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    st70 build help for a novice....myself

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    dynafan

    Posts : 1
    Join date : 2012-12-31

    st70 build help for a novice....myself

    Post by dynafan on Mon Dec 31, 2012 1:47 pm

    Question i've been toying with starting a build from scratch on the st70 chassis. the only components i have collected so far are the chassis, one pa060 transformer and two a-431 transformers. i'm looking for help in picking out the right board and other related parts. the only thing i've done at this point is to mount all the transformers.

    stewdan

    Posts : 161
    Join date : 2010-03-07
    Age : 78
    Location : Houston Texas

    Re: st70 build help for a novice....myself

    Post by stewdan on Mon Dec 31, 2012 3:10 pm

    Hi Dynafan --

    The PA060 is the "normal" power transformer for a ST70. However A-431 output transformers are usually found on the Dynaco Mark III 60 Watt Monaural Amplifier and can drive 4 ohm, 8ohm and 16ohm speakers.

    The ST70 uses 6CA7/EL34 Tubes, the Mark III uses 6550/KT88 Tubes and the original Mark II used 6CA7/EL34 Tubes. The Mark III and II use similar output transformers (A431 and A430), which only differ in the speaker impedance that they can drive. If your speakers are 8 Ohms or 16 ohms then you can use either A431 or A430.

    So, first I would suggest going to the Tubes4HiFi website and downloading the Build Manuals for the ST70, the Mark III and the Mark II amplifiers, since I am not sure of what eaxactly you are trying to build, an exact copy or a newer clone or a hybrid of some sort.

    Then you can compare the circuit diagrams and parts lists and determine what you need in the way of Dynaco "stardard" parts.

    Once you arrive at your choices, you can choose from the original Dynaco PCB or a Modern Version of same or one of the newer designs like the VTA board or the Triode Board or Curcio Board, etc. There are customer comments at the Tubes4HiFi Website about the VTA Board. I have found usefull comments about the Triode and Curcio boards at other audio forums.

    Hope this has been some help -- keep asking questions and keep us posted on your progress.

    Stew


    Bob Latino
    Admin

    Posts : 2413
    Join date : 2008-11-26
    Location : Massachusetts

    Re: st70 build help for a novice....myself

    Post by Bob Latino on Mon Dec 31, 2012 4:34 pm

    dynafan wrote: Question i've been toying with starting a build from scratch on the st70 chassis. the only components i have collected so far are the chassis, one pa060 transformer and two a-431 transformers. i'm looking for help in picking out the right board and other related parts. the only thing i've done at this point is to mount all the transformers.

    The A-431 transformers may be used on an ST-70 rebuild but the A-431 output transformers will not fit the stock spacing of the output transformer mount holes on an ST-70 chassis. You will have to drill eight 9/32" holes (4 new holes for each output transformer) on the stock chassis to mount these (larger) A-431 output transformers.

    Bob

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