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815 Class AB2 amp

600彩好的mr. and mrs. potatohead live!

600彩好的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%).

600彩好的i have not yet listened to them.

here's what it looked like on the breadboard:

the is a bit of an ugly duckling.  it is a pair of 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 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.

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