For somebody reading this thread wanting to build a nice push-pull amp amp from the Golden Age of Audio, you can also refer to the Eico HF-87, a true audio classic. I just finished building a stereo clone of the HF-87 with Edcor iron and it's performance is outstanding. I used a 12SL7GT instead of a 12AX7 to reduce the gain a bit and it worked out great. 550mVRMS can drive it to full power so it can be used with or without a preamp if you wish. Noise is extremely low and I have to put my ear right up to my horn tweeter to hear a faint hiss.
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You are listing the common cathode resistor for the pair of output EL34s as 235 Ohm, vs the original value of 165 Ohm. I wondered about this decision/choice, could you explain, please. Wrt the slight hiss you hear, perhaps rectifying and regulating the heater supply for the 12SL7 will further improve that. Good use of that 12Vac winding on the XRXW117 power transformer!
I own an Eico HF-87 and plan to add fixed bias, with the unused bias tap on the original power transformer. I will modify it in the near future according to Dave Gillespie’s GNF and EFB (Enhanced flexible bias) modifications. The feedback mod is specific to the original output transformer, but you might consider the EFB addition with a small bias transformer. See: https://audiokarma.org/forums/index...fications-to-the-hf-87-nfb-loop.937912/page-3
Btw, nicely drawn schematic, what software did you use? (It appears that the common PS schematic omitted one pair of EL34s).
I own an Eico HF-87 and plan to add fixed bias, with the unused bias tap on the original power transformer. I will modify it in the near future according to Dave Gillespie’s GNF and EFB (Enhanced flexible bias) modifications. The feedback mod is specific to the original output transformer, but you might consider the EFB addition with a small bias transformer. See: https://audiokarma.org/forums/index...fications-to-the-hf-87-nfb-loop.937912/page-3
Btw, nicely drawn schematic, what software did you use? (It appears that the common PS schematic omitted one pair of EL34s).
Francois,
The 235 ohm output stage resistor is common to both channels on some iterations of the HF-87, but some HF-87's use a 165 ohm resistor and the 12AX7 heater connections 4 & 5 to complete the circuit. This supplies power for the 12AX7 heater, plus elevates the 12AX7 heater to 10.5VDC.
The very slight hiss I am hearing is a hiss, not a hum. DC on the heaters in this amp will not improve anything. The hiss I am hearing if from excess gain in the amp design. I have Klipsch LaScala's so residual noise from any amp is easily heard. Again, the hiss is so slight, but I thought I would mention it. I can easily live with it. Just an observation.
I don't need more power, but may try a fixed bias version in the future. The Edcor output transformers work extremely well in this application. It would be a great experiment to compare the original Eico OPT's to the Edcor's.
I'll update the PS schematic, you are right, one pair of EL-34's are missing! Thank you.
The schematic was drawn in Circuit Maker, a fairly old software app.
Aha! I forgot about Eico’s trick with the heater if the first 12AX7 in series with the cathode resistor. Thanks for the explanation. I’m delighted to hear the Edcor CXPP50-6.5k output transformers work so well in this circuit.
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