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    Cathode to grnd. resistor

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    Pooch

    Posts : 53
    Join date : 2009-12-01
    Age : 66
    Location : Wa

    Cathode to grnd. resistor

    Post by Pooch on Wed May 22, 2013 2:41 pm

    One of my VTA mk4s blows the 1/2 watt 10 ohm Cathode resistor. Rectifier tube the problem? Would upping the wattage help? I have some 11 ohm 3 watts that I used in a Golden Tube SE40 that I upgraded. I'm sure these will work but will it throw my bias off by 10%? On the other hand I could stop being so cheap and get my tail down to RS for some 10 ohms;) As a side note, what effect on the sound would changing the value of this cathode resistor have? What if I went to 100 ohm? Or 1 ohm?
    Thanks, Gordy

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Cathode to grnd. resistor

    Post by Bob Latino on Wed May 22, 2013 3:28 pm

    Hi Gordy,

    There are those who say to use a relatively small (1/2 watt) bias resistor on the theory that if a tube goes "bad" that resistor will blow, cut off current flow to the tube and "save the tube". If the tube is BAD then the tube should be replaced. I use 10 ohm 2 watt bias resistors on all the VTA amp kits. A 2 watt bias resistor can withstand a "transient" issue inside a tube. Sometimes a (usually NEW) tube experiences a momentary "blip" or transient short between two elements of an output tube that are not normally connected. A 2 watt resistor can usually withstand this transient whereas a 1/2 watt resistor may blow. Many times the tube is perfectly fine after this momentary "blip".

    In your case, I would just get some 10 ohm 2 watt resistors for your two Mark IV's. Changing out this resistor will have no effect on the sound of the amp since this resistor is not directly in the signal path. This resistor is there to give each output tube a reference to chassis ground. Without this resistor in place, the tube will still light up but will not "work" without the reference to chassis ground. I would not change this resistor to a 1 or a 100 ohm resistor (although you could) because then your bias voltage would have to be different by a factor of 10 in order for each output tube to draw the required 40 milliamps of current.

    Bob

    Pooch

    Posts : 53
    Join date : 2009-12-01
    Age : 66
    Location : Wa

    Re: Cathode to grnd. resistor

    Post by Pooch on Wed May 22, 2013 6:03 pm

    Thanks for the info, Bob. I'll do as you suggested and get some 2 watts, 10 ohm. I'll run the tubes on the tester to see if they're OK.

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