Ampeg SVT 6 Mosfets

[ID Crisis]

Hello all. New to the forum, but have been repairing music equipment for quite awhile.

I have an Ampeg SVT-6 Pro on the bench. It appears that all the MOSFET's are shorted.

Has anyone seen this and is there something that causes this?

Thanks in advance!

[Steve Conner]

We see the solid-state Ampegs with shorted MOSFETs quite often. I believe they all use a similar output stage with N and P channel vertical MOSFETs. (IRFP250/9250 IIRC?)

Try using the search function to dig up previous threads on them: don't search for SVT6 as they're all more or less the same.

As for what could blow them: Same as any other solid-state power amp. General abuse, overheating, short circuit on the output, too low a load impedance, etc. They have current limiting, but it doesn't always save the devices. All the FETs in a side are in parallel, so if one is shorted, they'll all read short. You need to test them out of circuit.

I think this circuit is supposed to use specially selected FETs, but some people have had success just buying a bunch from Mouser etc. Again the search function is your friend. And don't forget to check the ballast resistors too.