Higher Voltage Filter Caps ?

[Deric]

I've got a 6G6A Bassman that needs filter caps. Stock caps were all 20u/525V . Schematic shows 470vdc at the first cap. I'm guessing I might likely see higher than that with modern mains voltage. I have F&T 22u/500v caps on hand but I'm concerned that may be cutting it close. I can use a pair of 47u (or larger) caps in series to get the voltage rating up for the first two but still think 500v may be to close for the third cap... Thoughts?

Not having much luck finding caps higher than 500v at the usual sources (Mouser, Digikey, CE etc.). Anyone know a good source?

Thanks!

[martin manning]

Can you get some measurements of the PT input and output voltages?

[Deric]

Came without a power cord so gotta wire one up first. Will do that later this evening.

[Synchu]

There are these 600V rated Weber elytics available here in EU.
http://www.banzaimusic.com/Weber-Elytic ... 00VDC.html

Niki

[didit]

Hello -

Just Radios stock axial caps up to 600V .. https://www.justradios.com/axialelectrolytics.html.

Best .. Ian

[Deric]

Hello -

Just Radios stock axial caps up to 600V .. https://www.justradios.com/axialelectrolytics.html.

Best .. Ian

Thanks for the reminder!! I always forget about Just Radios but have used them several times for hard to find caps.

[Deric]

Can you get some measurements of the PT input and output voltages?

Primary - 121vac
Secondary - 380-0-380
Bias - 55vac
Filaments - 6.87vac

[Roe]

frag jan zuerst sells nice FT caps rated at 550/600v:
https://www.die-wuestens.de/dindex.htm?/kond.htm

Also, the TAD audio series caps are great for fenders:
https://www.tubeampdoctor.com/en/shop_C ... 5x50mm_517
https://www.banzaimusic.com/TAD-AudioCap-20uF-550V.html

For 6g6s I prefer at least 47uf on the mains.

[martin manning]

So with that voltage (unloaded, I assume) you will get 540 DC. The reservoir is two parallel caps at 20u on the schematic, which you could replace with two 100u 350V in series and get more than enough voltage rating. Balancing resistors will reduce the DC by a few volts. The screen cap might be ok at 500V, but I agree it should be higher. There will be a transient peak due to the choke. Using modern caps you may have room to make that a series stack too, and that will reduce the voltage by a few more volts for the following caps.

[pdf64]

Fender's cheap solution to this issue was the introduction (to the receiving tube world) of the standby switch, which was initially placed between rectifier and reservoir. As you've got silicon rectification, no reason not to use the caps you've got and try that.
I think that Merlin's 47k standby bypass is beneficial.

[martin manning]

Notice on the schematic the voltage rating listed for the caps is “600 p.” Which may be referring to peak.

[R.G.]

First off, kudos for actually worrying about what happens in funny conditions, not just normal operation!

Second: Most electrolytics have a surge voltage rating, higher than the normal working voltage. You might look up the surge ratings for the caps you're thinking of using. Surge ratings are intended for situations like this.

Third: I was always suspicious of surge ratings. A manufacturer saying "sure, go ahead and use it beyond ratings. It'll probably work out OK. Maybe."

So I usually wind up doing a trade-off on trusting surge voltages versus cost/space of stacked 350V caps. There's no question about sacked 350V caps working.

Fourth: well, maybe there is a question about sacked 350s working. The DC voltages at steady state distribute inversely with the DC leakages of the caps. If one is leakier than the other, the less leaky one has more voltage across i. And transient voltages distribute inversely with capacitance, so the smallest-capacitance one eats more of the transient voltage. So always use equalizing resistors (which you were planning to do; again, good thinking) to swamp out the DC leakages and transient stuff.

Finally, I realized that equalizing and ballast/rundown resistors across caps make enough current to light an LED. If you stick an LED in series with the run-down/ballast resistors, you can SEE if the cap still has significant voltage on it. I put those on the Workhorse amps main PCB so service people could see whether the caps were charged and deadly or not.

[TUBEDUDE]

I've got a 6G6A Bassman that needs filter caps. Stock caps were all 20u/525V . Schematic shows 470vdc at the first cap. I'm guessing I might likely see higher than that with modern mains voltage. I have F&T 22u/500v caps on hand but I'm concerned that may be cutting it close. I can use a pair of 47u (or larger) caps in series to get the voltage rating up for the first two but still think 500v may be to close for the third cap... Thoughts?

Not having much luck finding caps higher than 500v at the usual sources (Mouser, Digikey, CE etc.). Anyone know a good source?

Thanks!

Solen has 630V or 1000V caps, but since they are metalized film, they are physically larger. The 22 uF/630 won't fit under a doghouse, so it's not good for stock Fenders without some creativity.

[Deric]

Thanks for all the replies! I've gone ahead and ordered 600v caps from Just Radios.

[R.G.]

Ah. Film caps.

I had this fevered-brain idea a few years back that I call The Immortal Amplifier. The general idea is to figure out which things inside an amplifier decay and die on their own, and either make them have indefinite life spans or extend the ones they have.

It's not a new idea. Roll Royce reportedly did much the same, running prototype cars on a torture track and machines until something broke, then redesigning that broken part to be stronger.

For rectifier/filter caps,this involved taking a step back from electrolytics to film caps, as they were used in the first part of the 1900s before electrolytics became a smaller, lighter alternative. Today, we have MUCH smaller, lighter, and longer lived film caps. Today's film caps come in plastic packages, and it's possible to make an array of film caps to make up a total 20-40uF at high voltage to replace electros. In fact, there are cases where distributing some of the first filter cap out around the chassis might be a good idea.

Then there are HVAC/motor run caps. These things have gotten small and cheap, and are fully designed for passing high currents a high voltage. Sure, they're bigger than electros, but by a factor of 2 or 3, not 10.

So I decided film caps had to go in the Immortal Amplifier. It's worth considering.