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    Converting Classic Hi Gain to Lo Gain


    Posts : 299
    Join date : 2010-11-10

    Converting Classic Hi Gain to Lo Gain

    Post by mantha3 on Sun Nov 21, 2010 11:21 am

    I recently purchased an ST-120 and I'm just starting the build.
    I'm a newbie to this... big time

    I was just reading a bit on the Lo VS Hi gain. My ST-120 is the Classic Hi gain I believe as it is to run the 12AT7 tube.

    I could see next Spring buying some nicer speakers 93+dB. I think the 93+dB it is better to go with Lo Gain over Classic Hi?

    I may even go with the PA3 (PAS3) build kit or the VTA PAS3-Z modification. I'd like a Pre-Amp and could see doing that as a late winter/early spring build. Given that the pre-Amp is tube based I thought I should convert the ST-120 to the Lo Gain if I go with the tube based pre-Amp?

    If you get the PAS3 kit with finished custom chassis for $499... What is the benefit of the VTA PAS3-Z 50$ upgrade? Sorry, newbie and I'm not registering it.

    So back to the main question.. What do you have to update on the PCB? I know you run 12AU7(Lo Gain) VS 12AT7(Hi Gain). I thought some resistors would be De-soldered and swapped. Can you not update the PCB and just roll tubes and put some 12AU7s in and get Lower Gain?

    PS - I did buy the 4 military-spec Russian PIO output coupling capacitors

    And sorry for having a post with 9 questions.


    Posts : 68
    Join date : 2010-02-09

    Re: Converting Classic Hi Gain to Lo Gain

    Post by xlr8 on Sun Nov 21, 2010 2:32 pm

    I was told by a guy on audiokarma the other day that the use of negative feedback alows the amp to compensate for lower mu tubes in a circuit that uses 12AT7 or AX7, so I'm curious about this as wel.
    Bob Latino

    Posts : 2639
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Converting Classic Hi Gain to Lo Gain

    Post by Bob Latino on Sun Nov 21, 2010 4:55 pm


    Put the amp together as you have it now. If in the future you do acquire some 93 dB speakers, then Email me and I can give you a list of which resistors to change to convert to the lower gain board. You will also, as you mentioned, have to use 12AU7 or 12BH7 tubes in place of the 12AT7 tubes that the classic standard gain board uses.


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    Re: Converting Classic Hi Gain to Lo Gain

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