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    ST-70 / Mark III power transformer heat (AGAIN)



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

    ST-70 / Mark III power transformer heat (AGAIN)

    Post by JunkyJan on Mon Jan 10, 2011 3:55 am

    Good day all

    I have lately been giving my Mark IIIs a break by running my ST-70 again. I have noticed that my ST-70 power transformer runs hotter than those in my Mark III amps - not much, but noticeably so. The ST-70 is on a top shelf same as was my Mark III amps "looking at the ceiling" as Bob puts it (sounds like Making Whoopee but never mind!).

    I have fairly sensitive hands, brought on by 20+ years of software development - I can not touch my ST-70 power transformer for any extended period, it is too hot.

    So... I went "researching" on this subject on the World-Wide Web and came up with the following (from various sources):

    1. Many tube amplifier power transformers tend to run hot - some were actually DESIGNED to run for extended periods at temperatures around 100 deg. Celsius (that's 212 Fahrenheit for you ex-Colonials south of the Canadian border Wink )

    2. The heat is not caused by "bad transformer design" as some people out there on the WWW think, but is merely an unavoidable consequence of a transformer having to deliver several B+ voltages of 300 - 450 volts, and at the same time also driving high-current tube heaters

    3. Some famous classic amplifiers, for instance the Marantz 5 or one of the earlier McIntosh amps (can't quite remember which) ran just as hot as the average ST-70 or Mark III power transformer

    4. Some amplifier power transformers don't take too kindly at being cooled down (forced-air cooling etc) as it is contrary to the design engineer's parameters - obviously it won't damage it but core temperature was taken into account when the transformer was designed, so a cooled-down transformer is probably not running exactly at its design specs... (for the non-engineers, electrical conductors will change resistance in relation to temperature - carbon-based conductors decreasing with rising temperature, but most metallics including copper will increase)

    Do anyone care to comment about these observations? I'm afraid it is so many years since I left engineering school that I have forgotten the maths involved in calculation Q and efficiency in transformers (been in software far too long, actually), not that I really care Smile - the above does seem logical / plausible to me.

    -- JunkyJan, BC Canada

    Bob Latino

    Posts : 2454
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-70 / Mark III power transformer heat (AGAIN)

    Post by Bob Latino on Mon Jan 10, 2011 7:24 am

    Hi Jan,

    An ST-70 power transformer will always run warmer than a Mark III power transformer because ..

    1. The ST-70 is driving TWO channels instead of one channel
    2. The VTA ST-70 has the heat from eight tubes in the surrounding area while the VTA Mark III has the heat from only five tubes.

    The original ST-70 had a stack lamination of about 1 1/2 to 1 5/8". With limited "iron" in the stack, all ORIGINAL ST-70's ran pretty warm. Even after 40 to 50 years, thousands of original ST-70's are still running fine. Your VTA ST-70 has a higher stack (about 2.1 inches), thicker gauge wires and runs cooler than any original ST-70.



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

    Re: ST-70 / Mark III power transformer heat (AGAIN)

    Post by JunkyJan on Mon Jan 10, 2011 6:56 pm

    Hi Bob

    I think you may be missing my point... The fact that my ST-70 runs slightly warmer than my Mark IIIs doesn't bother me in the least - I'm surprised that it doesn't run even hotter actually. The reason for my post was that I am curious as to forum members' opinions regarding my observations as to the causes of power transformer heat build-up in tube amplifiers in general.

    I think us "modern types" who grew up with transistors have a feeling of discomfort having a power tranny running at what we THINK of as a high temperature. An electrical engineer friend of mine was over a few weeks ago and said that "Surely something must be wrong - are you sure there is no short-circuit somewhere?" after examining my Mark IIIs.

    -- 'Jan

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    Re: ST-70 / Mark III power transformer heat (AGAIN)

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