The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Dynaco VTA tube amp kits, all products and all products




    Posts : 1
    Join date : 2012-07-21


    Post by jefe59 on Tue Dec 11, 2012 12:16 am

    Hello All,

    I've been working on a St-70 and have got it to work nicely with new quad cap,
    tubes, etc. The last issue I was trying to address is 60 hz hum that was a little higher than it should be. This evening I disconnected my PAS3 from the amp and then the amp was dead quiet. I reconnected the preamp and the hum returned. So the hum seems to be from the PAS3. The hum increases with volume level on the preamp. The PAS3 is mostly stock. I replaced the boards with vintage electron clone boards. The only real mod is a SDS cap board. I also added a grounded power cord with the ground attached to the power trans case. FWIW I live about 2 blocks from a very powerful FM radio tower which I can also hear slightly through the system.
    Just looking for any ideas on how to reduce the hum.

    Steve Derby
    Milwaukee WI

    Posts : 1460
    Join date : 2008-11-30


    Post by tubes4hifi on Tue Dec 11, 2012 12:55 am

    try the PAS3 with a cheater cord to break the ground connection, maybe you have a ground loop thru the system somehow.
    You could also have a ground loop if the power supply is grounded several different places. I tie ALL grounds to the output jack ground.
    If the ST70 has a two wire cord, try plugging it into the socket in the opposite direction.
    Make sure there is no "spaghetti" wiring in the preamp.
    I just worked on one a customer send me with hum, I removed two feet of wiring from it, that's all I did,
    and all the noise and hum went away. Two feet of extra wiring is alot of "antenna" for noise and hum pickup!!
    Especially since half of that was the AC power running all around and looped around the line amp board.
    The other half were all the signal inputs and outputs being too long.
    Stock Dynas also had a poor wiring scheme with no twisted pairs on any signals.
    I replace all input and output wires with tightly twisted pairs, the 2nd wire is grounded and acts as a shield.

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