Princeton Reverb clone monster bias cap

[Randall]

I have a Lewis Electric Roadrunner combo that is based on the AA1164 Princeton. It has an intermittant hum that sounds like a filter cap to me. One 20uF section of the can cap has already been bypassed with a big fat and stout F&T cap. I can't see the value with the way it is installed, but I'm betting it is more than 22uF. Regardless, jumpering a fresh 22uF cap across any of the can connections yields no results, so I do not suspect it at this point.

However, jumping a 22uF across the bias filter cap does make a difference. My issue is, the cap that is in there now is a 22uF 500v ! The schematic calls for a 25uF/50v, and I would normally bump that up to a 50uF/100v, or even 100/100v if that's what I had on hand. But a 500v cap? It is an IC brand btw, which I do not like. Wondering what do you think the rationale was for designing in a 500v cap in a 50v circuit? Unless it has been replaced, which I wouldn't know.

[Enzo]

Replaced sounds like most likely. Only other reason to use 500v instead of 50v would be we had no 50v on hand but had plenty of 500v.

[nosaj]

Pet peeve of mine. If your gonna replace the cap at least mount it with relevant info showing. I always take the time to make sure you can read a replacement just because it may be me that sees it again.
nosaj

[vintagekiki]

https://www.thefretboard.co.uk/discussion/102187/louis-electric-columbia-reverb-gut-shot

[Jon Snell]

22uF 50volt, of a decent make to ensure low ESR, is adequate. I suspect, even without seeing the amplifier, it has been replaced with a capacitor they had on hand and not original.

[SoulFetish]

Pet peeve of mine. If your gonna replace the cap at least mount it with relevant info showing. I always take the time to make sure you can read a replacement just because it may be me that sees it again.
nosaj

Im trying to be better about that

[Leo_Gnardo]

However, jumping a 22uF across the bias filter cap does make a difference. My issue is, the cap that is in there now is a 22uF 500v ! The schematic calls for a 25uF/50v, and I would normally bump that up to a 50uF/100v, or even 100/100v if that's what I had on hand.

I would swap in a 47 or 50 uF 100V cap without a moment's hesitation, and be done with it. I can only guess that somebody tried to fix or "improve" the amp, all they had on hand was that flakey 22/500 IC cap. Who knows maybe it was removed from some other amp and Mister Fixit figured "close enough for rock & roll."

[pdf64]

I have a Lewis Electric Roadrunner combo that is based on the AA1164 Princeton....

Has it got a HT fuse, ie between the CT of the HT winding and 0V common?
If the connection there was broken, it's possible that the voltage on the bias supply cap might drift higher than in normal operation.

[Randall]

It has no HT fuse.

I replaced it with an F&T 100/100v, and tightened the chassis screw it makes it's ground with. All is well.