JTM45 build - bias too high

Discussion in 'The Workbench' started by badoogie, Aug 30, 2020.

  1. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    Again as Neikeel and I have suggested, a stable steady bias is the corner stone of the power section. If the amp can't maintain a steady unwavering bias signal with no power tubes installed, then that portion of the amp will need investigated as to why it will not hold a steady bias voltage.

    Changing tubes and trying other shotgun approaches will only add to your frustration level if there is a bias problem.

    If the bias circuit is stable over a suitable period of time, then that is the first checkmark. After proving an empty power tube bias, then add the load into the circuit, which is the power tubes. With no signal injected, again the power section needs to be stable and consistent.

    Only then can you be confident going forward.

    That oscillation that started after you saw the spark is probably something compromised by the meter tip touching two components, or a single component and ground simultaneously . It could have been a short from plate to filament. Prove your bias, then add your load, and you'll be farther along.
     
  2. badoogie

    badoogie Member

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    Thanks for the replies. :thumb: How should I check the bias circuit? Without the output valves in there is no voltage at pin 8, so where in the circuit should I be measuring? :scratch:
     
  3. neikeel

    neikeel Well-Known Member

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    negative voltage on pin 5 that stays stable in the -50 to -35v depending on where you set the bias pot.
     
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  4. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    The voltage that you typically read on pin 8 is the cathode current, actually. You read it as voltage due to reading voltage across a 1 ohm resistor. (Ohms law 1volt = 1amp x 1ohm R).

    When you read the voltage, which value is equal to amperage due to the 1 ohm resistor and the 1,1,1 law then you can figure the dissipation of the power tube. Refer to the Testing Part 3: Biasing of the Valvestorm Metro instructions, paragraph "B" and "C".
     
  5. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    The first 5 paragraphs of this excerpt from the "ValveWizard" talk about the actual bias voltage needed for several valve types. This 50 volts that Neikeel quoted can be figured from the values shown as suggestions in paragraph 3 and 4.

    http://www.valvewizard.co.uk/bias.html
     
    Last edited: Sep 14, 2020
  6. badoogie

    badoogie Member

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    Thanks guys, I took some readings (power and standby on, no output valves) and pin 5 of V4 and V5 was -53V with the bias pot fully clockwise and -40V at the other extreme. I left the amp on for ten minutes and measured again, and got the same stable readings. I also took measurements at other points in the bias pot’s range, and the voltage seemed stable throughout.
     
  7. badoogie

    badoogie Member

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    And a picture of my poor sick amp...

    884C1587-5128-407D-BFE1-164E60D4D5DE.jpeg
     
  8. Rotorcraft230

    Rotorcraft230 Well-Known Member Gold Supporting Member

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    FA699553-CCFF-49E8-8A74-B2AD7A1842F7.jpeg Here is a picture of my Valvestorm 45 build. Mine shown each of the legs of the bias pot going to each cap. If your pot is like mine I’m guessing the other leg has to be connected but beaware our pots may not be the same
     
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  9. badoogie

    badoogie Member

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    Thanks Rotorcraft230, your bias pot looks more like the one shown in the Metro instructions. Does yours have an electrical connection to the cap on the right, or is that leg just for mounting the pot?

    My bias pot is wired as a variable resistor in series with the 68K resistor. The right leg is not connected to anything.
     
  10. Rotorcraft230

    Rotorcraft230 Well-Known Member Gold Supporting Member

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    So the pot that I have you can’t see the bottom side ( kinda hard to explain but I’ll try) it has 2 legs that make up the bottom and they are connected, each one of those sides are soldered to the 2 -bias cap turrets. The top you can see. I’ll look for a picture of the pots bottom side and post If I have luck.
    Mark
     
  11. Rotorcraft230

    Rotorcraft230 Well-Known Member Gold Supporting Member

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    1B5D07EC-6D06-414E-9699-ACDFFF7734A3.png Here is what mine looks like. The 2 legs that are pointing up are bent over and soldered to the -bias pot terminals. It’s hard to see but they are both part of the lower pot which is metal
    Make sense?

    edit: not sure if your design wants both caps connected to the underside legs. If it is Metro instructions that’s how I did mine and we have a very good steady and adjustable bias control
     
  12. Rotorcraft230

    Rotorcraft230 Well-Known Member Gold Supporting Member

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    I’ll get better pics when I get home tonight say around 7
     
  13. badoogie

    badoogie Member

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    Thanks Rotorcraft230, that’s kind of you. :thumb: Both of those turrets are connected to ground - shouldn’t the bias pot just have one connection to ground?

    As the bias is stable without the output valves in, what should I check next? :scratch:
     
  14. Rotorcraft230

    Rotorcraft230 Well-Known Member Gold Supporting Member

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    If both turrets are grounded then It doesn’t matter. The Designer of the ampmust have used both legs for pot stability
     
  15. badoogie

    badoogie Member

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    As the bias is stable without the output valves in, does anyone have any suggestions as to what I should check next? :hmm:
     
  16. South Park

    South Park Well-Known Member

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    Does the bias change when you hit the stand by switch
     
  17. Chris-in-LA

    Chris-in-LA Well-Known Member

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    So, the bias is stable without the power tubes. What happens with the power tubes in?
     
  18. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Btw, re-read post #21.
     
  19. neikeel

    neikeel Well-Known Member

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    Forgot about this thread.
    Assuming bias stable
    Pin 3 voltage is 440v or thereabouts
    Pin 4 is a couple lower
    Heaters are 6.3 or thereabouts
    Then a pair good tubes.
    Only concern is that bias is not stable on one socket and you have a leaky 0.1uF coupler.
     
  20. badoogie

    badoogie Member

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    Thanks guys, I decided to check the coupling cap on V4 first, as that’s where the bias reading was going high with the output valves in. I disconnected the grid side of the 0.1uF capacitor and turned the amp on (still with the output valves removed). The disconnected end of the cap measured around 0.1VDC, although this fluctuated around half a volt in either direction. Do you think I should try replacing this cap?
     

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