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815 Class AB2 amp
Mr. and Mrs. Potatohead live!
I have the stereo pair of 815 amps assembled. Testing is ongoing... on the 8 ohm taps, I get over 50 watts out, and still low THD at 1 watt (~0.02-0.04%).
I have not yet listened to them.
Here's what it looked like on the breadboard:
The 815 is a bit of an ugly duckling. It is a pair of 2E26 beam tubes in one glass envelope, designed for RF transmitting use.
The 815 and 2E26 have never been very popular as audio amps. They have a low plate dissipation rating, a low maximum screen grid voltage rating, and are not very linear when operated in class A or AB1 (mostly due to those restrictions).
However, if you operate them close to the published ratings in class AB2 (where you drive the grids positive), the performance is pretty impressive.
I get 50W out from a B+ voltage of 475V, and 0.02% THD at 1 watt. Using a Dynaclone A431S output transformer, the frequency response is within +/-0.3dB 20Hz-20kHz, with -3dB at 75kHz or so. More measurements, FFT, IMD, etc. coming soon.
The driver PCB uses a 6AC10 triple triode compactron, so the whole amp uses only two tubes (granted, multi-section tubes). It uses MOSFET source followers to drive the grids of the 815 positive (that's the black heatsink at lower right), and a "Maida" regulator to generate a 125V screen voltage (black heatsink at the left). A power supply at the left rear generates 475V, 235V, and -95V from a ClassicTone Marshall amp power transformer (40-18053).
Plate current for each plate is measured using an isolated current sensor and a PanelPilot programmable LCB panel meter. Since the cathodes are internally connected, you cannot sense the current there, and I wanted to be able to separately adjust the bias on each half of the 815.
All these PCBs are available on eBay.