Peavey Classic 100 dead head.

[tbonuss]

Hello All,

Through the help of folks here I was recently able to resurrect a dead Vox VT100 (again, thanks so much everyone), so I figured I'd try to fix something for which I actually have a schematic.

http://music-electronics-forum.com/a...-schematic.pdf
(Sure hope that's the right way to link that...).

1. No output from speaker jack except a slight popping / humming / clicking. Immediately turned it off.
2. Started on bulb limiter, light started and stayed bright (at the same intensity).
3. No output from the preamp / effects loop send.

Before I go any further I thought I should ask a very open-ended advice question: what should I do now?

Thanks in advance for any help at all!

Steve

[52 Bill]

Have you checked all of the fuses?

The first step would be to start testing power supply voltages. There is the high voltage for the tubes and then there is a lower negative voltage for the bias and the opamps.

If you are on a limiter, then all of the voltages will be reduced from the schematic voltages, but you should still be able to read all of the voltages.

[xtian]

Pull all the tubes out, and treat it like a brand new build, following the first-time start up tests, helpfully described here:

Paul Ruby Amplifiers

This will lead you thru confirming that the rectifier and B+ power supply is functioning properly. Your bulb limiter is useful during these tests. It should definitely be dim, after an initial flash.

[tbonuss]

Thank you 52 Bill and xtian for the help. This place is really great.

Verified all the fuses are good.

1. Discharged P/S caps.
2. Pulled Tubes
3. Started on bulb limiter--substantially dimmer than before.
4. Took P/S voltages as follows:

-Page 2
-Middle chain
-Just after rectifier, left to right.

Reading
----------
319 VDC -- Ouput Xmr (J57 / across C37)
319 VDC -- Screen (J61 / across C40)
310 VDC -- B++ (J62 / across C38)
310 VDC -- B+ (J64 / across C39)

-Page 2
-Bottom Chain
-Starting in the Middle

Reading
---------
-20 VDC -- 27V- (across C43)
-10 VDC -- 14V- (across C49)

[52 Bill]

Those voltages are a good starting point.

Reinstall the preamp tubes and monitor the limiter brightness. If all seems okay, add the outputs 2 at a time.

[tbonuss]

With the preamp tubes back in, it dimmed after about 1 second to what seems to be about "half way". With one pair of power tubes (V4&V5) the bulb was brighter and stayed consistently brighter. Not completely sure I understand what "dim" should be...with two tubes in, its certainly not "max" brightness as if there is a pure short.

[The Dude]

When you have just the preamp tubes in, are all of the filaments lit?

[52 Bill]

The brightness of the bulb will depend on the wattage of the bulb in the limiter and how much current the amp is drawing.

What you are basically doing is testing the power tubes, try a different pair and see if there is any of them that will cause the bulb to light up really bright.

Did you try to see if the amp will pass a signal with just 2 output tubes?

[The Dude]

In post #1 there was not output from the effects send, so I think we have a preamp problem......not? IMO, there's no reason to install the output tubes until we get the preamp working.

[tbonuss]

When you have just the preamp tubes in, are all of the filaments lit?

Barely.

Also--yessir--correct: no output from the preamp on the effects send.

[tbonuss]

What you are basically doing is testing the power tubes, try a different pair and see if there is any of them that will cause the bulb to light up really bright.

Roger. So we are testing the tubes per se, not the rest of the circuit. I get it now. Once we get the preamp verified / repaired, then we should go to that if it still doesn't work?

[52 Bill]

Roger. So we are testing the tubes per se, not the rest of the circuit. I get it now. Once we get the preamp verified / repaired, then we should go to that if it still doesn't work?

Yes, I missed the dead preamp part.

I would pull the output tubes and test the preamp without the limiter in circuit. The dim filaments can be caused by the limiter.

Do a quick look at all of the jumpers that connect the circuit boards.

[xtian]

Please say the wattage of the bulb in your limiter. Anything less than 60w may not even get the heaters lit when you have all the tubes installed.

[DrGonz78]

I would have a 100 watt bulb connected on a tube amp of that size and it might still light up but not very bright.

[The Dude]

...I would pull the output tubes and test the preamp without the limiter in circuit. The dim filaments can be caused by the limiter....

I agree. If the filaments light, I would next get a known good tube and start subbing out tubes to make sure it isn't something as simple as a bad preamp tube.