Hello all!

I’m back again with perhaps an interesting little discovery.

A client recently emailed me that he’d only be in town for a couple days, and his amp had gone dead. No lights, nothing. He sent me a link to a forum post showing the same symptoms he was experiencing, and asked if it was something I could fix. I said if the internet was correct then sure, no problem. This is of course heavily dependent on the internet being correct, and the local electric suppliers having the equipment I needed.

The issue at hand, was the tubes would no longer light up, and neither did the panel lights. The internet forum pointed to the rectifier bridge that fed the heaters and other low voltage circuits. Here is that rectifier:

It had de-soldered itself, and it cooked the hot glue they had installed on the wire next to it. Reading it’s label, it was a basic 8amp 600v rectifier. Big girl to be sure but nothing that special. They’re very popular on amps due to price and amp load capacity (Ampacity?). Anyhoo…it was now that I realized that this circuit was fused at 8amps, meaning that unless the designer felt he wanted things fused very far beyond the requirements, the rectifier was running at it’s limit for at least part of it’s life. The internet appeared to be on to something.

After hunting around Miami, the best I could come up with was a 1000v 8a square rectifier. Not the 12amp I wanted, but I only had 2 days to fix it, so I couldn’t feasibly overnight anything. Besides 8a was the factory rating so it should work fine. I mounted it up, making sure that nothing untoward was touching, and powered it up.

It worked! Sounded pretty good too. The effects, though digital, were very nice especially the controllable repeats on the delay. After a few minutes though I decided to see how hot the rectifier was getting. It was idling at 120C! (248F). The spec for the part says that it’ll run until 150C, but that’s too close for my comfort. A larger rectifier might help, but there were none to be found that would fit into the board without heavy modification. This is when I went back to my computer overclocking days, heatsinks! I’ve got stacks of the things! I took a hacksaw and cut a square from a video card heatsink, thermal greased and glued it into place. The idle temperature dropped to 99-100C(210-212F) Not ice cold but now well below the spec limit.

I know I know, it’s a kludge. Not even a pretty one to tell you the truth. It does however bring the temperature to a workable range that should keep the amp ruining for a good while longer than the original part.

The amp was reassembled and delivered to a very happy client who was flying out the next morning! Normally I prefer to have more time, but when you don’t have the luxury of time you just gotta make things work! I hope you’ve enjoyed reading about this mod/repair, and if you need something taken care of please drop me a line at rauldelp@ra2amps.com.