38-690 puzzle - Printable Version
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38-690 puzzle - Guest - 08-18-2009
I am starting to work on my 38-690, but right now just studying the schematics to get the overall flavor of the circuits. One oddity jumps out. There is a 6J5 labeled as Bass-Amp AVC, near the audio driver and output circuitry.
My diagram (from Chuck) shows the plate is tied directly to the cathode and ground. Is this tube doing anything? It looks like a dummy inserted to up the tube count! Or am I missing something subtle?? I haven't actually looked at the chassis yet to see if that is how it is wired.
Bill
Re: 38-690 puzzle - Ron Ramirez - 08-18-2009
This tube is connected as a diode; with the grid acting as a diode "plate".
Very interesting circuit. It is used to control the gain of the bass amp tube (6K7G) in much the same manner as AVC is normally used to control gain of RF and IF tubes, and sometimes mixers as well. A portion of the audio signal is taken from the plate of the audio driver tube (6F6G) and fed to the grid of the bass amp AVC tube, where it is rectified and sent on to the suppressor grid of the bass amp tube. This serves to control the amount of signal amplified by the bass amp tube.
Yes...very interesting indeed.
Re: 38-690 puzzle - Guest - 08-18-2009
I see, I see! Weird but neat...