I decided to build a low-cost SET amplifier to play around with.
I had a whole bunch of Russian 6F5Ps (Hi Mu Triode, Power Pentode combo) that I bought on an online-auction site simply because they were dirt cheap ($3.95 each).
As I understand it, 6F5Ps can be poor-man's 2A3s when they are triode-connected, although their power dissipation rating is smaller:
I was not keen on impressing the full 300V on the pentode, so I dialed back on the B+:
The anode power dissipation on the pentodes is a little over the spec, so I will have to reduce the B+ further...
While I was waiting for the irons from Edcor, I decided I would put the whole thing into a 10" x 6" X 2" Hammond chassis. I used the free software from Front Panel Express to plan for the layout of the amplifier. It appears that everything would fit into the chassis, but it would be a tight fit:
Now all the parts are ready to go:
The Front Panel Designer printouts are used as drilling templates:
Wiring in progress:
I was curious about the Plate-to-Plate feed back scheme mentioned in the "6V6 / 6SL7 / 5Y3GT SE Amp
" thread, so I tried it:
Here is a FFT spectrum analysis chart of the unmodified amp using another free software, Wave Spectra
(downloadable from http://www.ne.jp/asahi/fa/efu/soft/ws/WS140.ZIP
) with 1KHz sine wave to produce output at 1W at with THD of 0.1%
You can see the 60Hz hum, 120Hz PS ripple, 1KHz primary, and its harmonics. 2nd harmonic is at -62dB and higher order harmonics declines in magnitude.
Here is the corresponding FFT spectrum analysis chart with the Plate-to-Plate feedback resistor. I expected some decrease in the distortion (it's negative feedback, after all), but the FB actually increased the 2nd harmomic distortion by ~17dB without having much impact on the higher order harmonics! THD increases to 0.5%:
I suppose the addition of the P-PFB resistor would make the amp sound more prominently like a SET amplifier, at least in this application.
BTW, the Edcor OT transformers are superb. I couldn't believe how these relatively inexpensive transformers are so well-made, and sound so good.
The primary inductance was measured at 13H.
I will look deeper into the mechanism of what causes the profound increases in the 2nd order harmonic distortion.