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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.

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