I purchased one of these beauties last year. It ships without a schematic, but I was able to trace it out and lay it out in Eagle CAD. With help from others (including Ray Moth, who introduced me to the concept of a "grid stopper" and "parasitic oscillation"), it looked like there was room for improvement of the stock circuit. Square waves looked fuzzy with oscillation, becoming oddly distorted at higher frequencies.
I relocated the grid stoppers thus, and the "fuzz" on the square waves vanished. I also added a 12 pf capacitor in parallel to the 470K ohm feedback resistor, and upped the value of the EL84 cathode bypass capacitor from 180 uf to 220, but more on that in a moment.
(Along the way, I replaced the stock OPTs with Edcors, but that was probably premature)
Modified amplifier, 1 khz square wave:
10khz. I can get more picture-perfect results here if I increase negative feedback, but it seemed to come at the expense of midrange constriction, so I reverted back to the 470K ohm resistor.
20 khz. there was some ringing, and waveforms still looked pretty distorted at higher frequencies. So I added a 12 pf capacitor in parallel to the 470K ohm feedback resistor, and that smoothed over the gremlins nicely. I was surprised to see how such a tiny bit of capacitance has such a profound effect on it's performance. I may be able to get away with a bit less.
What a difference: The stock amp sounded lean, constricted and bass-shy, but the modded amp sounds nice and relaxed with plenty of bass bloom. Maybe a bit too much in fact. I may try downsizing the 0.1 uf coupling capacitor and see what effect that has. Sine wave frequency response still looks essentially flat down to 30 hz, but square waves don't look so hot 'till somewhere around 150 hz, so I don't think I'll be missing much if I roll off some of the lower bass frequencies.
The bulk of the mods were really simple: Just cut 2 circuit board traces, relocate 4 resistors, add 2 jumpers, 2 new capacitors, and replace 2 electrolytics. I'll post photos showing exactly which parts go where later in the week if time allows.
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Topic - Tecon Model "55" gets a makeover - 4season 20:58:25 09/07/09 (3)