Peavey Heritge VTX

[CKuzGuitars]

I just got one of these cheap because it has some issues. I have some amp knowledge but, this one has me stumped. I only get a hum out of the speakers - no sound. I get sound from the line out, so, I am assuming the preamp is OK. When I switch it to high power, one tube glows red hot. When I try different tubes, I get the same result. I am starting to think I have a blown OT. Any thoughts?

[Enzo]

The hi/low switch just changes the B+ voltage, I imagine at the low setting, there isn't enough energy to redplate the tubes.

So you get a red hot tube, and a different set also gets red hot. Are all the tubes red plating or just one, and if just one, is it the same socket with each set of tubes? or perhaps a pair of tubes, since they are wired in pairs.

Pull all the four power tubes, power up the amp chassis. Measure, is there B+ at pins 3 AND 4 of every power tube socket? Is there about +15v on ALL the pin 5s? Power off, measure resistance to ground from each pin 8. Are any pin 8 shorted to ground?

Check the health of those two large transistors in the corner. And are the two 3 ohm 5 watt resistors next to them OK?

[CKuzGuitars]

Enzo, Thanks for the help. I'll post an update after I run those tests. I did notice that the 100 ohm/ 5 watt resistors on the power tube board have been changed. They are 47 ohm carbon comp and look like they are about 1/2 to 1 watt. Is this someones attempt at modding the bias?

[CKuzGuitars]

Enzo,

B+ is 243 or 244 on all pins 3 + 4. All pin 5s are reading 14. Pin 8 on V1 and V2 read about 3.9 ohms. V3 and V4 seem to be shorted to ground - no reading at all on pin 8. The large transistors in the corner look OK but on of the 3 ohm resistors has been switched to 2 ohm.

I replaced the 47ohm resistors on the tube board with 100 ohm resistors before doing these tests.

CKuz Guitars

[Enzo]

So those drive transistors are shorted. Look at the schematic. From the cathode of the power tubes to ground, the circuit is through a transistor then through one of the 3 ohm resistors. If you read 3 ohms to ground from a cathode, then the transistor must be shorted.

Please don't use the phrase "no reading" when reporting a measurement, I don't know what it means. If the reading is open - as in the same as if the probes were not touching anything - that is still a reading. And if it shows zero ohms, that too is a reading.

I normally expect a transistor to measure open on a resistance scale when reading from collector to emitter. If you got an open reading, then the transistor is not shorted, it might in fact be OK.

Those transistors should always be replaced in pairs, even if one seems still OK. And if one of those resistorsw is now the wrong value, replace it as well. The Peavey part numbe4r for the traqnsistors is 70406465. I don;t think they have them any more, but it is worth asking. You could also contact the repair guys there and ask what they would recommend as a sub. The generic part is an MJ4247, which I think is an 8 amp 120v part. I bet an MJ15003 would work there.