Epiphone EA-65 circuit - wild and interesting

[Rob Mercure]

Folks,



While researching Gibson amps to help with another poster I reviewed the Gibson master service manual which I have in PDF format. I can?t remember where I got the file it?s pretty big (177,585 kb) but I remember that the location was provided either on an old Ampage post or on the current Forum. In any case while tooling through the schemos I came across the Epiphone EA-65 schematic and it?s truly ?interesting and strange? and I?m not sure what?s actually going on there ? so I?d like some other opinions before I again wind up tasting my toes.

The amplifier consists of a ? 6EU7 split load PI feeding another 6EU7 driver tube each triode feeding a 7591 output PP pair. The strange and interesting part is in the NFB loop which, moving ?backwards? from the OT secondary consists of three sub circuits in parallel: the first is simply a potentiometer wired as rheostast marked ?factory adust? and in parallel with this is a 200uf capacitor in series with an SS diode with the cathode going to the OT secondary which is in parallel with an indentical circuit with the diode reversed with the capacitor/diode midpoints tied together with a 68K resistor. Continuing ?upstream? this circuit feeds a lamp which drives an LDR; the LDR goes from the PI control grid to ground. Obviously as the lamp lights/darkens it modulates the signal into the PI via the control grid ? but what is it?s ultimate (and perhaps ulterior) purpose?

And, after we ?cuss and discuss? this circuit a bit I?ve spotted a couple of more truly wild schemos I?d like to dissect ? so if anyone can again point to the Gibson service manual site it would probably helpful.

Rob

[Enzo]

Schematic Heaven has it.

EA65, look at the title page. That is your "Photon Power Control" circuit.

The audio at the speaker is sampled off through those caps to light the lamp in the LDR over at the volume control. The LDR can't respond quick enough to serve as the NFB agent. The two large e-caps are wired parallel reverse polarity more or less, and to the left of each cap is a diode facing the appropriate way in series with them. This provides a signal proportional to the average output level at the speaker to the lamp in the LDR. The louder you get, the more it lights the lamp, and thus the more it shunts off the signal at the volume control.

SO it looks to me like a compressor or limiter of sorts. The parallel factory adjust control I am not sure about. Looks to me like it might tweak the speed of response or possibly a sort of threshold, as in set the lamp to not come on too early.

[Rob Mercure]

Well, that's what I was thinking - now I wonder how well it would work? Any takers?

Rob

[Enzo]

Who knows, you could probably remove it, but it might be there to fudge away some performance issue like farting at high levels or speaker cones over-excursing or something.