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Thread: 5c3 Biasing question

  1. #1
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    5c3 Biasing question

    I converted a bogen phono amp into a fender 5c3 deluxe. I used the origenal power and output transformer, tube sockets and gutted everthing else .I have 416v at the plate and 33v at the cathode with a 600 ohn resitor . I started with a 250 ohm cathode resistor ano ended up at 600ohm and can only get the plate dissipation down to 11 watts. I went through an old set of GE 6V6s already( nice sizzling sound after a while of playing) . Should i pick up a pair of JJ 6V6s tubes?

    any sudgestions welcome
    Baddog

  2. #2
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    Doing the math by the numbers you have there,I get 55ma's across that 600 ohm resistor.Now if you are measuring your plate volts to ground you have 22 watts,or if you subtract your cathode volts you get 21watts.I think maybe your GE tubes that fried had a short or some serious leakage before you put them in the amp and that is why they sizzled.I would try a fresh set of tubes and see what numbers you come up with.That 600ohm cathode resistor should run a good pair of 6V6 tubes in a much safer,if not somewhat coldish range.

  3. #3
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    Im going to pick up some JJ6V6s tubes because they can handle a higher plate voltage. How did you do the math by the numbers ? Acording to the Jim Jones Bias Table a 6V6 can only handle 12 watts .


    Here is what i did after checking the amp again with different( used ) tubes

    416v Plate
    - 34v Cathode
    382v ---------


    34v cathode
    Divided by 600 ohm resistor
    .056 ma

    .056 ma
    devided by 2
    = .028 ma

    .028 ma
    x 382v
    10.8 watts per tube

    Is this wright ?

    Thanks for the advice

    Baddog

  4. #4
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    I Notced when i start the amp up and check the voltages wright away i have a voltage reading on the plates of the 6V6 's of 500v that slowly drops down until the amp settles at 400ish volts. I only have 450v e-caps in the amp and was wondering if a standby switch would solve the surge problem and save my e-caps or should i buy 600v caps?

    this amp had a power indicator light with a resistor that went to the one side filliment voltage . I removed the resistor and now my readings on the plate of both 6v6's are the same. I have the amp biased at 9 watts per 6v6 tube and i think it sounds ok.

    The sizzling wasnt coming from the power tubes i think it may be a bad preamp tube . I also changed the the input signal to ground 5meg resistor on channel one to a 1 meg and it eliminated the problem on that channel. I did notice that when i turn the 1meg pot for channel one i get a squeek if i tork on the nob at all. Is this a tube microphonics problem ?

    Any thoughts or sudgestions welcome
    Thanks
    Baddog

  5. #5
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    Sorry,I forgot to divide the 55ma's by 2.That is why I prefer to measure the current across the OT directly.Just use an ammeter connected to one plate and the CT of the OT,much more precise.Your GE's were likely on the way out,a good NOS pair of GE's should certainly handle what you have there.I wouldnt be concerned about the 500v on start-up,the caps should be able to handle the temporary over voltage as they charge up and the tubes warm up till they draw full current.

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