Bogen/plexi conversion question

[orangechili]

Hi, I'm new to this forum and was wondering if anyone knows about converting bogen p.a amps for guitar use? My friend knows a little bit about electronics and helped me draw this schematic up. We're trying to modify the bogen chb 35a schematic to yield similar results to a model 1987 plexi. We're keeping the bogen transformer and trying to create a hybrid with a plexi schematic. I was wondering if someone could look at the schematic and tell me if it is set up right? I know if we use the bogen transformer we have to use the bogen schematic for the power supply. Any or all input would be more than welcome and greatly appreciated. Thanks

[PRNDL]

You're on the right path.
The first gain stage (Mic) needs some work ...
Put a 1M to ground on the jack, and a 47K grid resistor to V1.
Then move the master volume after the tone stack.

[Eli Duttman]

The DPST switch shown as standby is, in fact, a complete on/off switch. A standby switch will not remove power from the tube heaters.

To obtain the standby function, a SPST switch rated for 240 VAC and several Amperes service needs to be inserted into the red/yellow lead from the power trafo.

Replace the voltage doubler diodes with UF5408s. Replace the bias supply diode with a UF4007.

40 muF. caps. in a doubler stack look wimpy to me. More capacitance is definitely needed. The $64 question is how much more capacitance can the power trafo stand, without overheating. I'm guessing that an extra 33 muF. in parallel with the 40 muF. parts will be OK. No matter what, you MUST replace all electrolytic caps. in the amp. 'Lytics literally dry out over time.

Be sure to read the "sticky" thread about safety on this forum. A mistake with this stuff gets you DEAD.

[orangechili]

Thanks PRNDL, I will change the schematic up. I really appreciate the help.

[orangechili]

Thanks Eli, good call on the caps and standby switch. I will check out that "sticky" thread as well.

[Eli Duttman]

We're keeping the bogen transformer and trying to create a hybrid with a plexi schematic.

Something I neglected to mention before is the fact that 7868s are easier to drive than the EL34s found in some Marshall designs. AAMOF, the 7868 is as easy to drive as the EL84/6BQ5. So, replacing a 12AX7 with either a 5751 or 12AY7 might be necessary. The mu (amplification factor) of the 12AX7 is 100. The mu of the 5751 is 70. The mu of the 12AY7 is 40.