Conn 541 stereo 7868 amp to hot rod Ampeg B-15

[chris937]

I've been on a bit of a roll converting old organ chassis into guitar amps in the past two weeks and didn't want to lose momentum. This build started as a stereo 7868 amp from a Conn organ. I was shooting for a single channel with baxandall stack and a lot of juice. I just finished it as of right now and couldn't be happier.

This amp's front end and PI is nearly lifted from a B-15S. The main difference is in the power supply and using cathode bias on the output tubes. I'm also going to experiment with fixed bias down the road.

The PT and chassis are from a Conn 541 organ. The PT is a 100 watter from eBay. I spent too long fussing with layouts because I had wanted to use a pair of GZ34 rectifiers. That was for aesthetics as much as anything else so the idea was scrapped because it was just too cluttered.




Chassis stripped, heater wires run.



Finalizing filter cap layout



A preview of what's to come. (Sadly those knobs do not fit these pots.)



Chassis is mostly populated



Added a pilot light and some work you can't see.

[chris937]

First gain stage done, still need to do the tone stack and PI. NFB added.



First test run. 463 V plates, 435 V screens, 29V on cathodes (done in pairs) with a 47u and 200 ohm. I had to up that to 250 ohms to keep the cathode current under check.



I know it looks sort of messy but it's dead silent and intuitively laid out to me. (See my Spitfire build for a more cluttered example.)



After ten minutes of playing with a big dumb smile on my face I decided to bring it upstairs until my roomie wakes up. It was everything I had hoped for and just nailed the tone I wanted.