CtG electronics FCBN 51B question

[dbchamblee]

In looking at the schematic of this amp, I noticed that capacitors C13 and C23 in the cathode bias circuit are backwards with the positive side connected to ground. Is this right? The original Ampeg B-15NB schematics show a 50uF inserted correctly. The schematic is here: http://ctgelectronics.weebly.com/upload ... 5_orig.jpg

[Structo]

Can you post a higher resolution schematic?

[sluckey]

The cathode bypass cap in that schematic has it's negative lead connected to ground just like a B15.

But this amp can operate with fixed bias or cathode bias and...

The caps you mentioned are part of the fixed bias circuit. Positive end should connect to ground for those.

[dbchamblee]

Structo, that's the highest resolution on their website. If you save it as a pdf and then open it in a viewer you can zoom in. After a little more research I have seen other schematics with a similar setup I'm just not used to seeing electrolytic caps used reverse polarity. I thought they'd blow. I admit I don't totally understand what's going on in this circuit. I assume the caps must be seeing a voltage that's negative with respect to ground.

[JazzGuitarGimp]

From the looks of it (low resolution schematic), I'd say the diode in the bias supply is drawn backwards. Yes, the bias supply cap is seeing a negative voltage with respect to GND.

[roberto]

I have to say that I don't really like that solution.
Everytime you switch from fixed to cathode bias, you'll abruptly ground a cap charged with minus fiftysomething volts.

[dbchamblee]

roberto, I don't think you're supposed to change modes with the amp powered up, at least I wouldn't recommend it.

[roberto]

I agree, but a good design has to prevent possible actions that can cause reliability issues.

[mmmoser]

I built this circuit for a customer. The amp came out great and sounded quite nice- but I have to say I am not a Bass guy but believe the new owner when he tells me its sounding great.
If you want to check it out I have pictures of the build posted here:
https://www.facebook.com/media/set/?set ... 972&type=1