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Sound City L50 Bias Still Too Low

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  • #1

    Sound City L50 Bias Still Too Low

    I built a voltage doubler in a Sound City 50 and it still only gives me around -40V with the individual 10k pots all the way up. I used two 100uf caps with single diode as described in the Pittman book diagram.

    Does anyone have any suggestions on something different to try as far as values, or a different doubler layout? The Bias winding is getting about 30VAC measured across disconnected and I'm not sure what it should be stock. Everything else is schematic values as far as bias goes, 100k to a 10k off the wipers of the individual 10k pots...

    -Travis
    http://www.facebook.com/abbottamps
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  • #2
    It doesn't seem like your voltage doubler is doing its job; they are necessary with these amps / modern tubes.
    I'll check my copy of the book; what page is the circuit on?
    Whatever, I can't think how voltage doubling could be achieved with only 1 diode.
    Suggest you try the Fig 2 circuit in http://en.wikipedia.org/wiki/Voltage_doubler
    Pete.
    My band:- http://www.youtube.com/user/RedwingBand

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    • #3
      This is what I used for the voltage doubler on the 120 sound city.
      here's the link to the thread: http://music-electronics-forum.com/t...ght=sound+city
      Attached Files

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      • #4
        I was misguided in the Pittman book. The diagram for Sound City's and Hiwatt's is on page 156... but it shows the use of only one diode, which I thought was kind of strange considering all the other voltage doublers I've looked at... guess I should have looked at this one more.

        I reworked it for a second diode and it's all good now, voltage doubled. Without the second diode, the winding was going directly to the negative side of the series caps...

        Thanks for the help!
        http://www.facebook.com/abbottamps

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        • #5
          Originally posted by pontiacpete View Post
          This is what I used for the voltage doubler on the 120 sound city.
          here's the link to the thread: http://music-electronics-forum.com/t...ght=sound+city
          Would it be possible to repost the image that was attached to this post? It doesn't come up anymore. Thanks in advance!

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          • #6
            Sorry redcobra but I took that off of another thread. And I forgot what how the repair went.
            I guess the thing to do is search for the thread that had the original picture and ask there.
            I'll look around here, do you have a schematic that I can look at, maybe I can remember.
            I know it was a drawing of a voltage double.

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