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Looking For: Dual 30-30 @ 600 'Lytic for a Scott Amp

Discussion in 'Electronic Components' started by Watson A.Name - \Watt Sun, the Dark Remover\, Sep 30, 2004.

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  1. I'd like to say 'subject says it all' like so many other lamers do, but
    it's soooo much easier to just highlight it and copy and paste it into
    the body, like this: Looking For: Dual 30-30 @ 600 'Lytic for a Scott
    Amp

    A friend of mine is fixing a Scott amp and needs a dual 30 - 30 uF at
    600 VDC electrolytic, like the old Sprague or C-D twist lock aluminum
    cans common in those toob amps. (Notice I avoided that ever so trite
    'not uncommon', which should really be the old standard, 'not
    unusual').

    I have the feeling that it might be easier to clean out the old can and
    put a couple individual capacitors inside, or externally. If he were to
    use two 60 uF, 350 VDC caps in series for each section, what value
    resistor should he put across each of the two caps to balance any
    leakags? A hundred k? I see the SMPS caps, which are 220 to 470 at
    200V, typically use anywhere from a 330k down to a 150k for the bleeder
    resistor. My old Radio Amateur's Handbook shows, for three 100 uFs in
    series, a 20k, 10W across each cap. (I don't know why they need to be
    that high power, unless a cap were to fail and a lot more of the DC were
    to appear across the remaining two caps.)

    Any other solutions? Thanks.

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  2. Tim Wescott

    Tim Wescott Guest

    (450V)^2 / (20k-ohm) = 10W. Hmm. Usually the purpose of a
    low-resistance bleeder was for safety, to bring the supply voltage down
    to a safe level quicker when the power was off. Inside the can you just
    need to overcome and balance the capacitor leakage, which you can find
    from the cap datasheet.

    Antique Electronic Supply has dual-section caps going up to 525V, but
    not 600 (www.tubesandmore.com _not_ www.toobsandmore.com).
     
  3. Yeah, thanks. From my memories of a long time ago, I can't ever
    remember seeing a multisection 'lytic of more than 525V, and anything
    above 450V being very rare. I might have seen an oil filled cap in a
    large aluminum can that was chassis mounted, rated at something like 8
    uF, 600V. But that's not an electrolytic. But hey, those were the
    values that he gave me. :p
     

  4. IME, even if you can find the multisection can caps, they are either
    prohibitively expensive, or else they are prohibitively expensive and NOS.
    And getting a NOS electrolytic cap is like getting a NOS battery, or a NOS
    jug of milk.

    Looks like Antique Radio Supply has multisection cans going up to 525V.
    Gee, I wonder if he actually needs the full 600. Can he disconnect the
    power transformer and measure the unloaded AC voltage? Multiply by 1.4 and
    add 10%, for absolute worst case of overvoltage, no load, and no rectifier
    sag, I think.
     
  5. Thanks for the info. I asked him what the output tubes were, and he
    said e wasn't sure. So I'm not sure if he really wants to delve into
    this project. Like as in spend time on it other than just replace the
    capacitor.

    You know, some people look at this kind of thing as a waste of time, and
    if they can't just get it done in a few minutes with a new part, then
    it's gonna end up in the scrap metal pile. ;-/
     

  6. I just fixed up an early-60's McIntosh receiver for a friend, that needed a
    new cap or two. Tubes in the receiver section, semiconductors elsewhere.
    Beautiful sounding unit; measured better than .01% THD+N at rated power, if
    I remember! And "rated power" according to the service manual (imagine
    that, a service manual for a piece of consumer equipment!) meant continuous
    sine wave power into a 4 ohm load; I tested it, and it lived up to the
    claim. It just sat there singing away, no problem.
     
  7. Macs were notoriously conservative. I remember listening to a pair of
    Macs driving Bose 901s, and seeing the meters peak at +3, so I guess
    they were putting out a thousand watts(!)

    Odd that the receiver you worked on was solid state but with tubes in
    the receiver - must've been from the '70s. But at least it wasn't a
    room heater. I've got a Scott RS250 receiver stashed somewhere that
    someday I'll fix when I get the time and inclination.

    Well, I've strayed from the subject far enough. Maybe I should've
    xposted to rec.audio.tech or something.
     
  8. IIRC this one had supply rails around 40V. So, it could in principle dump
    250W avg sine into a 4 ohm load, per speaker, or 125 for an 8 ohm load. It
    was rated for 40W per channel (regardless of load) - obviously a rating
    based on heat dissipation ability, rather than supply capability. Don't see
    that very much these days!


    It sure looked older than '70s; I would have guessed mid 60's. A lot of
    point to point wiring with terminal strips, small PCB's here and there for
    individual subcircuits. Multi-section can electrolytics. Supply ripple
    rejection based on multistage R-C filtering, rather than regulation. No
    IC's anywhere. Carbon comp resistors, some mica capacitors. (There, see,
    we *are* talking about components!)

    And despite being solid state, it was definitely a room heater, even with no
    load. I'd guess (based on warmth) that its quiescent power consumption was
    around 30W.

    I don't remember what the output transistors were - that would have helped
    date it.
     
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