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JR20 Question

 

Things seem pretty quiet on the list lately, so I thought I'd ask a question:

What is the function of the "Bass Correction" switch, on a JR20 tone cab?

Looking at the schematic, it appears to switch a resistor (R35, 1K) in and out of series with the cathode resistors on the bass channel preamp tubes (V7 + V8, 6SJ7s).

Does that just function to change the bias on those tubes?? Or...?
 

The bias is not changing since the resistor is between the cathodes and not between the cathodes and ground. Since the tubes are being driven out of phase with each other it will affect the gain of the stage. Kind of a neat hack to change the gain of both sides of the balanced amp at the same time with only a single pole switch..
 

If I recall correctly PR40 cabinets had a "room size" switch, which altered the bass response to suit different sized rooms.?


On Sat, 2 Mar 2024, 16:12 Richard Brinkerhoff, <richard-brinkerhoff@...> wrote:
The bias is not changing since the resistor is between the cathodes and not between the cathodes and ground. Since the tubes are being driven out of phase with each other it will affect the gain of the stage. Kind of a neat hack to change the gain of both sides of the balanced amp at the same time with only a single pole switch..

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