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Hard for me to believe, but I've spent almost two years of on-again, off-again work with this amp. It sure has taught me a lot about how amps work!
As delivered by the factory, the printed circuit board is laid out incorrectly, leaving the amp without functional grid stopper resistors.
An externally hosted image should be here but it was not working when we last tested it.
But I went a bit further than simply correcting the wiring error. Though the basic circuit is mostly unchanged, the "final" version of the amp features upgraded output transformers (Edcor XSE15-5K), larger cathode bypass capacitors (Elna Silmic), and most importantly, the 1k ohm metal film resistors used as grid stoppers have been replaced with carbon composition types, selected for their high frequency performance. These are placed about 1 cm away from the tube sockets for best performance. Stock 0.1uf coupling capacitors were okay, but for fun I changed these to 0.1uf 716P polypropylene "Orange Drops" which didn't make a really big difference, but it certainly didn't hurt either. Tubes consist of stock Raytheon 5755 at the input, and Russian 6P14P-ER outputs, of which I have a good-sized supply.
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1 kHz square wave:
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10 kHz: I seem to have a resonance at around 120 kHz causing the extra "blip" in the sides of the wave.
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20 kHz: The high frequency artifact is more visible, but should be completely inaudible. The sides of the wave are going more triangular, but all in all, I'm really pleased by what I'm seeing here.
An externally hosted image should be here but it was not working when we last tested it.
How's it sound? Wonderful! "Air" and detail, but with guts too, and the harmonic rightness you expect from single-ended, all from good quality yet inexpensive parts.
Tecon Audio Model "55" product page