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Thread: Reeves Custom 50 PS (Hiwatt Custom 50 clone) EL34 Tube Bias Adjustment

  1. #1
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    Reeves Custom 50 PS (Hiwatt Custom 50 clone) EL34 Tube Bias Adjustment

    Hi there,
    I am new to this bias adjustment concept. I read a few websites and followed two pages of instructions by Reeves Amplification.
    Unfortunately Bill at Reeves mentions after finding out the plate voltage (pin 3 to ground) -> 499 volt, I use the bias calc website to find out the amount let's say at 70 percent (35 mA) and measure pin 8 to ground but when I asked him the pin 8 to ground will read voltage not current, he didn't give me a clear answer.
    This amp has a power scaling (PS) so at least amount it is supposedly amp bypass, and at highest amount (dialled clockwise) it is at 100 percent (no power scaling).
    There is a 1 ohm resistor between pin 1 and pin 8. Yesterday I realized the voltage across the 1 ohm resistor gets to a high point around 8 or 9 o'clock (at 10 to 20 percent power scaling) but when I turn the power scaling further clockwise (less scaling and more of the real power), the voltage drops from let's say 35 or 40 toward 12, 10, 7, ... and at max PS setting (no power scaling) it actually gradually drops below 7 toward 5, etc. I was surprised. Today I realized I'm supposed to measure the current to the ground, not necessarily pin 1. Is it possible that pin 1 is not grounded, therefore the charge from the plate, cathode, etc. influences the charge on pin 1 (suppressor grid) and that causes the drop in the current between pin 1 and pin 8?
    If so, in order to measure the cathode current, can I ground pin 1 with a piece of wire with alligator clips and do the measurement afterwards? I don't understand why Bill said I can just hook on the multimeter from pin 8 to the ground. How does the volt measurement from pin 8 to ground correlates with the cathode current to the ground?
    Thanks for helping me out to clarify this.
    Cheers,
    Arya

  2. #2
    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by arbous65 View Post
    I don't understand why Bill said I can just hook on the multimeter from pin 8 to the ground. How does the volt measurement from pin 8 to ground correlates with the cathode current to the ground?
    There's a 1 ohm current sensing resistor between pin 8 and ground. Every millivolt you measure on this resistor corresponds to one milliamp of current.

    Basic electric formula, Ohm's law: V = I*R. Voltage equals the product (multiply) of current times resistance. Since the resistance is 1 ohm that makes the arithmetic super simple.

    Hopes this helps. The rest of the Reeves, including power scaling, is beyond my understanding.

  3. #3
    g1
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    Check the wiring and make sure pin 1 is connected to ground. Double check with your meter set to ohms. Pin8 should measure 1 ohm to ground, pin1 should measure 0 ohms to ground.
    Quote Originally Posted by Justin Thomas View Post
    In my ideal world, I'm not too loud - your room is too small!

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    Hi there,
    Sorry I didn't pay attention. Pin 1 is indeed connected to the ground. The voltage across the 1 ohm resistor was falling down as I turned up the power scaling (voltage increase to the plate) and it was really odd so I thought the suppressor grid is saturating. Then I realized the power scaling acts weird when bias is set too low. When I maxed power scaling (full power) and adjusted the bias to 35 millivolt, then it became stable and reduction in power had a proportional relationship with the cathode current i.e. potential across the 1 ohm resistor.
    Thanks. It's all good now.
    g1 and Leo_Gnardo like this.

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by arbous65 View Post
    Thanks. It's all good now.
    Only 2 posts and you already collect a "like." Life is good when you pay attention to directions!

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