Ampeg BA115 blown MOSFETs

[Randall]

Amp blows mains fuse and lights up bulb limiter, A little in circuit troubleshooting shows both output mosfets shorted. I remove them, and sure enough, both are shorted every which way. Now the limiter doesn't light up, so I take it off it. My supplies are a little low at +/-38v and + 15.6 and -16.6v. They start out about .5 volt low and slowly climb to the max over several seconds.

So, I could use a little help here, as I have not ever been very good at this mosfet stuff. Questions are, why did both of them die? And what else should I be looking at? All the diodes quick check good in circuit.

Q1, 3 and 4 give good looking diode checks in circuit, no shorts.

https://ampeg.com/support/files/Sche...(103XXC_1).pdf

[The Dude]

I wouldn't worry about a few volts difference on supplies. It won't make any difference at all. Why did both mosfets go? There could be any number of reasons. Fuse didn't blow quick enough, shorted load, etc. There is a series current path through both FET's, so if one shorts, there's a good chance the other will, too. Check G-S zeners, driver transistors, R33, R49, and R61. Replace the outputs and any other bad parts you find. Then fire it up on a variac or LBL. There's a good chance the fuse saved the rest of the circuit and replacing the FET's will do the job, but best to be thorough to prevent ruining a new set of FET's. I like to bring up voltage slowly with a variac on SS amps with standard power supplies while monitoring current. If you don't have a variac, a LBL will at least limit current. And, of course, don't hook up a speaker until you verify no DC on the output and the amp is working.

[nevetslab]

Nice thing about only having one N-Ch/P-Ch MosFETs here, is you don't have pairs that have to be matched.