5F6A Power Supply Cap

[rymac]

The Fender reissue bassman uses 2 100uF rated at 350vDc wired in series with 2 220K resistors at the B+.
The original circuit has 2 20uF wired in parallel. What's the voltage rating on these two caps?
Which setup works/ sounds best? I see on Hoffman Amplifiers 5F6A Bassman uses 2 20uF in parallel.
Sprague makes a 80 µF @ 450 VDC that is the same size as the 100uF @ 350vDc. Could you use two 80uF in series to get closer to the original setup?

Rymac

[Enzo]

The schematic says 20uf/600v, so you have 40uf @ 600v for the first filter node.

600v caps are not common, you can find them, but they cost a lot.

Two 100uf caps in series makes a 50uf cap, and as long as there are equalizing resistors across them the voltage ratings add. SO those 100uf/350v caps make essentially a 50uf/700v cap. And for Fender those two caps are probably cheaper than the 600v equivalents

The difference betwen a 40uf and a 50uf cap is not a lot for the circuit. The extra 10uf from the original would have been well within the part tolerances of that long ago day anyway.

SInce the circuit is operating at something like 430VDC, a 500 volt cap would work just fine in place of the original 600v part. I bet that is what Hoffman uses.

There is no "better," either way works out about the same and requires two caps of similar value. Don't look at 40uf as some magical critical value, it is just a common amount. You could just as easily use a single 40uf/500v part or possibly even a 450v part if that's all you have. Those are working volts ratings, not surge ratings. Just check the B+ is not over 450 then.

If it were me, I would look at price. Perfectly good 22uf/500v caps are out there for a couple bucks or so, compare that to odd 600v caps or old odd value Spragues of unknown age. And the 100/350s are also easily found at reasonable cost to go the reissue route

[MWJB]

The main trouble with the Sprague 600v caps is that they're much longer than the 500v caps, a real headache for Fender style chassis, not so much of a problem for a scratch build if you allow a larger chassis.

Fender also used 500v 20uf caps on Tweed bassmans.

Don't use the 80uf 450v.

Unless you are using a PT of your own spec, you are going to see more like 470vdc (+/- 10v) on an original 50's/Hyboer/Victoria/Kendrick...etc, etc, PT. You might see 500v+ from a reissue (I've seen up to 540vdc on one build that used a Super Reverb PT, depending on bias). As Enzo says, 2x100/350v (450v are easier to find & usually cheaper than 350v) are cheaper & easier to install than Sprague 600v equivalents.

Hoffman acknowledges both filter wiring styles on his site, his schems show 20uf/500v.

My preference is for the series caps, it sounds better, less mushy, more dynamic. If you're converting an RI/using a high voltage Fender style PT (over 350-0-350VAC), then do the same at the screen supply.

[rymac]

Thanks for the info.

Rymac

[Diablo]

I ran into this cap problem with my Weber 5F6A build. The two 20mfd/500V caps in parallel fried after 30 seconds. I replaced them with two 100mfd/450V in series and a couple of 220K ohm bleed resistors. My plates measured 462V, so the 20/500 caps have no reserve voltage capacity. I'm not sure who made the bad caps, but I used Illinois caps as replacements.