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    ST-70 Transformer Primary Impedance 4300 Vs 3400



    Posts: 2
    Join date: 2010-08-03

    ST-70 Transformer Primary Impedance 4300 Vs 3400

    Post by CoffeeTones on Tue Aug 03, 2010 12:07 pm

    Hi, what is the effect upon sound using the different primary impedance OTs? I wish to use 6L6, KT66, KT77 and EL34L tubes in the same amp, with around 420 - 440 plate voltage and proper bias.

    Load from these tubes in AB push pull, seems to range from 3400 to 5500 ohms. Am I using the correct data?

    Which primary impedance would be best for middle ground with these output tube?

    Mathematically, 4400 looks good but I'd like some experienced advise.


    Bob Latino

    Posts: 2231
    Join date: 2008-11-26
    Location: Massachusetts

    Re: ST-70 Transformer Primary Impedance 4300 Vs 3400

    Post by Bob Latino on Tue Aug 03, 2010 7:27 pm


    The primary impedance on the original Dynaco A-470 output transformers was 4300 ohms. This works out well for EL34, 6L6, KT66, KT77, KT88 or 6550 output tubes. The original A-431 output transformer, as found on Dynaco Mark III's, also had 4300 ohm primaries.

    If the impedance is lowered slightly to 3400 ohms or raised somewhat to 5500 ohms IMHO it won't have any substantial difference in the sound of the amp.


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