Vibroclone filter caps

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by tonehoney, Aug 29, 2005.


  1. tonehoney

    tonehoney Member

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    I am "Vibroclonig" a 1971 Bandmaster Reverb and would like to know what values of filter caps I should use.

    The original Bandmaster Reverb used (5) 20uf filter caps. In other posts, I've read that increasing the capacitance can increase the tightness of the bass, but on the website
    http://www.lynx.bc.ca/~jc/fenderAmps.html, here's what he says,



    "It is now time to discuss a mystery aspect of the power supply, Because electrolytic caps were costly way back (producing huge savings at the end of the year for a factory that cut costs right), today one can afford to replace the 70yF caps by 100uF bringing the resultant B+ capacity to a typical 50uF - this tightens up the low end a bit and gives the amp a slightly more solid feel without stiff'ing it ... installing overly large values is a bad idea as it can fry the power tranny slowly over time if streesed to high enough levels ... the preamp caps in general are not altered in value since the preamp is very sensitive to them and looses it's "dancing" quality if strapped down too hard ...

    ...namely the preamp power distribution network ... first the caps, if they're too small in value ghosting and motorboating effects may be induced - if they're too large the amp will sound really dead unless the tnwrok resistance level is high (as in Marshalls) ... in Fenders it is often a mistake to bring the cap values up from their 20-22uF values, some say 30-32uF is good too but generally speaking 50uF and higher filter caps are tone killers in amps that have otherwise the capacity to have the bias dance a little with the signal, a very subtle aspect of tube gear that is overlooked ... "

    What values should I use to get that "big bottom" without losing tone?

    Thanks,
    Jim
     
  2. tonehoney

    tonehoney Member

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    Oh, btw, I'm switching rectifiers to a 5AR4.
     
  3. TheAmpNerd

    TheAmpNerd Member

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    If you are building a clone, then use what the factory did.

    http://www.ampwares.com/ffg/

    Find your amp there.

    Brown used 16uf caps, two paralled for the mains = 32uf
    Black used 20uf caps and two seriesed 70 mains = 35uf,
    in the AB763

    I've not had any problems using up to 68uf for the
    mains and 22uf for the rest in most Fender amps
    using the GZ34.

    With very good new caps you shouldn't have any problems.
    Don't go with cheap filters. Also make sure you replace all
    the bypass caps (25uf/25V).

    *** Your bass response will be fine ***

    If need be, change the little .001uf cap from the channel
    mix resistors (220k, 220k) to .01uf or .02uf. You can do some
    other stuff too. Tweak it to taste. These are fun and simple
    amps to work with and mod, thanks to straight foreward layout
    and eyelet boards.

    The more amps I work on the more I appreciate the 50s - 70s Fenders.

    Keep the bias filter at 100uf or below, these were 25uf brown
    and 50uf black for their bias filters.

    Enjoy and have fun.
     
  4. tonehoney

    tonehoney Member

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    Thank you for your replies! For AmpNerd, where on the schematic do I find the power supply layout/values?
     
  5. tonehoney

    tonehoney Member

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    Never mind, Reading IS Fundamental!
     
  6. tonehoney

    tonehoney Member

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    For AmpNerd, where can I buy 70uf filter caps? I've looked at Antique Electronics and at Hoffman with no success. I'm using Sprague Atoms.
     
  7. RussB

    RussB low rent hobbyist Silver Supporting Member

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    Most folks use the Sprague 100uf/350v caps. There is some mention of it at the Hoffman site.

    Some even use the 80uf/450v units

    I use the 100/350's myself
     
  8. tonehoney

    tonehoney Member

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    Thanks to all regarding the cap question! I appreciate your knowledge and help.

    Jim
     

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