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01-09-2023, 03:20 PM
Hi Guys
There is some missing info from the original post. I believe the reference circuit is for the 5E7 bandmaster, which has a standard Fender gain stage with 1k5 on the cathode and 100k Ra as the front-end of the PA, followed by a concertina. The feedback resistor is 56k as stated, tied to the cathode and 5k pot dialing in 100nF for presence. The stock PA gain is 38 - higher than even a Marshall typically has.
Are you changing Ra? or leaving it at 100k.? If the latter, then the open-loop gain may be reduced.
Gain with 68k series and 3k3 shunt is about 22, similar to a closed-loop Marshall.
Gain with the 50k pot at max is about 38. Not a big sweep but noticeable. In my STUDIO amp the Vibe control does what this pot does, and is five times the value of the fixed-R to give closer to open-loop performance at its end.
The presence pot has to be about 10x the shunt-R value to dial out the cap, so 50k is okay here although some of the sweep will not seem to do anything.
Because of the PA front-end configuration, stability is better than with a Schmitt unless the layout is particularly terrible. Did you begin with a stock Marshall?
Have fun
There is some missing info from the original post. I believe the reference circuit is for the 5E7 bandmaster, which has a standard Fender gain stage with 1k5 on the cathode and 100k Ra as the front-end of the PA, followed by a concertina. The feedback resistor is 56k as stated, tied to the cathode and 5k pot dialing in 100nF for presence. The stock PA gain is 38 - higher than even a Marshall typically has.
Are you changing Ra? or leaving it at 100k.? If the latter, then the open-loop gain may be reduced.
Gain with 68k series and 3k3 shunt is about 22, similar to a closed-loop Marshall.
Gain with the 50k pot at max is about 38. Not a big sweep but noticeable. In my STUDIO amp the Vibe control does what this pot does, and is five times the value of the fixed-R to give closer to open-loop performance at its end.
The presence pot has to be about 10x the shunt-R value to dial out the cap, so 50k is okay here although some of the sweep will not seem to do anything.
Because of the PA front-end configuration, stability is better than with a Schmitt unless the layout is particularly terrible. Did you begin with a stock Marshall?
Have fun
