Bogen CHB-20A Conversion with Ampeg Gemini II Preamp Schematic

[Platefire]

I'm going to go ahead and post this. I think I've got the schematic bugs out of it. If you see any issues with it, let me know.

Generally speaking in this conversion the Bogen power amp and power supply is stock Bogen. From the input jack up to the Phase Inverter has been changed to the Ampeg Preamp/Tone Stack. I rewired the 6EU6 to use a 12AX7 simply because they are more plentiful. Also changed one bias resistor as noted on schematic to achieve -16V bias as indicated on Bogen schematic. I added a standby switch.

The amp has a nice clean headroom and the tone stack does a very good job with guitar tones.
The 6GW8/ELC86 tube are kind of rare and expensive, so you might check that out before you pick up this amp. At about 6 to 7 on the pre-gain that amp starts to break up with the MV up 100%. Up to that point you get nice clean loud tones. To get an earlier breakup at lower volumes the MV works good to dial that in as needed. Platefire