"odd" cap can?

Discussion in 'Amp Tech Center' started by marshman, Nov 3, 2014.

  1. marshman

    marshman Friend of Leo's

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    I need a single, large-diameter 100uf cap can in a minimum of 450VDC for a resto project and can't seem to find one at my usual source, AES, nor do I see any at Mojo. For reference, the clamp perfectly fits the dual 100/100 and 50/50 500s from JJ that I have, but I hate to 'waste' a 100uf section if I don't need one, and as I understand it, linking a 50/50 will get me 100uf but halve the power rating, and that's no good.

    I ordered an axial F&T for the purpose but it's waaay smaller than the clamp can hang on to. Should I just order a smaller clamp?

    Any other ideas?
     
  2. muchxs

    muchxs Doctor of Teleocity

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    50uf and 50uf in parallel will give you 100uf at whatever voltage the individual caps are rated for.
     
  3. Wally

    Wally Telefied Ad Free Member

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    ^^^^^...there it is.
     
  4. marshman

    marshman Friend of Leo's

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    Ok, great, now I just gotta figger out how to do that. Thanks.
     
  5. Wally

    Wally Telefied Ad Free Member

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    Parallel....+ to +, - to -.....the two sections become one capacitor....100mfd @ the original votlage.
     
  6. marshman

    marshman Friend of Leo's

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    The old can was unmarked--just one tab painted black and one painted red, with color-coded wiring. The BL wires came from the PT, the red off the "PCB". I'm hoping to get in there with a good light and my copy of the schematic and deduce which is which from that, but it'd be nice to know the answer.

    Thanks for the help.
     
  7. BobbyZ

    BobbyZ Doctor of Teleocity

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    Or yank all the nasty cancer causing stuff out of the cap can and stick the F&T inside.
    A little RTV will hold it in place.

    Having said that it's not something I do if I don't have to. Also on the real old cans like early 60s and back they are full of roofing tar. (or something like it)

    I'd do the 50/50 parallel.
     
  8. BobbyZ

    BobbyZ Doctor of Teleocity

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    It's got two wires and two tabs? You sure there's only one cap in that can?

    Unless it has a sepret ground tab all the cans I've messed with are grounded to the can ant that's soldered to the chassis.
     
  9. Wally

    Wally Telefied Ad Free Member

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    Got a picture? As Bobby notes....it seems odd.
     
  10. marshman

    marshman Friend of Leo's

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    The original cap is metal-bodied, but wrapped in plastic shrink-wrappy sorta stuff, presumably to insulate it. It has 2 tabs, one of which has a dab of black paint on it, to which two black wires ran in different directions (one from the PT, the other towards the board), and a red-painted tab, from which a red wire ran to the board.
     
  11. marshman

    marshman Friend of Leo's

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    Ok, this is it...It's already been removed from the chassis, unfortunately, but there ya go.
     

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  12. Wally

    Wally Telefied Ad Free Member

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    That black tab is the 'negative', I would think.....sncne there are no grounding tabs on that can. You say htat there are more thanone black wire...and that 'they' go to the PT. Perhaps one of those wires is center tap going to ground...from that post to actual groudn with the other black wire??? Trace those wires...the black ones. I think you will find that they go to ground. The red post is your positive, and that is B+ that is hitting that PCB. Soemwhere you will find the rectificcation on the red side of this cap.
    IF thsiis correct, then a 50/50 in parallel will fix you up. Ground to ground...however that is made. Tie the 50mfd post together, and that is where that red B+ wire contacts.

    IF those black wires dont' do what I think they do, then ignore everything I am saying. (;^)
     
  13. marshman

    marshman Friend of Leo's

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  14. jazzguitar

    jazzguitar Tele-Afflicted

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    Plate voltage?

    Ouch!!!

    Do I see this correctly, 660 volts on the 6L6GC plates?

    Does the amp eat tubes?
     
  15. marshman

    marshman Friend of Leo's

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    I'm pretty sure that the 660 is divided between the pair, or roughly 330 per tube, which jives with the 30-ish watt output. When I bought it, it had a set of like-new RCA Black Plate 6L6GCs and a trio of also like-new Mullard 12AU7s in it...they're probably worth enough to pay for the amp and repairs.
     
