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The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


3 posters

    ST-70 quit working

    daveshel
    daveshel
    daveshel


    Posts : 169
    Join date : 2011-11-06
    Location : Tucson AZ USA
    • Post n°1

    ST-70 quit working Empty ST-70 quit working

    Post by daveshel Sun Nov 06, 2011 8:50 pm

    My (rebuilt) ST-70 is not working. The initial symptom was that it would start out OK then stop making sound after about 10 minutes. Now it has no output. All the tubes light up except the 5AR4/GZ34 rectifier tube. I looked to see if any caps or other components looked burnt or damaged and everything looks good. Seems to me the likelihood is that the rectifier tube has failed and simply needs to be replaced. Is this a typical point of failure or do I need to look further?

    Thanks,
    Dave, new guy on the forum.
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    Bob Latino
    Bob Latino
    Bob Latino
    Admin


    Posts : 3263
    Join date : 2008-11-26
    Location : Massachusetts
    • Post n°2

    ST-70 quit working Empty Re: ST-70 quit working

    Post by Bob Latino Sun Nov 06, 2011 9:24 pm

    Hi Dave,

    Yes - your GZ34 rectifier has probably gone bad. I should also mention that a GZ34 rectifier can also be bad even if it lights up. The thing to do is to check the bias voltage. If you get NO BIAS reading and no sound then usually the rectifier is bad.

    If you put in a new rectifier and that one blows pretty quickly, then probably there is an issue with the high voltage system .. Usually a partially shorted quad cap .. In that case - replace the quad cap ..

    Bob
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    daveshel
    daveshel
    daveshel


    Posts : 169
    Join date : 2011-11-06
    Location : Tucson AZ USA
    • Post n°3

    ST-70 quit working Empty Re: ST-70 quit working

    Post by daveshel Mon Nov 07, 2011 1:07 am

    Thanks, Bob.

    So if I was to measure the BIAS voltage I might avoid wasting another rectifier tube? My quad cap could be 40 years old.
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    Bob Latino
    Bob Latino
    Bob Latino
    Admin


    Posts : 3263
    Join date : 2008-11-26
    Location : Massachusetts
    • Post n°4

    ST-70 quit working Empty Re: ST-70 quit working

    Post by Bob Latino Mon Nov 07, 2011 5:27 am

    daveshel wrote:Thanks, Bob.

    So if I was to measure the BIAS voltage I might avoid wasting another rectifier tube? My quad cap could be 40 years old.

    Dave,

    If your quad cap is original it could be as old as 52 years, since the ST-70 came out in 1959. I would replace it. Kevin at Dynakitparts sells replacement quad caps for the ST-70 ..

    Bob
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    Roy Mottram
    Roy Mottram
    Roy Mottram
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    Posts : 1839
    Join date : 2008-11-30
    • Post n°5

    ST-70 quit working Empty Re: ST-70 quit working

    Post by Roy Mottram Mon Nov 07, 2011 4:57 pm

    I'd replace any quad cap over 10 years old, and I'd do the rectifier tube diode mod which I've now posted as a "sticky"
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    • Post n°6

    ST-70 quit working Empty Re: ST-70 quit working

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