JMP50 Clone Build, Part 9
Final setup and initial testing on my JMP50 clone
While the rest of the world—at least everyone in the US—was watching the Superbowl, I was finishing up my new JMP50 clone build. After the pictures were taken, I use a compressor to blow all the stray bits of solder and strands of wire out of the chassis and got ready to test per George’s instructions. There was a lot of junk in the chassis and I wanted to make sure I got it all out so some little piece of wire didn’t short something out when the amp was turned on.
Initial power-on revealed a big problem - no voltage anywhere. No heater voltage, no HT voltage. Nada. I had reversed the twisted leads that run from the on/standby switches to the HT fuse and mains voltage selector. Easy to fix and no harm done. A few minutes later, I had this corrected and began testing voltage.
Almost everything was within normal range for an amp without tubes installed. The exception was V1, which showed nearly 2x the correct voltage on one pin and no voltage across pin 3. This was because in my late-night building, I had forgot to add a jumper between two cathode resistor/cathode bypass cap turrets that are done differently on a typical 1987 build. Again, this was an easy fix once I figured out what was wrong. After that was sorted out, all the voltages were within their normal ranges. I began installing tubes and the voltages all stayed in their normal ranges. So far, so good. When I tried to bias the EL34s with a 47k bias resistor, I was only able to bring them up to about 28mA. A quick change from 47k down to 33k let me bias the tubes at 40mA, so that was good too.
So, how did it work??? How did it sound??? To be continued!