AB763 Filter Cap Orientation

[bobloblaws]

Awhile ago I bought myself 67/68 silverface Twin Reverb with the AB763 circuit. One of the first things I did was replace the original Mallory filter caps. I'm quite certain I oriented the polarity of the new caps as I found the old ones but in any case it's now obvious I didn't check the schematic at that time. Just now I opened the amp up again to check the bias (no issue, I just remember it being biased on the cold side and wanted to try tweaking it a little hotter) and opened the cover to look at the filter caps (I couldn't remember what brand I had put in and I was curious). Having worked on a number of similar amps in the meantime I noticed the polarity of the outermost 70uF cap was backwards. Per the schematic its negative terminal is supposed to connected to ground but I had it in backwards. I haven't played the amp a whole lot and certainly haven't cranked it up since I live in an apartment building, but the amp seems to work fine with no significant hum or any other apparent problem. My knowledge of how polarized caps work in a circuit is limited so I'm curious what you guys would have expected the result of this SNAFU to be.

B.

[Tom Phillips]

...67/68 silverface Twin Reverb with the AB763 circuit...opened the cover to look at the filter caps...I noticed the polarity of the outermost 70uF cap was backwards. Per the schematic its negative terminal is supposed to connected to ground but I had it in backwards... I'm curious what you guys would have expected the result of this SNAFU to be.

I agree that the outermost 70uF cap negative terminal is supposed to connected to ground. For clarity (Picture worth 1,000 words), attached is a photo of a 1964 Stock doghouse parts board. I would just replace the cap with a new one. It does not matter how hard you played the amp. Just applying the high voltage would damage, or at least degrade, the cap. In fact, I would just replace the first two caps. They are wired in series and the high voltage is supposed to split equally between them. The two 70uF/350V caps are wired in series (stacked) to make the equivalent of one 35uF/700V cap.

Cheers,
Tom

[bobloblaws]

I agree that the outermost 70uF cap negative terminal is supposed to connected to ground. For clarity (Picture worth 1,000 words), attached is a photo of a 1964 Stock doghouse parts board. I would just replace the cap with a new one. It does not matter how hard you played the amp. Just applying the high voltage would damage, or at least degrade, the cap. In fact, I would just replace the first two caps. They are wired in series and the high voltage is supposed to split equally between them. The two 70uF/350V caps are wired in series (stacked) to make the equivalent of one 35uF/700V cap.
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Cheers,
Tom

Sounds about right Tom, I will replace them both (and hook 'em up right this time! :-))