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    Quad cap... DOH.!! -and questions..


    Posts : 71
    Join date : 2010-09-03

    Quad cap... DOH.!! -and questions..

    Post by Dingojazz on Fri Mar 16, 2018 11:30 pm

    See ST 70 posting below, 'won't bias'

    ... Well, come to find out, when I did the VTA rebuild 5 yrs ago, I installed the quad cap to match the original.. Evil or Very Mad

    Per Bob's post; ACTUALLY, with replacement cap, the 80 uf tab becomes "1", 40 uf "2"... etc.  Criminy!  

    So, three questions;  First, how has this amp worked (apparently quite well) for over 5 years..!?!?
    Second; can I assume this can cap now toast, and place order a new one?  
    Third; Is anything else now damaged?

    Thanks, bom

    Bob Latino
    Bob Latino

    Posts : 2745
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Quad cap... DOH.!! -and questions..

    Post by Bob Latino on Sat Mar 17, 2018 8:48 am

    Dynaco use a 30, 20, 20, 20 uF cap and (for some reason ?) used the quad cap numbering as used in the left side photo for the four sections of the quad cap. The #2 section was the 30 section and sections 1, 3 an 4 were all 20 uF.

    The VTA ST-70 and ST-120 both use an 80, 40, 30, 20 uF quad cap. (right side photo) The 40 section is # 1, the 80 section is #2, the 20 section is #3 and the 30 section is #4. The 80, 40, 30, 20 is marked 1, 2, 3, 4 as shown in the photo because that is the sequence DIRECTION that the DC voltage travels. The DC from pin #8 of the rectifier tube sends the DC FIRST to #1 which is the 40 section, then to # 2 which is the 80 section, then #3 which is the 20 section, then #4 which is the 30 section. The 40 section is placed FIRST because you don't want the rectifier to work into a large value cap first thing. Note that the quad cap will only measure each value "correctly" when the cap is out of the circuit (no wires or resistors connected). If you measure the quad cap values in circuit, all sections will measure higher because of bleed over through the choke, resistor(s) and jumper wire. Lastly, do NOT do capacitance measurements with the amp ON. The quad cap on the Dynaco and VTA ST-70 amps has about 435 - 450 VDC. You will either smoke the fuse in your capacitance meter or smoke the meter itself.

    Lastly, to answer Dingojazz's question > You can probably wire the quad cap differently than what is shown in the two photos and the amp will still "work" - it just won't be an optimum wiring.


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