transformer shunt method biasing weirdness

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by brad347, Sep 10, 2006.

  1. brad347

    brad347 Member

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    After fixing the pro junior in the "stumped" thread, I was biasing it up and decided to use the transformer shunt method. This method is new to me. For one of the power tubes it worked smoothly... got the tube biased up to a nice coool 23 ma. When connecting my lead to pin 7 (plate) of the other EL84, however, I get this oscillation... a squealing tone that happens as soon as my probe touches pin 7 and is present whether or not the other probe is even connected to anything. I even tried connecting the other probe to the OT center tap before touching it to pin 7, same thing, as soon as my probe comes into contact with pin 7 it's squeal city.

    What the hell? Anyone care to field this one??
     
  2. John Phillips

    John Phillips Member

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    The current via the meter is either disturbing the circuit enough, or coupling electromagneticly to something else in the circuit, that it causes self-oscillation.

    DO NOT USE THIS METHOD. It can not only do this, it's dangerous, and also potentially inaccurate. It's quick, but that's about the only thing in its favor.

    Passing the current through the meter is dangerous because the meter has a very low resistance when used to measure current. If you touch something you don't mean to with one of the probes, you will discharge the full plate voltage of the amp through the meter and whatever component you accidentally touched (or to ground) - very likely destroying at least one of them. It's potentially inaccurate because it disturbs the circuit operating conditions, and even if it doesn't cause full self-oscillation, can give spurious readings (and there's also no guarantee that any self-oscillation will be audible, it might be outside the audio range, so you might not notice).

    The RIGHT way to do it is to use the meter to measure the voltage drop across the OT primary. First, with the amp turned off, measure the resistance of the OT primary (it's best to check both sides since they're not usually quite identical), then take your voltage readings with the amp on, and divide the voltage by the resistance to find the current.

    With the meter set to read voltage, its resistance is very high, so if you do touch something else you won't draw a significant current - although if the probe tip itself bridges two pins on the tube socket you're still in trouble, so it isn't totally foolproof. You're also not disturbing the circuit as much, if at all, and although it's not impossible that the presence of the meter wires will cause oscillation, it's much less likely. It's also overall the most accurate method, since it neither alters the circuit nor includes the screen current (as any form of cathode-current metering with either a bias kit or a built-in resistor does).


    Sorry for the lecture :).
     
  3. brad347

    brad347 Member

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    Thanks John. I was aware of the dangers but not the innacuracy issues. Just in case I had my wife come down to the basement and observe while I did it just in case someone had to call the ambulance ;) Luckily I didn't die :D

    Do I have to disconnect anything to use your method?
     
  4. brad347

    brad347 Member

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    oh well, she wouldn't get anything. What money we do have is all hers. God bless her.
     
  5. TheAmpNerd

    TheAmpNerd Member

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    Brad,

    One word of caution. If she is a bean counter, she could
    be looking to minimize expenses! Your untimely demize
    would put a cap on these expenses.


    After all, you ARE a musician with a house yes?
     
  6. hasserl

    hasserl Member

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    Tip, get the slip on type alligator clips for your meter probes, so you can clip one of them to one point allowing you to work one handed. Whether you clip one to ground and measure the voltage at the center tap and each leg of the primary, or clip one to the center tap and measure the voltage to each leg of the primary, do the measurement with one hand holding one probe and the other probe attached with a clip. Much safer than sticking both hands inside the live amp!
     
  7. brad347

    brad347 Member

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    :D no, no house... we rent for the time being. But expenses... yeah i'm certainly one of those! :jo
     

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