Ampeg BA115 blown output section - Thermal problem?

[glebert]

Sold an Ampeg BA115, never really liked how hot the back plate got, but figured it was as designed (I had fixed a different problem on the amp). Back plate actually says "Don't touch, this gets hot" or something. Couple weeks later it comes back as not working. Both power FETs appear blown, maybe the driver BJTs as well. Ordered new parts, but in the meantime I am wondering if anyone has tried to add a heatsink or fans to this amp.

Also wondering if this guy's use of the amp is going to be a problem. He is using as a guitar amp, and I know he is in a practicing, gigging band so if he is driving distorted guitar through it for hours on end I am afraid I am going to fix it, he'll break it, and I will have to give him a refund anyway.

Greg

[The Dude]

I've had all sorts of bias problems with these. Some not enough and you get crossover distortion. Some too hot and boom. I've actually had to change the bias trimmer in some because it was intermittent. I'd check that when you get it repaired again. My best guess is that it was biased too hot or bias was intermittent.

[glebert]

Great info, thanks for the reply.

Greg

[glebert]

I've had all sorts of bias problems with these. Some not enough and you get crossover distortion. Some too hot and boom. I've actually had to change the bias trimmer in some because it was intermittent. I'd check that when you get it repaired again. My best guess is that it was biased too hot or bias was intermittent.

Question for you, Mr. The Dude

When you say intermittent did you see the bias jumping around? At idle, or over temp, or with vibration, or something else?

Thanks,
Greg

[The Dude]

I saw the bias jumping around and it was very difficult to adjust- even at idle. On some, a minuscule adjustment would make bias jump extremely high or low depending on which direction you were turning. Those ended up needing bias trimmer replacement. On others, setting up to factory specs resulted in crossover distortion and I had to bump the bias ever so slightly. With this particular amp, it's a good idea to give the amp a few whacks while monitoring bias to make sure it's consistent and holding.