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Biasing cathode vs plate

Discussion in 'Amp Central Station' started by Matthewdormer, Apr 16, 2021.

  1. Matthewdormer

    Matthewdormer TDPRI Member

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    So I posted a thread asking about how to bias my amp myself and in the end I purchased a small probe that reads through a dmm. I have been able to get readings for a voltage (416v) and a current (sitting at ~21). I have a drri with groove tube 6v6’s.

    I have read somewhere that with the tool I have (it is a 1ohm resistor) that I am reading the cathode current, but apparently to bias I should be worrying about the plate current. But also I have read that there maybe isn’t a big difference in 6v6 between plate current and cathode current.

    Can anyone clear this up for me before I can start using my amp comfortably. It is currently set up at around 21mA at 416v assuming the tube can handle 14w. This would mean my current dissipation is at 62%. But I am not sure if that is accurate with my uncertainties in what I am measuring. Here is a photo of the tool and rough instructions:
    https://ibb.co/Nrrs9Lv
    https://ibb.co/C1Lg9gX
     
  2. 2L man

    2L man Tele-Meister

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    If your amp has screen resistors you can measure voltages over them and calculate screen current and deduct it. If there is one resistor you need to measure/calculate all pre amp current and decuct it and divide by power tube count. You can also look tube datasheet where often are few examples and compare your voltages to it and you get an idea what screen current is. But most of users just trust what amp builder has set them. Or deduct estimate 5% out of cathode current.

    Often you find that matched pair of tubes have more bias difference than screen current is and when amp sounds good you just use them because it does not matter much on instrument amps.
     
  3. Paul G.

    Paul G. Friend of Leo's

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    Unlike launching a rocket to Mars where accuracy is paramount, biasing an amp is somewhat nebulous. There is no exactly correct bias. Depending on load, input voltages, frequency and many other factors, tubes operate over a range. Setting the bias just determines the ballpark for safe operation with lowest unpleasant crossover distortion. The difference between the cathode current and plate current will be in the range of 3-5%. If you bias to a safe operating range that should not make much of a difference.

    I bias AB push-pull to about 60% MPD, then play and see what I hear. I may go up, I may go down, but as long as I'm in safe operating range (<70%) I'm happy. Don't forget, you may be plugging into wildly different voltages in the seedy club than at home.

    My point? It's fine to use cathode current, as long as you stay within normal operating range.
     
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