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    using variac, pros and cons



    Posts : 3
    Join date : 2009-03-16

    using variac, pros and cons

    Post by leitmo on Mon Mar 16, 2009 6:15 am

    hi everyone!

    is my first post here,

    i'm asking myself if it could be useful or useless using a Variac with my ST-35 from dynakitparts. Lot of people are building variacs to prevent their tubes from'll be useful with dynaco / dynakits??

    thanks for listening

    Bob Latino

    Posts : 2453
    Join date : 2008-11-26
    Location : Massachusetts

    Re: using variac, pros and cons

    Post by Bob Latino on Mon Mar 16, 2009 1:36 pm


    What you might use with your ST-35 in place of the variac is some type of "soft start" device which delays the application of B+ high voltage until the tubes have warmed up. It is useful though not absolutely needed with the ST-35 power amp and the SCA-35 integrated amp. Both amps have solid state rectifiers and apply the B+ high voltage almost instantaneously. Some say that the application of high voltage before the tubes have warmed up can cause "cathode stripping" on the output tubes and reduced life on these tubes. Personally I have never seen this and the fact that there are still many ST-35's and SCA-35's out there working fine gives testament to the fact that maybe this issue is over hyped.

    You can make your own "soft start" AC outlet by wiring an outlet so that one side of the AC line has a thermistor in series with the line. The thermistor acts then as a current inrush limiter slowing the rate at which the full 120 volts AC is applied. The thermistor used should be capable of handling 5 or 6 amps continuous.

    Dynaco amps with tube rectifiers have a built in delay with the use of a GZ34 tube. This tube is indirectly heated and takes 15 seconds or more to warm up before applying high voltage.



    Posts : 3
    Join date : 2009-03-16

    Re: using variac, pros and cons

    Post by leitmo on Tue Mar 17, 2009 10:12 am

    thanks for your reply Bob!

    unfortunately i don't have SCA-35 pre, just a solid state kit. If ST-35 doen's need it i won't build it.


    Posts : 406
    Join date : 2008-12-05

    Re: using variac, pros and cons

    Post by PeterCapo on Tue Mar 17, 2009 12:44 pm

    Actually, Kevin at sells a CL90 thermistor that should be a good choice to mount right inside the Stereo 35, in series with the power transformer primary. Kevin can provide detailed installation parts and instructions.

    Variacs are useful for adjusting the voltage from your wall if it is too high or too low. I believe the Dynaco amps were okay up to 124VAC line, but beyond that you can expect problems.


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