Amp 3: Experimental Power Amp
My second amplifier was a bit of a disaster. I disassembled it and used its transformers and its choke to build a power amp. Apparently I was planning on building a preamp to go with it at some point.
When I first powered the amp up, the fuse blew immediately. This became frustrating as I could not diagnose the problem: there seemed to be nothing wrong with the circuit. It turned out that I was using the wrong fuses — I had grabbed 500 mA fast blow fuses instead of the 3 A time delay fuses.
The amp is a poor stand-alone guitar amp. I tried hooking it up to my Yamaha PSR-420 keyboard and it made the keyboard sound a lot nicer. The PSR-420 is a consumer model that my parents bought for me when I was young.
Lessons Learned
- Increasing the screen resistors on the output stage is a cheap and easy way to adjust the output level of the amplifier.
- Triode, pentode, and ultralinear operation sounded much more similar than I thought they would.
- Choke-filtering a plate supply can give the power supply unwanted impedance.
- Feedback loops can make an amp oscillate. I had to disconnect the feedback loop in this amp. By adjusting the master volume, I could sweep the oscillation frequency from about 1 Hz to about 1 kHz.
- I still haven't gotten the hang of cutting square holes with a nibbling tool. In the future I'll buy toggle switches instead of rocker switches.
- Eyelet boards are fantastic, but get bigger eyelets — maybe 1/8 inch?
- Shoulder washers are great for cutting down on ground connections.
- Put the power supply on circuit card too, even if you can do point to point.
