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. W7UJ

    W7UJ XML Subscriber QRZ Page

    I just noticed that my Henry amp's meters suddenly seem to be showing nothing on the plate current meter and shows high grid on the multimeter. Still putting out power but somethindg seems a miss. Ideas?

    Ed W7UJ
  2. KE4AMQ

    KE4AMQ Ham Member QRZ Page

    tube going bad
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    "Nothing" for plate current can't be a tube going bad. If it's still putting out power, there is plate current.

    Something went wrong with the metering, if the amp is putting out considerable power.

    If it's putting out very little power, then first thing I'd check is the cathode fuse (F101 I think it is in the 3K/5K but I didn't look lately). Should be about a 3A fuse; if that blows, then there won't be any plate current or any plate current indication but LOTS of grid current and output should be either "nothing" or very little (just whatever leaks through).
  4. W7UJ

    W7UJ XML Subscriber QRZ Page

  5. W7UJ

    W7UJ XML Subscriber QRZ Page

    I thought it was a metering problem as well but I came across this in the manual: NO PLATE CURRENT WITH EXCESSIVE GRID CURRENT - This is a sure indication of a break in the high voltage line between the power suppy and the tube. You MUST unplug the amplifier from the AC line and trace the circuit with an ohmeter to find the break. I guessing this might be what is going on???
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    That would normally produce no output, or very little output, nothing like what the amplifier can normally provide.

    What kind of output power do you indicate?

    Did you check the CATHODE FUSE which is in a fuseholder on the rear panel?

    If that opens, it's the same as having no plate voltage and you'll have the same symptoms.
  7. W7UJ

    W7UJ XML Subscriber QRZ Page

    Seems like the output power is normal , the cathode fuse blew last August and I replaced and seemed to be putting out plenty of power, so I thought it was all good. I hadn't used it since then and the other day I went to use it on 20 cw and saw no idling plate current and the grid was way high on the meter. I usually try to keep the grid under 400ma. I don't want to destroy the tubes with high grid.
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    "Seems like the output power is normal..."

    What does it measure? The 5K can easily output 1500W with a lot of room to spare. Is it doing that?
  9. W7UJ

    W7UJ XML Subscriber QRZ Page

    Ok just checked it 20w input = 507 watts, 70w input =10015 watts (seems low for that amount of drive) Grid meter almost pegged.
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I think you meant 1015W, not 10015W...but if so that's about 12 dB gain (70W drive) which seems about right.

    Check the back to back diodes across the plate current meter...if one is shorted, the meter can't read anything but the amp will still work fine.

    Can you get the grid current to go down if you increase the LOADing (rotate LOAD control)? If the LOAD control does not dramatically impact grid current, something's wrong -- because it should.
  11. K6CLS

    K6CLS Ham Member QRZ Page

    ... Any idea why the fuse blew, twice?
  12. 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.
  13. 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.
  14. K6CLS

    K6CLS Ham Member QRZ Page

    ahhh, ok. I misread. but I still wonder about blowing the fuse once.
  15. 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

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