AB868 - Q re. cap on V3

[Stringrazor]

I'm near the end of my SFDR recap job. I've found an existing cap on V3 that seems to be installed backwards. It looks non-original (the only one like that in the amp). It's a 35uf/25v with the + end on the ground eyelet (near the front controls) and the - on the eyelet that goes to the tube socket. The schematic I have says that's wrong but the amp has always worked fine. V2 is the reverb pre but I've never had a problem with reverb. If the cap is wrong, why do things work ok?

 

[VaughnC]

V3 is usually the reverb driver tube and the cap would be the cathode resistor bypass. The - side of the cap should be at ground and the + side of the cap should be on the cathode of the tube because that's the voltage polarity developed across the cathode resistor. Being installed backwards has probably strained the cap so I would just replace it.

The only electrolytic that should have the + side on ground is the bias supply filter cap.

 

[Stringrazor]

Thanks! Just soldered it in. Now to hold my breath and fire it up...

Success!

 

[aeolian]

This is wild. the silverface I just reworked to BF had this same cap in backwards. It does look like someone went though part of it at one time. It's so obviously backwards, I can't imagine what sort of conventional wisdom had folks doing this.

 

[TheAmpNerd]

Originally posted by aeolian
This is wild. the silverface I just reworked to BF had this same cap in backwards. It does look like someone went though part of it at one time. It's so obviously backwards, I can't imagine what sort of conventional wisdom had folks doing this.

No wisdom at all.

You should see the amps I get in
with mains filters wired incorrectly in parallel in some Fender amps. They work but that is about all you can say abou tthem. I'm surprised the caps didn't blow up or something.