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    ST-120 output transformers

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    Brinkman

    Posts : 14
    Join date : 2009-01-01

    ST-120 output transformers

    Post by Brinkman on Sat Mar 28, 2009 11:39 pm

    Hi Bob. I'm gonna throw a bunch of questions your way:

    I'm curious as to how the output transformers in your ST-120 kit compare to the outputs of the stock ST-70 (core materials, laminations, bandwidth, winding ratio, UL tap %, etc...).

    I'm also wondering if you've ever experimented with mono-blocking them Mk.III-style, and if so, could a Mk.III-compatible driver board be used? What would the requirements be for a pair of power transformers in this configuration? Could the Mk.III chassis be used?

    Sorry, but I've been wondering about this for a while now....

    Best,
    Benjamin

    Bob Latino
    Admin

    Posts : 2377
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-120 output transformers

    Post by Bob Latino on Sun Mar 29, 2009 7:00 pm

    Benjamin,

    The ST-120 output transformers (and power transformers also) are custom wound for me by a USA company. They are larger than a stock A-470 output transformer as found on the ST-70. They weigh about 2 pounds more than a stock A-470 output transformer and are 3 7/8" tall, 3 1/4" wide and 3 1/8" deep (about 1/2" taller, 3/8" wider and 1/4" deeper than a stock A-470 output transformer). They are interleaved/layer wound with 4200 ohm primaries and 40% ultralinear screen taps and have M6 silicon steel grain oriented laminations. The company that makes them tells me that the cores will not saturated unless they see about 90 watts. A stock A-470 output transformer, as used on the ST-70, will start to saturate the core at 40-45 watts. The bolt pattern on these output transformers will not fit a Mark III chassis.

    The ST-120 is basically equivalent to a pair of Dynaco Mark III's with VTA driver boards although the ST-120 runs about 25 volts higher on the plates (500 volts DC on an ST-120 vs. about 475 volts DC on the plates of a Mark III) of the output tubes than the Mark III's. The VTA board on the ST-120 also has proprietary changes to components vs. the VTA driver board on the ST-70. The circuitry of the ST-120 also has a supplementary cap module for extra B+ storage and a set of bypass capacitors whose values were worked out by trial and error with an oscilloscope. The ST-120 also has the convenience of a single chassis and triode/pentode switches.

    I have a few customers that use them as monoblocks and a few customers that biamp with them. See photo below showing a pair of ST-120's biamping a pair of fairly inefficient Magnepan MG 3.6R speakers.

    Bob



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