Amp Pros - Blackface Filter Caps Question

cooljuk

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I did find some casual talk about this in other forums but not from anyone I know or trust, and it conflicts with what I have always been taught, so...

A Super Reverb AB763 schematic shows a rating of 525v for the three 20uF filter caps in the dog house. The schematic also shows voltages of 465v in that in that general neighborhood of the schematic.

I can't find any electrolytic caps (other than some unknowns from Hong Kong) rated at 525v+ that will physically fit (Sprague Atom has a 600v but it's 4" - too long).

All the amp-specific recap kits use 500v caps from various brands. Some photos I found of original Fender caps were Mallory caps, rated at 500v.


I'm not a total dummy and I know at least two things:

1) wall voltages have only gone up since the 60's.
2) you can go over, but not under, voltage ratings for caps.


It seems backwards to me that the common practice is to run 500v caps, here. What gives?

Thanks!
 

cooljuk

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Or an easier question might be -

Anyone know of a 550-600v 20uF-ish axial electrolytic that's not much more than 3" or so long?
 

ehb

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From what I see, the voltage is max 410vdc below the 4.7KΩ and the 1KΩ voltage divider and 465vdc above the voltage divider.... 500vdc still has a 90v/35v fudge depending on which side.... If I am seeing correctly....




1586385694098.png
 

Gary

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I use F&T caps in all my repairs and 500V are fine in the application. Modern Spragues are nothing like the originals, except for the name printed on them. With Sprague caps, you’re paying for the name not the quality.
 

cooljuk

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From what I see, the voltage is max 410vdc below the 4.7KΩ and the 1KΩ voltage divider and 465vdc above the voltage divider.... 500vdc still has a 90v/35v fudge depending on which side.... If I am seeing correctly....




View attachment 453435
I did see that, but the top of the schematic says "voltmeter values shown + or - 20%." Plus, the back of the amp says 117VAC and I've got about 125VAC mains here on the primary of the PT.

I can take a reading of those spots on the actual amp, or even have a scope track the peaks while running it.

The common practice is really to just use underrated power values on caps? It seems to be that way. I am just cautious as it goes against my general practice. I know caps are probably underrated on the wrapper but it feels funny, ya know?
 

Soul Tramp

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Cooljuk, I've run into this before. There's little you can do but go with the 500V caps. I use F&T wherever possible, then TAD, then Richey. But there are other quality brands. Be sure to use a quality cap since you're pushing the upper voltage limit. I re-capped my Bassman with 500V F&T caps.

I agree with Gary on the Spragues. Your paying for bragging rights.

Did you look at the Sprague C-SA20-600. It's 1 1/8 x 2 3/8?
 

RAG7890

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Cooljuk, I've run into this before. There's little you can do but go with the 500V caps. I use F&T wherever possible, then TAD, then Richey. But there are other quality brands. Be sure to use a quality cap since you're pushing the upper voltage limit. I re-capped my Bassman with 500V F&T caps.

I agree with Gary on the Spragues. Your paying for bragging rights.

Did you look at the Sprague C-SA20-600. It's 1 1/8 x 2 3/8?
...............3 best Cap options.

F & T

F & T



Oh.............& F & T

:cheers2:
 

Soul Tramp

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Thanks, man. Much appreciated.

Unless I have quarantine-brain (likely), or this spec sheet is wrong, C-SA20-600 is 1" X 3.875" though.


I must be the one with quarantine-brain, you are correct. I just tried to find the site where I got my dims, but can't remember where I saw them. Last night I was just googling around the web looking for an option for you. But clearly I was wrong. Sorry!

Don't know if you're interested, but if you'd like to keep your amp looking factory original you can stuff the new caps inside the old Mallory cap housings. I did this with my Bassman so it would retain it's vintage look. I have pics if you'd like to see what I did.
 

cooljuk

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Don't know if you're interested, but if you'd like to keep your amp looking factory original you can stuff the new caps inside the old Mallory cap housings. I did this with my Bassman so it would retain it's vintage look. I have pics if you'd like to see what I did.
I would love to do that! It's just the very type of nerdy / artsy project that I'm into. I've done that on Gray Tigers in guitars for customers before. Unfortunately (at least aesthetically), this amp was re-capped around the 1980s and the original casings for the Mallory electrolytics are gone.

I've looked a bit online, to see if I could find some old ones of appropriate values to gut and use but didn't come up with more than one or two. If you have some more from repair work you'd like to sell, reach out. I even thought about having some rubber stamps made and finding some appropriate diameter cardboard tube to DIY my own with modern caps inside. That's a bit much for a personal project, but if there's a market for that I would consider doing it.

But, ya, in any case I'd like to see your Bassman and how you did it. That's a level of detail that I appreciate!
 

cooljuk

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A modern Sprague dissected....

View attachment 453605

I had heard that was the case, with the Atoms being smaller inside than the case is, but never opened one up. I wonder why they do that?

Do you suppose I could open up a 600v Atom, cut the casing down to be an inch shorter, and put it back together to fit in the doghouse?
 

Soul Tramp

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I don't do amp tech work so I don't have much in the way of vintage spare parts. But I'll take a look to see what I do have.

It would take an amp tech with a sharp eye to notice the difference. I also did the electrolytics inside the chassis that are part of the bias supply.


Cap_1.jpg


Cap_3.jpg


Cap_4.jpg


Cap_6.jpg


Cap_7.jpg
 

Soul Tramp

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My Bassman was pretty much trashed on the outside, but unmolested inside. As well as the recap, I stripped and re-did the cabinet. It now looks very sharp, I think quite a bit better than the original.
 

cooljuk

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That's absolutely beautiful. So beautiful, you might leave a note on the inside of the amp explaining the date of the "cap rebuild" so some other tech down the road after you're gone doesn't see "originals" and get clipper-happy and toss them out.

I did something similar with these:



If you do come across a set of the Mallory electrolytics that would be appropriate for my 66 Super, let me know and I'll buy them. I'd love to do the same re-stuffing project.
 

Soul Tramp

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Beautiful! It's nice to know I'm not the only one wacky enough to do such things that will be hidden away and not seen to appreciate.

I don't know if it's craftsmanship, artistry, or some compulsive disorder, but it makes me smile knowing it's done to my satisfaction. It's the same reason it takes me about 50 hours to build one of my amps. And it's the same reason I put 11 coats of shellac and lacquer on my tweed cabs.

I'll check tonight to see what I have for old Mallory caps.
 


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