Tube amplifier

This is a Williamson-style push-pull audio amplifier designed around the RCA type 815 transmitting tube as the output stage. The 815 was designed during WWII as a VHF transmitting tube for use aboard aircraft, but is suitable for audio use as well. It is a dual beam-power pentode, making it convenient for push-pull use.

I ran across a pair of 815s in a surplus shop and thought it would be fun to learn a little about vacuum tube electronics and design an audio amplifier around them, partly because they are an obscure part without an audiophile cult following and thus cheap, and partly because they look like little robots with their two plate caps.

There aren't any (solid state) semiconductors in the signal path, but for convenience I used silicon rectifiers for the plate supply and modern semiconductor voltage regulators for the screen and bias supplies. Plate voltage is around 450V, screen voltage around 200V, and grid bias around -20V. Plate current with no input is around 14mA per pentode. Audio power output before distortion starts is around 15W (per channel) at 1KHz. It's plenty loud.

Future modifications will involve an IR remote for controlling the volume and input selector, and digitally instrumenting various operating parameters like plate current, screen current, bias, and output power level.