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    Square waves

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    ramon68

    Posts : 87
    Join date : 2009-04-12
    Age : 76
    Location : naples fl

    Square waves

    Post by ramon68 on Fri Jun 17, 2011 5:40 pm

    There is a pictorial of a Stereo 70 build shown on both Bob's and Roy's websites. There is a picture of a 10khz square wave from an old MkIII and a new ST120.
    The difference is dramatic. What accounts for it, wider bandwidth OT, better power supply, better driver stage? Would a new MkIII do any better against a ST120?
    I'm thinking about MkIIIs (M70s actually), or a ST120.

    Bob Latino
    Admin

    Posts : 2415
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Square waves

    Post by Bob Latino on Fri Jun 17, 2011 8:09 pm

    The better square waves on the VTA ST-120 are mostly attributable to the better driver circuit and wider bandwidth on the VTA ST-120. The original Dynaco Mark III used a single 6AN8 pentode/triode driver tube. This single driver tube design was done by Dynaco as a economy measure so that only one driver tube could be used. IMHO you really need a "tube and a half" (THREE triode sections) to really create a driver circuit that is both wideband and has equal drive (is linear) at all drive levels. The ST-120 has a very linear drive system that even at full power maintains both wide bandwidth and low phase shift. Below is a chart done of the ST-70's bandwidth for both channels at FULL POWER done by an ST-70 kit builder who has the proper test equipment. The ST-120's bandwidth is pretty much the same. (but actually slightly wider).

    Left channel at full power



    Right channel at full power



    The full VTA ST-70 test link is below ..

    VTA ST-70 test link

    Bob

    JunkyJan

    Posts : 103
    Join date : 2008-12-09
    Location : BC, Canada

    Re: Square waves

    Post by JunkyJan on Sun Jun 19, 2011 6:18 pm

    ramon68 wrote:There is a pictorial of a Stereo 70 build shown on both Bob's and Roy's websites. There is a picture of a 10khz square wave from an old MkIII and a new ST120.
    The difference is dramatic. What accounts for it, wider bandwidth OT, better power supply, better driver stage? Would a new MkIII do any better against a ST120?
    I'm thinking about MkIIIs (M70s actually), or a ST120.
    You can build a set of Mark IIIs to the same specs, Ramon68 - here is a link to the VTA driver board version for the Mark III: http://www.tubes4hifi.com/MK3.htm

    I have gone that route by building two Mark III amps using the VTA drivers ( http://dynacotubeaudio.forumotion.com/t514-dynaco-mark-iii-build ), however up-front you need to know that it is not as cost-effective as the single chassis ST-120 approach though. You do gain a bit in physical separation by going with monoblocks - you have to decide for yourself which option you can live with (without causing Wife Aggro too much ;-) ).

    -- JunkyJan


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