The “Amp on a Board”
Because I could. This piece of wood uses 18FY6 and 17C5 or 19FX5 tubes to achieve an astounding 1.5 watts downhill with a tailwind. How to do this without, say, wasting money on it? That’s today’s contest: Build the best amp for under $20. I think the most expensive part was the pine board from Michael’s. But good pine improves the sound so much… Ok, back to reality. The power supply is two big 17V transformers back to back. The filaments of the tubes are connected to the 17VAC windings, and the 120VAC on the reversed second transformer is rectified and filtered for the 165VDC plate supply. Everything else was precision calculated on a napkin, except the coupling capacitors. They were randomly chosen and math on the napkin made sure they were just good enough to work. The input tubes are in those shielded sockets which are soldered to the ends of the ground buss “T”.
Does is work? Yeah, it actually isn’t bad. No hum or noise, but obviously not loud. Play it gently! Here’s the art:
Yes, the 150uF and 33uF are just connected in parallel. 33uF was not enough, so in the spirit of the design I just pasted that 150uF on there, but placed it closer to the stages it was filtering for. That was the last of the audible hum. Output transformers are some $5 items from AES. Please, DON’T BUILD THIS unless you can do it FOR $20. It just wouldn’t be right.