old 5F6-A filter cap ratings--replacement advice??

[gtrnstuff]

I did try a search in the forums, and read the tech pages about Fenders, 5F6-A, etc.  But I still have a question.
I recently acquired a working 1960 5F6-A chassis.  Sounds great, no obvious issues, amazingly enough.
I'm concerned about the original filter caps, so I'm going to be replacing those.
The first 4, in the can outside the chassis, are 20/600.  I found some Sprague Atoms with that rating that were too big.

With the standby switch "off", the upstream side measures 540v. "On" is 490v.
I gather that the 20/500 Spragues that would fit would be living on the edge.

Where do I go from here?
Are Weber caps OK? Looks like the rating and dimensions would match.
Thanks in advance...

[tubeswell]

Filter caps need to be rated at a voltage in excess of the B+ DC running voltage. Traditionally with electrolytic caps voltage rating, there was typically a rule-of-thumb margin-of-error of about an additional 20% (give or take) above the B+, which allowed for 'production variability' at the factory.  (Hence you see '600V' caps specified where the B+ is around 430-460). These days caps tend to be rated more conservatively, so you can get away with 500V caps where you get a B+ of 460, but you still need to watch the start-up surge, especially in the reservoir cap that happens when you flick the standby on.   If your amp is set to idle with a B+ of 460, the start-up surge could take the reservoir cap up to 490, but if your idle voltage was 480, then the surge could be  500-and-something, and whilst you might be okay, you might not. When the caps gets stressed to maximum, there is more chance of the dilectric compound breaking down, which could cause your B+ to short to ground.

If the B+ is as per the (original) Fender 5F6A schematic, then the 500V rating should be okay.

[Mars-Hall]

I found out recently that Luxe makes repro's of the old orange caps.  I think it's by special order.  You can message them through thier ebay store.  There stuff is expensive but for your amp, it would be worth it.  

Another option is TAD makes a 22uf 550v cap, I've special ordered those through Mojotone.  Check their site they may stock them by now.

Also Weber stocks a 20uf 600v cap as well.

Outside of these options you could arrange the power section caps in series, ala blackface.  You would need the balance resistors as well.  Not recommending this, only giving you all the options I'm aware of.