I have built a Bogen CHA-20 into a guitar amp using a schematic with my own design. I added 2 12AX7 tubes to add reverb and vibrato. I have drawn the schematic for the amp. The reviews of the amp are on Harmony Central amp data base as I have tried 3 different schematic designs. I would be happy to email a schematic attachment if you wish to see what I have done. I tried using a higher cathode resistor on the power tubes and the current (which is "self biasing") essentially stayed the same. I went from the original measured 328 ohms to 430 ohms with virtually no change. I contacted a friend who is off the chart knowledgeable about amp building and he suggested leaving the original resistor in. I also added a "tweed" switch to break the grounds on the tone stack and a pentode/triode switch to lower power. My friend plays a 5E3 deluxe and agreed the sound of his and mine is similar except that mine has the added reverb and vibrato. tubenit
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