EH Scott Radio Enthusiasts

The Fine Things are Always Hand Made

OK folks...question: Here is the schematic of the outboard volume expander for an AW23. I was working on one earlier today and noticed a mod that I had not seen before. If this mod isn't factory, it was done a <long> time ago - near its original production. The mod is simple - a 20ufd/25VDC cap is wired from the 6A7 cathodes to ground (+ side on the cathode). It is soldered right on the wiper of the balance potentiometer. Any thoughts on what this might do? Should I remove it/leave it out when I restore it?


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Comment by Kent King on March 9, 2023 at 6:16am

From the “Instructions for Expander Adjustment Using the New Type 6A7 Tubes”, Scott noted this:

“5. Replace the 6A7 tubes in the expander with new ones we sent you. These tubes will have a letter instead of a number marked on the small sticker.”


I was working on an expander recently and found these two tubes in it….I’m sure these were originally factory marked. They both tested good, but not quite the same, one was very slightly less than the other in my I-177 tester…good enough to keep them together though!

Comment by Norman S Braithwaite on March 6, 2023 at 9:51am

Possibly to keep the stage gain within tolerable limits over the range of bias.  It removes negative feedback generated through the 300-ohm resistor.

Mike:  please contact me regarding Scott Escutcheons!


Comment by mike hadley on March 5, 2023 at 5:13am

Possibly an attempt to alter the frequency response of the 6a7 but there would also be an effect on overall gain this would give a loss of control from the 76 tube because the cathode potential of the 6a7 would be higher.

Another thought, has the potentiometer become noisy an attempt has been made to stop the noise.

I think that with all of the availible set data returning the expander to factory is the way to go


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