Early Ampeg V4: breaks up around 3 and now won't turn on

[Jonny toetags]

Have a friend that bought a V4 what was modded with a pentode/triode switch and a new powertransformer.
He didn't try it and brought it straight to me.
The only work I've done is properly wire the switches (removing the polarity), remove aux plug, proper ground wire and bias trim pot.

Amp is running some robust 6L6's
All filter caps are original (everywhere)

I'm told the amp is breaking up pretty early, has some oscillating noise at times and now won't turn on after he last tried it.

[52 Bill]

When you added the bias pot, where did you set the bias?

Until you get the amp back there is no way to tell what went wrong. Dead amp probably means dead fuse, could mean dead power tube, etc.

[nsubulysses]

Are all the power supply voltages there? If all the caps are original this would be the first thing to check. Especially with oscillation and early breakup. All those old caps should be replaced regardless of if they test fine or not.

Check the voltages at pins 3, 4 an 5 of the power tubes and make sure there is high voltage on pins 3 and 4 and a negative bias voltage on pin 5 (can't remember exactly but should be around -30 to -60V. Also check the cathode bypass caps in the preamp. If they are failing/failed the preamp would be biased poorly and could explain the early breakup. Check pins 1 and 6 (plate voltages high voltage) and pins 3 and 8 (cathode voltage should be small voltages) of the preamp tubes. ALthough there is also the pentode/triode mod which I don't really understand and the new PT, so not sure if this is part of the problem.

Is the fuse now blown?

[Jonny toetags]

I should be getting it back any day now.
As an ex-V2 owner, I mentioned to him that it might need a lot more work seeing as all the caps are original.

[audiopete]

Could have popped the fuse with too high bias. These amps need to be set cold so the tubes can handle the 500V+ that these amps deliver. No modern guitar tube is rated over 500V today. These Ampegs were built with 7027A which was a 600V tube. My advice to Ampeg owners is snag a pair of NOS 7027s for best reliability.

I had a client, that I think is no longer a client, because he is mad that the Winged C 6L6s I put in his V4 lasted 9 months and then redplated. I told him when I first saw him this amp was a tube killer but somehow it's my fault now... I told him to buy a modern amp so he bought a Blackstar and now the V4 sits with no power tubes in it. Go figure.....