5C3 deluxe biasing

Discussion in 'Amp Tech Center' started by capohk, Aug 7, 2020.

  1. capohk

    capohk TDPRI Member

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    Checking bias on this amp for the first time - the OT resistance for the two windings is quite different. Does this indicate a problem in the OT windings?

    [​IMG]
     
  2. Wally

    Wally Telefied Ad Free Member

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    No. It is rare to find an OT with closely matched resistances; and I have seen a much wider spread than that.
    As for the results of your measurements, I do not use this method and would have to swap positions of those two 6V6s just to assuage my curiosity as to whether or not the disparity followed the tubes????
     
  3. Jon Snell

    Jon Snell Tele-Meister

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    The primary DC resistance is always different between one side and the other. The AC resistance or impedance is the same though.
    If you want to set the bias current, place a 10R 2W resistor in the cathode path to ground. That will give you an accurate reading.

    Just measure across the 250R resistor.
     
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  4. dan40

    dan40 Tele-Afflicted

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    Also be sure that your plate voltage measurement used for the bias calculation is measured from pin 3 (plate) to pin 8 (cathode). Just use the second calculator on this page and plug in the numbers that it asks for.

    https://tedweber.com/bias-calc/
     
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  5. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    It *is* interesting that more current seems to flow on the OT side with higher resistance. I like Wally's idea to swap the tubes and see what happens -- if the current draw evens out or the imbalance goes the other way, you'll know that the tubes aren't well matched. No big deal since this amp is running nice and cool anyway.

    One other thing to check is the voltage on each 6V6 grid. It should be around 0, but if one is drifting positive, that tube or socket may be dirty between the pins or internally gassy, causing the tube to draw a little extra current. I'm curious to hear more. :)
     
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  6. capohk

    capohk TDPRI Member

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    I looked at this again today - did the "Tube Dissipation Using Cathode Resistor Voltage Drop" part of Rob Robinette's bias calculator and got some readings that look about right. Re-measured a bunch of stuff based on your advice.

    [​IMG]

    [​IMG]

    Seems biased cool? The tubes are a matched pair of JJs so not surprised they end up being pretty close. Sounds great, so I guess I'm done. Thanks for all your comments! As always, such a great place to ask questions.
     
  7. Jon Snell

    Jon Snell Tele-Meister

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    Still easier to measure across a 10R resistor.
     
  8. Wally

    Wally Telefied Ad Free Member

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    Capohk, you have some cold tubes there.
     
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  9. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Good man, I'd stop there too. Your B+ is only a few volts shy of what's on the 5B3/5C3 layout, and those showed a cathode voltage of 18V, for a dissipation of just under 12W each.

    I doubt that time spent trying to make it run hotter would make it sound any better than when they were on Leo's bench. And your tubes will live long happy lives. :cool:
     
  10. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    The B+ seems a little low. Please provide the heater voltage. What is the voltage at the wall as well?

    How does the 5C3 sound with the cold bias?
     
  11. capohk

    capohk TDPRI Member

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    Coming back to this. wall voltage is ~223V. Using a step-down transformer 220>110. Heater voltage is 6.0VAC

    B+ is 338V, Plate voltage measured from my bias probe is 314V, plate current is 26.8mA

    Rob's bias calc gives this as 65% plate dissipation which is pretty cold for cathode bias. I've put the original 1954 RCA tubes back in. Unfortunately, I've blown the P12R speaker so my only other speakers are a Weber 12A150 and a C12150 - a bit different from the original one so sound comparisons are a bit difficult.
     
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