What voltage rating for AB763 doghouse caps?

[glebert]

I've got two blackface Super Reverb amps in my queue with original (bulging) caps. The 20uf caps are rated for 525V with a post-choke voltage of 460V per the schematic. Tubesandmore.com recommends 500V rated caps for this, but considering that 460V may be higher with modern mains voltages is that high enough? I made the mistake of letting the shop manager order caps and he got 600V Spragues that are too big to fit in the doghouse.

[g1]

Another consideration is when the tubes are removed for troubleshooting, the voltage will rise even more. Node C may also be at risk. Here we are close to 125VAC line voltage, so I would want to measure at that with tubes removed.

[Helmholtz]

With all tubes removed there's no load current and all filter caps should charge to the same voltage as the reservoir cap when measured in standby.

[Justin Thomas]

As I'm not necessarily a "purist" when it comes to making old amps work reliably unless it's a "museum piece," I'd have no issue putting a couple of 47uF/4-500V caps in series in the doghouse - the reservoir cap is probably already like that & I'ddo it for the second cap too if needed. I know radial caps can be had for stupid cheap & they're much smaller, and even F&T axial caps are smaller than originals. Might require some creative construction techniques or heck I'd make a new turret board for myself because I'm weird like that...

I hate Sprague Atoms because they're ridiculously over-sized & even more ridiculously over-priced.

Jusrin

[Jon Snell]

As I'm not necessarily a "purist" when it comes to making old amps work reliably unless it's a "museum piece," I'd have no issue putting a couple of 47uF/4-500V caps in series in the doghouse - the reservoir cap is probably already like that & I'ddo it for the second cap too if needed. I know radial caps can be had for stupid cheap & they're much smaller, and even F&T axial caps are smaller than originals. Might require some creative construction techniques or heck I'd make a new turret board for myself because I'm weird like that...

I hate Sprague Atoms because they're ridiculously over-sized & even more ridiculously over-priced.

Jusrin

They are large because they last for years and years.
Small electrolytics tend to crash and burn because they cannot stand the ripple current required in older valve amplifiers.

[glebert]

As I'm not necessarily a "purist" when it comes to making old amps work reliably unless it's a "museum piece," I'd have no issue putting a couple of 47uF/4-500V caps in series in the doghouse - the reservoir cap is probably already like that & I'ddo it for the second cap too if needed. I know radial caps can be had for stupid cheap & they're much smaller, and even F&T axial caps are smaller than originals. Might require some creative construction techniques or heck I'd make a new turret board for myself because I'm weird like that...

I hate Sprague Atoms because they're ridiculously over-sized & even more ridiculously over-priced.

Jusrin

Owners on these amps definitely don't want any changes from original.

OK, I know that people have to be recapping amps of this vintage. What caps are you using for the 20uFs?

[Justin Thomas]

Normally I just use F&T 20uF@500. That's a working voltage. I'm not usually doing a bunch of testing without tubes for more than a short period of time. Had them in my 61 Concert for years, regularly used. Had about 505V on the plates & 500V on the screens. Never blew a cap or a tube or anything.

I'm not saying it's "best practice" or anything but I've also never seen a bunch of folks here saying their caps were exploding (unless they were 450V or something & assuming they used at least decent ones).

Jusrin

[The Dude]

Anything over 500V is going to be large, expensive, and hard to find. I'm with Justin. You could get a couple 47uf/350V radials and put them in series for less money and without sacrificing reliability. Also, you shouldn't have problems fitting them in the doghouse.

[DrGonz78]

It worked fine on a Musicman and Ampeg amps. But the B+ voltage on those amps were very high. Between the two caps you want, for lack of a better definition, balancing resistors to spread the voltage evenly across each cap. Are your tubes matching? Check! Caps balanced? Check!!

[g1]

They are large because they last for years and years.
Small electrolytics tend to crash and burn because they cannot stand the ripple current required in older valve amplifiers.

I believe it has been shown that they are just sticking regular radial caps in a larger shell. There may be good reasons for their use as far as appearance and physical layout, but I don't think they have any better specs than what radials are capable of. And there is a very high cost premium for the axial casing.