Ampeg BT25

[carldacorte]

Been working on a couple problems at the same time

I've got an Ampeg BT25 solidstate bass amp that was dropping volume as I was playing or hitting hard notes. The amp is from 1969 and has never been serviced from what I can tell so I figured the first thing I would start with would be the filter caps.

I couldn't find matching caps... or even caps that came close to the values in the amp. (There were two rated 2500mfd @ 90v and one rated 2500mfd @ 80v.) So what I did was wire two 4700mfd @ 55v in series for each cap. This should give me a rating of 2350mfd @ 110? And that should work fine right?

Well, I powered the amp on after swapping the caps and I saw smoke... Opening up the amp again I saw the diodes in the rectifier were fried. What did I do wrong?

I checked my polarity, my solder joints. Nothing is getting crossed, and nothing is shorting to ground.

Any help is greatly appreciated!

[Jazz P Bass]

Capacitors in series

Done correctly, this should have worked.
Did you wire the caps + to -, + to - ?
Here is a link to Mouser.
I would get the right caps, seeing that you now need rectifiers.
Aluminum Electrolytic Capacitors - Snap In

[carldacorte]

Looks like I installed the caps backwards... Re-wired them and rebuilt the rectifier (including the ancient traces that were destroyed when the rectifier blew). I also re-flowed all solder joints throughout the amp which took care of the "deep" and "bright" swithces not working. The amp sounds great now!
It's still a little noisy at times and will still have the issue with the volume dropping (though not nearly as bad as it once did.) I've noticed that if I turn the amp on with the volume down, not play anything through it, and wait a minute, and then turn the volume up and play, the amp will be fine and won't have the volume drops.

Any idea as to what would be causing this?

[Enzo]

Some advice. FIlter caps are not critical value items. 2500uf and 90v are not standard values. Looking for exactly that will be fruitless. And making up something to that exact value is a wasted effort. 2200uf and 3300uf would be standard values or even 4700uf. And 90v is an odd one, but 100v is common. So perhaps a 3300uf/100v cap would be easier to find.