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    VTA / MKIII bias resistor

    wgallupe
    wgallupe

    Posts : 138
    Join date : 2014-05-18
    Age : 66
    Location : Central Mass.

    VTA / MKIII bias resistor Empty VTA / MKIII bias resistor

    Post by wgallupe on Fri Sep 25, 2020 4:33 pm

    Hi, Looking for some help and guidance...

    One of my MkIII monoblock amps made a loud 'pop' and then went silent. After some basic diagnostics (power tube swap, changed the rectifier) I found that the V1 power tube socket would not bias correctly with either of the two tubes. Voltage readings got up to around 45vdc!!! The corresponding 10 Ohm bias resistor looks fine (not burnt) but measures zero! V2 resistor measures fine at 10 ohms. Couple questions:

    What causes the resistor to fail?

    Did the high bias voltage damage the power tube or anything else?

    What type and wattage is the resistor and where can I buy some replacements?

    Thanks in advance,
    Wayne
    Bob Latino
    Bob Latino
    Admin

    Posts : 2959
    Join date : 2008-11-26
    Location : Massachusetts

    VTA / MKIII bias resistor Empty Re: VTA / MKIII bias resistor

    Post by Bob Latino on Fri Sep 25, 2020 5:17 pm

    wgallupe wrote:Hi, Looking for some help and guidance...

    One of my MkIII monoblock amps made a loud 'pop' and then went silent. After some basic diagnostics (power tube swap, changed the rectifier) I found that the V1 power tube socket would not bias correctly with either of the two tubes. Voltage readings got up to around 45vdc!!! The corresponding 10 Ohm bias resistor looks fine (not burnt) but measures zero! V2 resistor measures fine at 10 ohms. Couple questions:

    What causes the resistor to fail?

    Did the high bias voltage damage the power tube or anything else?

    What type and wattage is the resistor and where can I buy some replacements?

    Thanks in advance,
    Wayne

    Hi Wayne,

    If the tube in that socket has a problem, then it can take out that 10 ohm resistor. Even though that resistor blew, it is not the fault of the resistor. I would just replace the 10 ohm resistor and replace the tube that was in that socket and you should be OK.

    Bob
    wgallupe
    wgallupe

    Posts : 138
    Join date : 2014-05-18
    Age : 66
    Location : Central Mass.

    VTA / MKIII bias resistor Empty Re: VTA / MKIII bias resistor

    Post by wgallupe on Sat Sep 26, 2020 6:43 am

    Thanks for the help Bob. I believe its a 2 watt resistor. Does it matter which type? ie: metal film, metal oxide, etc. I found some 10 ohm 2 watt 1% tolerance metal film on Amazon...

    Bob Latino
    Bob Latino
    Admin

    Posts : 2959
    Join date : 2008-11-26
    Location : Massachusetts

    VTA / MKIII bias resistor Empty Re: VTA / MKIII bias resistor

    Post by Bob Latino on Sat Sep 26, 2020 8:01 am

    wgallupe wrote:Thanks for the help Bob. I believe its a 2 watt resistor. Does it matter which type? ie: metal film, metal oxide, etc. I found some 10 ohm 2 watt 1% tolerance metal film on Amazon...


    Yes - A 10 ohm 2 watt 1% resistor is appropriate here .. It does not matter if it is a metal film. metal oxide or carbon film resistor.

    Bob
    peterh
    peterh

    Posts : 1434
    Join date : 2012-12-25
    Location : gothenburg, sweden

    VTA / MKIII bias resistor Empty Re: VTA / MKIII bias resistor

    Post by peterh on Sat Sep 26, 2020 2:12 pm

    Metal film will evaporate faster while the "composition" resistors ( where the full mass consists of
    resistive material) will take some time to get warm, and considerable longer time to burn off.
    It's this burning off where the resistor transforms to an isolator that protects the output
    transformer when a tube decides to flash-over. A fuze will also take longer time to react
    then a "burnable resistor"

    In fact even a 1w metal film would do just fine.
    wgallupe
    wgallupe

    Posts : 138
    Join date : 2014-05-18
    Age : 66
    Location : Central Mass.

    VTA / MKIII bias resistor Empty Re: VTA / MKIII bias resistor

    Post by wgallupe on Thu Oct 01, 2020 2:50 pm

    Just following up for others to have as future reference...

    I followed Bob's advice above and all is back to normal.

    Thanks again to Bob and peterh!!!

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