B++ Should be 290DC : have 402DC!!!
B+ Should be 276DC : have 401DC!!!!

The Plate and Screen voltages variances don't worry me much. But the B++ and B+ do make me wonder.
Now the "stupid/naive/ignorant" part, right now I hadn't placed the tubes (I know I know the circuit wont be complette without it), I'm affraid that this voltage can damage the tubes.
Could this variance be fixed when I place the valves on, or could this be a death sentence to the tubes?

All the rest measurements are pretty much fine and new filter caps are in place, so should I try the tubes without being affraid of losing them?

Thank You

[MWJB]

Install all the tubes before measuring the voltages, without the tubes drawing current voltages will read high. B+ & B++ will drop significantly with the 12AX7s installed.

[jmaroto]

Thank You for the quick response, You were totally right, I just intalled the tubes and my B+ and B++ voltages droped to 276 DCV and 258 DCV. Now the only missing thing is find out why the reverb is not working but it's not a must.

[Jazz P Bass]

Reverb not functioning can be narrowed down to the tank & cables or the send/ return circuit.
Start with the tank.
Measure the resistance of the input & output transducers in the tank.
Then verify that the cables are good (ohmmeter)
The send half of the circuit can be measured at the cable.
With a signal applied to the amplifier input & the reverb control at 10 you should get a nice healthy Vac reading at the cable end.
The return circuit can be tested by applying a signal to the return cable.
A signal generator & a scope would be most helpful.
But you can get by with a DVM.

[jmaroto]

HI Jazz P Bass, what cable do you reffer. This amplifier was pretty bad I had to rebuild a lot of the connections jump wires and some cables. This specific model doesn't have the option of FX loop I think it is one of the first generation of the Classic Series. The rest of your troubleshooting is quite clear but I don't have a clue what specific cable apply to the reverb circuit.

Thank You

[Jazz P Bass]

Uhm, the cables that connect to the reverb tank.

[jmaroto]

Well, I think that resolves the mistery of the reverb pot that didn't worked. There's no cable from jumpers J6 / J7 / J8 / J9. I though it was the cable for the Loop FX and since the model I have don't include that feature, but I was wrong. Anyway I had to made the pilot light cable and the "LEAD/NORMAL" cable too, one more cable wouldn't make the difference.

Thank You for show me the elefant Jazz P Bass.

[Jazz P Bass]

Better make that 2 cables.
Input & Output.
To tank input: J6/Tip J29/ Ring
To tank output: J31/ Tip J30/ Ring

[jmaroto]

Thank You for the info, cables are in place and reverb seems to be fine, (low prescense but I think it's part of the model).

I was testing the amp this morning and found that the clean channels works perfect, but when the Lead is ON and PRE or POST are higher than 6, a high squeal appears overtaking any sound from the guitar. Even when nothing is connected to the amp.

I though the V1 from the preamp was microphonic (this are JJ Tubes completely new), swapped in all kind of positions and no change. Tried inverting the polarity on the OT but still the same. Additional I don't see the preamp tubes light bright, they have just a soft orange in all the three.

Anyone have any advise I can try.

[Enzo]

I have never seen a Classic 30 without an FX loop. Only model I know that didn't have one in the early versions is the Classic 50.

[jmaroto]

I just found out something new, when the lead channel is on, and I tap the input jacks the squeal change pitch dramaticly, depending of the PRE and POST controls positions it goes away. I was checking the Geofex debuging page and on the possible solutions of squeal noise mentions that jack shorting can be the cause.

Does anybody had tried this solution, changing the input jacks?

Also, I was checking pots values and PRE should be 1M but is reading 2.8M, and POST should be 10K and reading 8.9K