Suddenly my V1a plate voltage is high

Discussion in 'Shock Brother's DIY Amps' started by Jewellworks, May 23, 2020.

  1. Jewellworks

    Jewellworks TDPRI Member

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    I've had this 5F2a that I've been experimenting with, (different tone caps, adding a voltage divider and high pass caps between V1a and b) and when measuring voltages, I'm seeing my V1a plate voltage almost 30 volts higher than my V1b voltage. Which is really odd because I didn't change anything in that part of the circuit. Still 1 feed coming off the B+, and splitting between the 2, 100k plate load resistors. I checked the resistors and they're both still 100k-ish, but just for testing, i swapped the V1a resistor for another one. Same result. 30volts or so higher than V1b. I even removed the voltage divider/cap just to be sure. (even tho that's after V1a's plate, and feeding V1b's grid) - Same result.
    I can't imagine how it could be the tube itself, but there isn't anything else to check... Is there????
     
  2. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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    I guess a schematic with voltages would help. This would be the minimum:
    B+, plate voltage, bias voltage, grid voltage.

    Without more precise information we can only guess.
    Is the amp still functioning properly despite the higher plate voltage? With the current information the V1a might not be conducting at all resulting in no voltage drop at all over the plate resistor. Tube might be biased differently.
     
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  3. Jewellworks

    Jewellworks TDPRI Member

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    [QUOTE="Tom Kamphuys, post: 9857033, member: 143510" ] tube might be biased differently.[/QUOTE]

    That's it! I forgot i changed the cathode resistor from 1.5 to 2.7. I didn't think it would make that much of a difference in voltage off the plate though.
    sorry, I'm new at this and still figuring out how all these pieces parts work together.
     
    Last edited: May 24, 2020
  4. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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    Just for some closure, plotting the load lines for the two cathode resistor values (using some pretty standard values for the other parameters) seems to result in roughly a 30V difference in plate voltage (Va) between the two situations:

    1_5k.png 2_7k.png
     
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