High Gid Current

Discussion in 'Amateur Radio Amplifiers' started by NE7X, Jun 5, 2011.

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  1. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Normal plate idle current is about 100-200mA, but of course you can change this with the bias.

    Offhand I would think 650mA Ig would blow the grids out of the tubes, that far exceeds their max ratings under full drive condx.

    You're likely measuring cathode current, which is Ig + Ip combined. Something is miswired.

    I doubt the tubes could withstand 650mA Ig at all without vaporizing pretty quickly.
  2. NE7X

    NE7X Ham Member QRZ Page

    I checked and checked again, I do not see any wiring errors in the BIAS circuit. I physically traced every wire, end-2-end, and used an ohm meter to check connectedly. The 10V zener also checks good, I tested on variable bench DC supply, turns on right at 10V. The BIAS circuit is pretty simple, maybe 6 components total.

    Interesting point, cathode and grid current combined. The filament is the cathode. What would cause combined current? Could there be a parasitic oscillation ? If so, how would I check/isolate? All the .01 by-pass caps were replaced with new 1KW disks from the guy who I got the amp from. He was trying to fix this issue for over six months and gave up, so I pick up the amp from him.

    73s, Thomas NE7X…
  3. W1QJ

    W1QJ Ham Member QRZ Page

    OK, Does that amp have a safety diode on the B- rail? In the Ameritron amps if the diode on the B- rail goes short then the grid current meter reads the same as the plate current meter. This sounds like what is happening. CHECK THE SCHEMATIC OR CHECK TO SEE IF SOMEONE ADDED A DIODE TO THE NEGATIVE RAIL. IF THERE IS ONE, I BET THAT IS THE PROBLEM. You have to lift it to check it.
  4. W8JI

    W8JI Ham Member QRZ Page


    You are measuring the COMBINED grid and plate currents and calling it Ig. You are measuring cathode current with your test meter, NOT Ig.

    To read how a metering system works, read this link:


    You are wasting time checking the zener at this point, because you say you have a pinning grid meter. You can short the zener and the grid meter won't change significantly. One of the problems is you are calling cathode current grid current (Ig), and this makes it difficult to understand what you are really doing.

    If you go back to my post a little while ago, I pointed out what affects the grid reading in the Henry. That aside, I'm not sure what the problem really is or if it is the grid meter at all because the currents you give are clearly the cathode current and NOT the grid current.

    So which meter is pinning? Is it really the grid current setting, or is the the plate current meter that pins?

    Don't get into parasitics or any drama like that. It sounds like your amplifier has something wrong with the metering circuit. Not bias, not parasitics, not tubes. It either has a wire connected wrong, an accidental short or open in the metering circuit, or a bad component like a shunt.

    Read my web page on metering, and you will see how that circuitry works.

    73 Tom
  5. W1QJ

    W1QJ Ham Member QRZ Page

    Just for the hell of it check d106,d107 and d108. I am assuming you rewired and shunted everything correctly.
  6. W8JI

    W8JI Ham Member QRZ Page

    This is already dangerous stuff Lou. He has brought leads out of the cabinet to measure cathode current. The entire things bothers me, because the amplifier obviously has a component or wiring error. Nothing clamps the negative rail of the Henry HV supply to chassis at the supply or for a tube arc, so it would be pretty easy to get 3000 volts on the negative side of the supply (and out on the external meter) if something is mis-wired.

    Back around 1975 I was almost killed by using an external meter, and I was being careful. That was my very last use of an external meter. :)
  7. K2XT

    K2XT Premium Subscriber QRZ Page

    Hey Tom, Have you ever met Lou in person? Ever visited our spot at Dayton and drank a beer and ate peanuts in the shade of our tent? If not, why not? It has been described by some as the most fun they have the entire Dayton weekend.
  8. NE7X

    NE7X Ham Member QRZ Page

    I keeping calling it "Ig" because this is the position the front panel meter switch in.

    When I key the amp, with no drive, the meter reads 300ma. I inserted in series with the zener wire an external meter and it reads the exact same as the front panel meter, 300ma. I could be incorrect, however my assumption was nothing wrong with the front panel meter shuts since the external meter reads the exact same values as the internal meter.

    On my other two Henry amps, when I have the meter switch in the Ig position and key with no drive, the meter reads zero. So why does this amp show 300ma when the meter switch is in the Ig position keyed with no drive? After reading W8JI's web site and all the comments here, its looks like the meter is actually reading cathode current and not grid current? Am I correct?

    Tonight I will go through the meter circuit wiring harness. Since this other guy worked on this amp trying to fix this problem for over 6 months, it hard to tell what changes he may have made. So all bets are off.....

    Thomas NE7X...
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Thomas it does sound like your meter is reading Ic and not Ig.

    What does the PLATE CURRENT (Ip) meter read at the same time? I'm guessing it's going to be about the same.
  10. W1QJ

    W1QJ Ham Member QRZ Page

    Thomas, Please, take one minute and check D106. Thanks
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