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11-09-2022, 07:43 PM
Hi Daryl
I was just looking at the Fender Super Champ XD schematic to cut and paste the PA section and this one is different from the usual inasmuch as it uses a tweed circuit: a concertina splitter preceded by a gain stage. That is actually quite a nice sounding circuit with the right values.
The feedback loop is R23=2k7 (series) and R17=47 (shunt). The ratio here is 57.5, so a gain of 58.5 which is pretty high. This could make the tone a bit woofy or dark. It is odd that so much gain is being demanded from the PA considering the number of opamps preceding it.
The output tubes share a cathode current-sense resistor and there is only one bias pot. A useful mod would be to split the cathodes and add a second resistor, then split the bias feed and add a second bias pot in parallel. This allows hum-balancing of the output stage, which in itself is good with an added benefit of improving the bass response through the OT (not that you might hear that).
Have fun
I was just looking at the Fender Super Champ XD schematic to cut and paste the PA section and this one is different from the usual inasmuch as it uses a tweed circuit: a concertina splitter preceded by a gain stage. That is actually quite a nice sounding circuit with the right values.
The feedback loop is R23=2k7 (series) and R17=47 (shunt). The ratio here is 57.5, so a gain of 58.5 which is pretty high. This could make the tone a bit woofy or dark. It is odd that so much gain is being demanded from the PA considering the number of opamps preceding it.
The output tubes share a cathode current-sense resistor and there is only one bias pot. A useful mod would be to split the cathodes and add a second resistor, then split the bias feed and add a second bias pot in parallel. This allows hum-balancing of the output stage, which in itself is good with an added benefit of improving the bass response through the OT (not that you might hear that).
Have fun
