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    ST70 Capacitors

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    heyraz

    Posts : 58
    Join date : 2011-10-26

    ST70 Capacitors

    Post by heyraz on Wed Oct 26, 2011 10:44 am

    A few questions about capacitor selection please. (I'm new to this so please excuse if I get the terminology wrong and please correct me too, that's how I'll learn). I am building a LO GAIN VTA70v11 board.
    1. The 2 "Heater" Caps (.02 uf ceramic): I've read ceramics are noisy, is there any value in using vintage pio's if I have them?
    2. On the VTA70v11 board, what are the "upgraded" values for C13,C14,C15,C16,C17,C18? from the standard 88uf on board?
    3. C9 and C10 (cathode resister bypass capacitors?) are omitted on the VTA70v11 board. Please explain.
    Thanks in advance, I'm sure I'll have more questions as I go along.
    Rich
    PS-Do you have any info on the Sound Valves VTP-101 preamp? I've been looking for a schematic and can't find anything at all. I read an older post that mentioned how loud the VTP-100 was at it's lowest setting. My solution to that problem was to substitute a 5751 tube at V4, the effect was fantastic (I'll never go back to 12ax7 at V4). Much more dynamic, clearer, forward, subtle, everything improved. Thanks again.

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST70 Capacitors

    Post by Bob Latino on Wed Oct 26, 2011 12:16 pm

    1. The two .02 ceramic caps on the 7 lug terminal stip on lugs 5, 6 and 7 just ground the center taps of both heater legs to the chassis. These are not in the signal path. Since they only look at about 3.15 volts you can use really any .02 or .022 uF cap 50 volts or higher here.

    2. On C13 and C14 you can go up to 120 uF @ 450 volts. On C15 and C16 you can use up to 47 uF @ 450 volts. C17 and C18 are for the bias system. They are two 100 uF @ 100 volt caps. Dynaco use two 50 uF caps here. You don't need any more capacitance here .. the two 100 uF caps are plenty big enough for their intended purpose.

    3. C9 and C10 were found to be "not necessary" to the operation of the board. In fact their omission drops distortion a little and reduces gain somewhat ..

    I don't know where you may get a schematic for the Sound Valves VTP-101 preamp? Maybe a forum member may have one ?

    Bob

    tubes4hifi
    Admin

    Posts : 1261
    Join date : 2008-11-30

    Re: ST70 Capacitors

    Post by tubes4hifi on Wed Oct 26, 2011 7:56 pm

    I'm not 100% sure but I think the VTP-101 is a copy of the rare PAS3 series 2.
    That is a 3 tube phono plus 3 tube line stage (all 12AX7) powered by a SS power supply.
    I have a poor but readable copy of it if you want to take a look at it,
    email me at info@tubes4hifi.com to request

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