Henry 5k appears to be showing no plate current and shows high grid

Discussion in 'Amateur Radio Amplifiers' started by W7UJ, Nov 25, 2021.

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  1. K6CLS

    K6CLS Ham Member QRZ Page

    ... Any idea why the fuse blew, twice?
  2. W7UJ

    W7UJ XML Subscriber QRZ Page

    Yes 1015 watts. The load control has very little impact on the grid current. It just moves a little bit about a half a number on the scale. So from .630 to .600.
  3. W7UJ

    W7UJ XML Subscriber QRZ Page

    It only blew once in August... I didnt use it till last week and then I noticed the issues on the meters.
  4. K6CLS

    K6CLS Ham Member QRZ Page

    ahhh, ok. I misread. but I still wonder about blowing the fuse once.
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    To quote my old Chinese friend, Sum Din Wong, it sounds like the amp has a problem besides the plate current meter not functioning.

    If you drive the amp with 70W, peak plate tuning and loading for max output, and then increase loading, the grid current should go down and the output power reduce slightly. And 70W drive should not produce anything like 600 mA grid current.

    Of course, possibly the metering is pretty screwed up and the indicated grid current is not correct. There are several resistors that function to make both the plate and grid current meters read properly and if any of them have drifted off value that can impact meter readings. Henry amps were not known for this problem, but anything is possible.

    Re the plate current meter not reading anything, two possibilities that spring to mind are a shorted diode (there are two of them across the meter, in parallel with the meter terminals) which would let the amp work fine but kill the meter readings; or possibly a blown meter which may have failed when the cathode fuse blew because to blow a 3A cathode fuse indicates "something arched" either in one of the tubes or somewhere in the plate compartment downstream from the meter -- if that fault was considerably >3A, it may have caused one of the protection diodes to blow, or even damage the meter itself.

    I'd check both.
    Last edited: Dec 1, 2021
  6. W7UJ

    W7UJ XML Subscriber QRZ Page

    Thanks Steve for your help, I ll get someone to check it out and let you know what I find.

    Thanks again Ed W7UJ
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    It would be good to know what caused a 3A cathode fuse to blow, even once.

    Usually an arc. If that's just dust in the RF cage, or exploding a bug or something it's no big deal...but if it's an ion arc inside one of the tubes, that can mean the tube is failing and should be replaced.

    3CX1200A7s are extremely rugged tubes and I've never actually seen one fail, but anything's possible.
  8. K1LKP

    K1LKP Platinum Subscriber Platinum Subscriber QRZ Page

    Last edited: Dec 2, 2021

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