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Thread: PS Filter Capacitors

  1. #1
    Supporting Member gbono's Avatar
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    PS Filter Capacitors

    I need 20uF/600V filter caps for a Fender Concert amplifier. Digikey didn't list an axial aluminum electrolytic above 500V, Mouser was sky high pricing and forget Allied.

    Tubesandmore have Sprague Atoms for $9.86.

    Any other sources would be appreciated.......

  2. #2
    Senior Member Daver's Avatar
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    If you really need the 600V rating go to Weber Speakers I've used his caps before with no problems.
    Dave

  3. #3
    Supporting Member gbono's Avatar
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    Thanks Dave, 600V is what's on the schematic. Weber is out of stock on the 20uF but they have 40uF's.

  4. #4
    Supporting Member Dave Curtis, dB AudioTech's Avatar
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    I just checked the invoice on last one I did a few years ago (a '62); The two 20/600 caps (that I used) before the standby switch could be a single 40/600, as they're in parallel. The five others I used were all 20/500, and the amp is doing quite well. FWIW, the original caps were all rated at 525 volts. What does your B+ measure before the STBY switch with that swith open? That's the voltage the caps should be rated for. IDR what this one measured, and I don't have time to dig through the bench notes...

    (another)
    Dave

  5. #5
    Old Timer tedmich's Avatar
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    what about 2 x 47uf/450v in series with 220k 1W metal oxide equalizing resistors?

    Allelectronics has Nichicon $1 ea


    cut leads but hey..$1!

    full leads, 105C $0.79
    http://www.goldmine-elec-products.co...?number=G16918

  6. #6
    Supporting Member Dave Curtis, dB AudioTech's Avatar
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    2 47uF caps in series equal 23.5uF



    I'd just get a 47 or 50uF axial:

    High Voltage ELECTROLYTIC Capacitors - Axial Leads

  7. #7
    Old Timer
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    The typical approach is to rewire the main filters as per a AB763 Super Reverb & use either 2x100uf 350v, or 2x220uf 350v wired in series, each cap bypassed with a 220K 2W resistor.

    Be sure to remove one of the jumpers between the first 2 caps on the board, use the remaining free eyelets as B+ entry & ground points. Wire the 2x220K into a "V" shape, free ends go to ground & B+ eyelets, the point of the "V" gets a jumper wire to the junction of the 2 caps.

    B+ is typically 495v (+/-) with a reasonable current load, occasionally more. With the room vacated by the original screen supply cap, it's easy enough to wire 2x47uf 350/450v in series (bypassed with 2x220K, as per the mains) for the screen supply, with smaller, modern caps they'll usually fit between the 2 eyelets that held the old 525v cap. A dab of silicon will hold the cap bodies to the board.

  8. #8
    Supporting Member Dave Curtis, dB AudioTech's Avatar
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    Quote Originally Posted by MWJB View Post
    The typical approach is to rewire the main filters as per a AB763 Super Reverb & use either 2x100uf 350v, or 2x220uf 350v wired in series, each cap bypassed with a 220K 2W resistor..
    But if 600V caps are available, why stack a pair?
    Dimensions of High Voltage Elevtrolytic Capacitors

    110uf is going to stiffen up the power supply and change the response of the amp (new PS caps will likley change it anyway). The OP (or his customer) might not want it that stiff.

    BTW, bleeder resistors are a modern requirement (it's in the NEC), as HV power supplies are required to dissipate voltage down to safe levels (less than 60V, IIRC) in less than a minute after shutdown.

    FWIW, the 600V "Beaver" cap in a recently-serviced Sunn 200S was just 2 series caps in a cardboard tube; no balancing/bleeder resistors.

  9. #9
    Old Timer
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    "But if 600V caps are available, why stack a pair?" No reason not to use the 600v caps, but most of my customers would prefer to see recognised brands (Sprague, F&T) of cap in the pan when they pick up the amp, also gives them a comfort level if selling on e-bay etc, when chassis pics are displayed. Other customers don't give 2 hoots...

    110uf isn't exactly going to make the amp unrecognisable, most folks prefer the difference. These blonde & brown amps with all 12AX7 don't generally have as much headroom as the later BF amps, a bit more front on the note/dynamic response is usually welcome.

    The bleeder resistors primarily even out the voltage accross the 2 caps, bleeding the HV is a happy accident.

    I haven't tried the 600v caps, but when I have A/B'd 500/550v caps in parallel, compared to 2x100uf in series, the parallel caps are noticably mushier, more so than the 10uf difference might suggest (though they typically also measure 40uf in circuit). So I stick to what I know gets the results, after all it only means using 2 more resistors, a bit of wire & a couple of minutes pulling the jumper wire from under the board.

  10. #10
    Old Timer tedmich's Avatar
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    I believe most relabeled off brand tube amp specific caps rated to 600V (JJ/Ruby etc) are simply the common 450VDC caps just with a shorter rated lifetime; most EL caps will handle higher voltages just with a shorter lifetime. IME using two series 180uF CDE 381LX caps take 1"x2" of board space, handle 500-600VDC very well and last +12,000h and cost <$10 (and 90uF sounds great!)

    Or you could shell out $23 for a Vishay/Sprague Atom 600V 20uF and if you think there is a meaningful difference between 20-22-47/2 microfarads on +/-10% caps you are being a bit silly IMHO.

  11. #11
    Supporting Member Dave Curtis, dB AudioTech's Avatar
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    No reason not to use the 600v caps, but most of my customers would prefer to see recognised brands (Sprague, F&T) of cap in the pan when they pick up the amp, also gives them a comfort level if selling on e-bay etc, when chassis pics are displayed. Other customers don't give 2 hoots...

    IME, the only problems I've had with new caps have been with Atoms. Once, I thought every preamp tube I subbed in was microphonic; it turned out to be one of the 20/500 Atoms in the doghouse ('76 ProReverb). I've had leads just about fall off some of the smaller 25/25 units I've used. I'm not saying they're not good caps, I just don't think they're worth the extra $. I also give the customer the option, and more often than not, they go with the cheaper ones.

    Also, most of those 600V caps I linked to are rated at 105C.

    The bleeder resistors primarily even out the voltage accross the 2 caps, bleeding the HV is a happy accident.

    That's why they're also called balancing resistors.
    My point was that the "happy accident" is a requirement (new manufacture, of course).

    I haven't tried the 600v caps, but when I have A/B'd 500/550v caps in parallel, compared to 2x100uf in series, the parallel caps are noticably mushier

    Have you tried using snubber caps across them?

    So I stick to what I know gets the results, after all it only means using 2 more resistors, a bit of wire & a couple of minutes pulling the jumper wire from under the board.

    Go with what you know; no problem there! The OP was looking at $1 radial caps, so I figured he didn't want to spring for Atoms. I haven't had any problems (yet) with the caps I've been getting from the links I listed; yes, they are almost certainly chinese (or some other 'nese), but unfortunately, that seems to mean less and less these days.

  12. #12
    Old Timer
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    "Or you could shell out $23 for a Vishay/Sprague Atom 600V 20uF" the Sprague Atom 20uf 600v caps are too big to fit in a Fender cap pan. You could probably find a away to fit them in the main chassis, but again, when it comes to resale, it's not going to give a potential buyer as much confidence.

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