  16. Wally

    Wally Telefied Ad Free Member

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    marshman, that schematic shows two 50mfd/350caps tied together ...and I am going to think that they are in series because that 350V rating is not enough for that voltage in that amp. IF so, that arrangement results in a capacitance of 25mfd/700V. This may not be your amp. OR...someone may have already replaced the original cap/s...or there might have been a change at the factory that this schematic doesn't show. You might want to make sure what amp you have and what schematic belongs with it.
    Does that cap can in your amp have only one cap spec'd out on the side? IF so, what are the specs?
    IF those RCA Blackplates have been subjected to anything except proper operation in this amp, they may not be as new as they look! (8^O FWIW, I would not waste RCA blackplates in this amp. There are more 'worthy' amps, ime. YMMV..

    Pease put a meter on those black wires and check for continuity to ground. That would help you understand what is there....better than looking at this schematic, imho...because this schematic may or may not be your amp. IF you find that that black wire does not go to ground, then you might think that this amp has been 'recapped' improperly. Does the amp work at all?

    I don't think that there is a way to 'split'' that B+.....the power tubes get their voltage from that OT priimary winding...and that voltage will be on both tubes, ime. I can't read that little schematic too well....but if that B+ is anywhere beyond 500 volts, it is stiff. 600+ VDC....that is getting way on up there. Look at the voltage on the preamps....35vdc to the plates....hardly enough to get them to work. The orignal premaps were probably in great condition.

    Have fun...
     
  17. marshman

    marshman Friend of Leo's

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    I am pretty sure that this is, at some level, a variation of my amp. There were NO50/350s, they had been subbed out with, apparently, a 22/350 and a 50/150. The work looked original, though I've no way to confirm that. I am replacing them with 50/450s. There are also 2 little disc caps that have been added to the circuit, one on top and one on the bottom of the board. I have no reason to suspect they don't belong there and as such do not intend to bother them.

    For what it's worth, I have removed a 10/50, a 22/350, a 10/350, a 50/150 and a 100/350. All replacement caps are rated for at least 450 VDC, except the 10/50, which went to a 16/50.

    Well, as near as I can decipher, there appears to be a connection of 660V to the OT and one of 330V going to the tubes. If that means there are 660VDC on my plates, I'd better have a look at that. I think I actually checked that when I bought the amp and measured something like 430 in there, but I don't recall what or where I measured that...for some reason, pin 3 is my first stop, so probably there.

    The amp did function reasonably well when I acquired it, but there was a considerable bit of hum then, as well. Being as the amp is probably early 1970s vintage, I have literally left it sitting, awaiting fresh caps and a 3-prong cord. Last months' AES Cap Sale finally got me off that proverbial fence and started working. I was hoping to use a multi-can, but I don't want to have extraneous wires running about given my lead-dress skills.

    Here's the thread I started when I first acquired this beast last year, along with some other tidbits and a few paltry gut-shots.
    http://www.tdpri.com/forum/amp-central-station/432620-nad-univox-1235-a.html
     
  18. Wally

    Wally Telefied Ad Free Member

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    Marshman, as I noted...that schematic is barely readable by these eyes on this screen. So....whatever votages are written there are fairly obscure. I took a look at that link.....maybe you should have simply resurrected that thread. IT seems that muchxs has intimate knowledge of this one......why wouldnt't he, right?
    Once again....IF that 'can' you have there is a single 100mfd, then a 50/50 can will yield a 100mfd cap...wired in parallel. There will be no more wires than are already there. There will siply be a connecting wire between the two posts in that 50/50 can to tie the two caps together in parallel.
    A meter will tell much about that circuit...without the schematic, ime. IT is fun to analyze what is there without a schematic...slower but fun.
     
  19. marshman

    marshman Friend of Leo's

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    I could've brought that thread back, but all I really wanted to know was "Why can't I find a single radial cap @100uF/450+VDC?" Not saying everything hasn't been helpful or that I don't appreciate the help, but I just had that one question, and that discussion evolved into this one. Hopefully this afternoon I'll get. A chance to go back and wire that big cap in and see if there's smoke.

    I seem to have a decent, slightly fuzzy copy of the schematic on a clipboard down on the workbench...if I can find it on one of my harddrives, and it's actually easier to read, would anyone want to see it?
     
  20. Wally

    Wally Telefied Ad Free Member

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    Have you established the ground for that cap??
     
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