Drake T4-X resting plate current creeps up

Discussion in '"Boat Anchor" & Classic Equipment' started by K1APJ, Aug 10, 2019.

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  1. K1APJ

    K1APJ XML Subscriber QRZ Page

    Just picked up a decent T-4X. Works with reasonable output on all bands, but the resting plate current creeps up, even when the unit sits idle with no drive. I can think of a few things that can cause that, wondering if there is a common issue. Transmitter and PS have been re-capped. Yes, I am lazy, have not made any measurements yet, just wondering if this is a frequent issue with a known solution.

    Thanks again,
     
  2. K2XT

    K2XT Premium Subscriber QRZ Page

    I am not aware of any, but that matters not.
    Take to bottom cover off and check the bias voltage on the pins of the finals to see if it is drifting down. Then the screen and the plate.
    If the bias is changing it could be a leaky capacitor from the plate of the driver.
     
  3. K1APJ

    K1APJ XML Subscriber QRZ Page

    Yes, that makes sense. Thank you for your input.
     
  4. K2XT

    K2XT Premium Subscriber QRZ Page

    Please come back with voltage readings when you get them.
     
  5. WA1GXC

    WA1GXC Ham Member QRZ Page



    First guess is that one or both of the 6JB6's well on their way out. Replace in pairs Always discharge B+


    When I receive new T-4X [ ] I always check the 15 ohm cathode and 68 ohm screen resistors, clue as to whether the tubes as received

    have had a difficult life If they've drifted high and/or not equally, the answer is always, Yes.


    G L
     
  6. K1APJ

    K1APJ XML Subscriber QRZ Page

    I spent a few minutes on this tonight. Control grid bias is -54 and rock solid, doesn't move at all. Screens are about 235, didn't check the plates.

    I started to suspect the meter shunt, so I set up a power supply to send current through the whole cathode circuit. The 3.3 ohm shunt is indeed high, about 4 ohms, so the meter is reading quite a bit high. Cathode resistors are a little unbalanced but not bad. I did not check the screen resistors.

    First order of business is to fix the metering, while I'm at it I will replace the cathode and screen resistors with closely matched pairs.

    I get better than 100 watts out even on 10 meters, so I did not figure that the tubes are completely hopeless, but once I am sure the rest of the circuit is correct I may spring for new bottles.
     
  7. K2XT

    K2XT Premium Subscriber QRZ Page

    You definitely don't need new finals. That is outstanding.
     
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The T4X is very old but idling current drift was not a known problem.

    It's actually so old, it's one of the Drakes I never had! Must date to the mid-late 60s.

    Indeed, resistors drifting over time and temperature will create various issues.
     
  9. K1APJ

    K1APJ XML Subscriber QRZ Page

    Yes, at this point I suspect that the metering circuit itself is drifting, but I will change the cathode and screen resistors as well.

    Thanks to all. Will post updates here, should be able to get some resistors tomorrow.
     
  10. WA5VGO

    WA5VGO Ham Member QRZ Page

    Gassy 6JB6’s can cause the current to creep upward.
     

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