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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.