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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


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    Dynaco St-70 Driver Board Resistor

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    rudes


    Posts : 61
    Join date : 2012-03-07

    Dynaco St-70 Driver Board Resistor Empty Dynaco St-70 Driver Board Resistor

    Post by rudes Fri Sep 15, 2023 3:28 pm

    Any idea what would cause the 47 ohm resistor to explode? The 7199 tube tests good.

    Dynaco St-70 Driver Board Resistor Screen11
    Bob Latino
    Bob Latino
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    Join date : 2008-11-26
    Location : Massachusetts

    Dynaco St-70 Driver Board Resistor Empty Re: Dynaco St-70 Driver Board Resistor

    Post by Bob Latino Fri Sep 15, 2023 5:07 pm

    The Dynaco ST-70 came out in 1959. Some of the amps out there now are 60+ years old. A big problem with the original Dynaco ST-70 driver board is those resistors are carbon composition resistors which have a tendency to go up in value as the years go by. This throws off the circuit parameters and can cause problems (like a blown resistor). If the board is an original Dynaco PC-3 board, I would replace the entire board. Dynakitparts sells an original type board or you could also use the VTA replacement driver board.

    Bob
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    rudes


    Posts : 61
    Join date : 2012-03-07

    Dynaco St-70 Driver Board Resistor Empty Re: Dynaco St-70 Driver Board Resistor

    Post by rudes Fri Sep 15, 2023 5:48 pm

    Hi Bob,

    Actually the board is a few years old with metal film resistors. Before I replace the resistor, I was just wondering what could of caused it to “pop” the way it did.
    Bob Latino
    Bob Latino
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    Dynaco St-70 Driver Board Resistor Empty Re: Dynaco St-70 Driver Board Resistor

    Post by Bob Latino Fri Sep 15, 2023 6:26 pm

    rudes wrote:Hi Bob,

    Actually the board is a few years old with metal film resistors. Before I replace the resistor, I was just wondering what could of caused it to “pop” the way it did.

    OK - the board is not an original Dynaco PC-3 board. Yes - replace the resistor BUT > If that resistor pops again then replace the 7199 on that channel.

    Bob
    WLT
    WLT


    Posts : 186
    Join date : 2013-07-13
    Location : Rochester NY

    Dynaco St-70 Driver Board Resistor Empty Re: Dynaco St-70 Driver Board Resistor

    Post by WLT Sat Sep 16, 2023 10:35 am

    With so little voltage amp current going thru the resistor it has a large derating factor. The 620 ohm resistor would have had a problem before the 47 ohm. However it is also in the feedback circuit. If the 1000 ohm feedback resistor is shorted it would have a very different load added to the idle current. Isolate the 1000 ohm resistor and check it. Or at least make sure it did not get shorted somehow.

    It may have been just a bad resistor.
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    rudes


    Posts : 61
    Join date : 2012-03-07

    Dynaco St-70 Driver Board Resistor Empty Re: Dynaco St-70 Driver Board Resistor

    Post by rudes Wed Sep 20, 2023 3:31 pm

    The 1K resistor checks OK. Replaced the 47 ohm resistor and amp is fine. I've never seen that resistor fail.
    WLT
    WLT


    Posts : 186
    Join date : 2013-07-13
    Location : Rochester NY

    Dynaco St-70 Driver Board Resistor Empty what type

    Post by WLT Thu Sep 21, 2023 8:55 am

    OK. Bad resistor. Normally the metal films are pretty reliable. I still prefer the carbon films.

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