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    Quad Cap Question

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    kaner

    Posts : 88
    Join date : 2011-09-20

    Quad Cap Question

    Post by kaner on Wed Dec 21, 2011 10:49 pm

    I have a 40/40/20/20 quad cap that I would like to use with a VTA board in an ST-70 rebuild. Will it be enough? What am I giving up? Thanks for any help.

    tubes4hifi
    Admin

    Posts : 1292
    Join date : 2008-11-30

    Re: Quad Cap Question

    Post by tubes4hifi on Wed Dec 21, 2011 11:39 pm

    as long as it's rated 525v or better, that's more cap than the original 30-20-20-20 so sure, why not?!

    kaner

    Posts : 88
    Join date : 2011-09-20

    Re: Quad Cap Question

    Post by kaner on Wed Dec 21, 2011 11:58 pm

    It's rated at 525V. Thanks!

    j beede

    Posts : 328
    Join date : 2011-02-07
    Location : California

    Re: Quad Cap Question

    Post by j beede on Thu Dec 22, 2011 12:34 pm

    I think the stock ST-70 used a 30uF segment on the rectifier side of the choke... If you are using tube rectification you might check to see what the Cmax is for your particular tube. If you are running a 5AR4 or GZ-34 Cmax is usually 60uF. Several poeple on this forum seem to prefer to keep that load lower than that. I have had very good results with pre-rectifying the B+ using a pair of 1n4006 diodes ahead of the 5AR4 in my Mark IIIs. Have you tried this?

    quadaptor

    Posts : 59
    Join date : 2009-05-04

    Re: Quad Cap Question

    Post by quadaptor on Tue Dec 27, 2011 2:48 pm

    What is the proper orientation for this cap in a VAT amp? I've heard the 40/40/20/20 quad cap is coded different than the originals. Also, can somebody explain the best way to drain the electricty from a quad cap to prevent electrical shock when replacing? Thanks

    tubes4hifi
    Admin

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    Join date : 2008-11-30

    Re: Quad Cap Question

    Post by tubes4hifi on Tue Dec 27, 2011 3:06 pm

    I would connect one of the 40uf sections to the output of the rectifier (input to choke) and the other 40uf section to the output of the choke.
    You can use about a 100 ohm resistor to drain the voltage from the capacitor, do each section twice to fully remove any stored voltage.

    quadaptor

    Posts : 59
    Join date : 2009-05-04

    Re: Quad Cap Question

    Post by quadaptor on Tue Dec 27, 2011 4:06 pm

    Thanks - do you simply ground the capacitor to the chassis and then touch each post on the quad cap? I've listened to Dynaco amps for years and built one of your VTA ST-70s but I really don't consider myself an electrician. I also want to install a set of Russian capacitors in the board. Should I discharge the existing caps the same way. Again, many thanks!

    tubes4hifi
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    Re: Quad Cap Question

    Post by tubes4hifi on Wed Dec 28, 2011 12:44 pm

    yes to both questions

    burnedfingers

    Posts : 31
    Join date : 2009-05-06

    Re: Quad Cap Question

    Post by burnedfingers on Thu Dec 29, 2011 9:03 am

    quote
    If you are running a 5AR4 or GZ-34 Cmax is usually 60uF

    Yes, according to the tube specification sheets 60 mfd is maximum. Real life practice would dictate that the first cap value be lowered since the level of quality between the older rectifiers and the newer ones leaves a lot to be desired. Lowering the first cap to 40-45 mfd lowers the chance of sparking/ark over that wrecks the 5AR4. Have used a 60 mfd value in the past which resulted in several failures even with name brand rectifiers. Past experience my recommendations would be to steer away from the JJ brand as well as those made in Japan and China. Note* JJ does make some very nice triodes and I am especially fond of the 12At7 that I used in my VTA board I had a few years ago.

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