Gibson GA40

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Gibson GA40

Postby bingo boy on Thu Jun 01, 2017 11:56 pm

Hi, I haven't been here in a while. I have been struggling with a Gibson ga40 amp build. My mistake when I used the rewritten copy of the Joe Piazza schematic. It wasn't till I asked a friend to give me his takes on the circuit. On both volume controls the wiper is tied to the plate of the 5879 tubes. I am surprised I didn't pick up on that error. I also observed the filter supply ties the plates and the screens of the output tubes together. The power transformer supply voltage a/c is 300 volts, yet the rectified d/c voltage is 315 volts. Hmm? I went with 6l6"s as my working voltage is 450 volts with a 300 volt a/c supply. No screen resistors and no grid stopper resistors. The tone control is set off of the north rail of the phase inverter. I completely reworked the power amp insomuch the first filter supplies the plates and after the choke to the screens. Then the phase inverter has it's own supply to which it also feeds the depth control with about 400 volts. Hmm? The phase inverter uses a 12ax7. I have had nothing but problems trying to get this thing going. So, I am asking if anyone has built one of these amps, and what were their experiences. Maybe my transformer is too big for this amp, 300-0-300 @ 145 mils. Thanks for your reply... GULP! Ken
bingo boy
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Joined: Fri Feb 01, 2013 3:06 pm

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