Ampeg SVT 350T cuts out

[jannesser]

I recently bought an Ampeg SVT 350T combo amp (not to be confused with the 350H head). It sounds great and had a problem with a poti, which I have already replaced.

After playing for a while (approx. 30 to 40 minutes), the output cuts out intermittently. When I hit a string hard and turn up the preamp gain, the output comes back for a few seconds, but disappears again. The preamp output does not seem to be affected, so I guess it is a power amp issue, possibly caused by heat development.


I really like this amp and don't want to give it up. Does anyone have experience with this problem or suspect that a certain MOSFET may have aged poorly?


I'm grateful for any suggestions and willing to share photos of the circuit if that helps.


Kind regards,
Jannes

 

[agedhorse]

Has nothing to do with the MOSFETS, or their aging. That’s not how they work.

The first thing to try is using a patch cord to jump from the effects send to effects return jack.

Otherwise, unless you are (really) experienced and have the necessary test equipment (and know how to use it), it’s time for a QUALIFIED service tech to PROPERLY diagnose it.

It’s also possible that your repair attempt is the root cause of this problem, either the repair or something you did during your attempt.