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Creeping Grid Current

Discussion in 'Amateur Radio Amplifiers' started by W5DM, Dec 30, 2010.

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  1. W5DM

    W5DM Ham Member QRZ Page

    Recently bought a Command Technologies Commander HF-1250 (single
    3CX800A7). When tuning up, cautiously increasing drive power until about 20 ma grid current is reached while holding down the key, the grid current will, without increasing drive power, slowly increase at an ever increasing rate--but power output and plate current do not change. Increasing loading in the output will allow more drive power to be applied--but increase drive power too much and the grid current increase wil begin again. I changed out the tube and put in what is supposed to be a good medical pull--same grid current creep. Two tubes with the same failure mode?
    I have built a few amps in my time (been a ham since 1937) but have no experience with a high gain (and fragile grid) tube like the 3CX800A7.
    FWIW the grid current meter is 100 ma full scale and, according to the schematic, shunted by a 500 ohm resistor.
    Could someone please give me a clue to what is going on?
    73 Paul W5DM
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I think the HF-1250 is no longer made, now that Palstar has acquired Commander. Is Palstar still supporting this product?

    Before taking any guesses, I'd want to know "did it always do this, since it was new?" or is this a new symptom? And if it's a new symptom, when did it occur?

    Frankly, the symptom you describe usually is a bad tube and I've seen this occur in many different amplifier designs, using different tubes; however it might be caused by something else. If you just key down without any drive at all, do you see plate current also drifting?
     
  3. KH6AQ

    KH6AQ Ham Member QRZ Page

    Does the grid current creep happen during normal operation? If not, I wouldn't worry about it.
     
  4. W8JI

    W8JI Ham Member QRZ Page

    Paul,

    I'm going to assume the metering circuit is good and they picked the right resistors.

    Was Commander smart and did they do good engineering and does the amp has a fast electronic grid current shutoff, or were they foolish and omitted something critical to protecting the tube to save a few bucks??

    I'm not sure if that amp has a good grid overload shutoff system or not, but ANY amp with a gold plated grid tube should have one. It takes literally just seconds to ruin a 3CX800 from excessive grid current.

    As for the current creep, there are two primary causes.

    The first cause is internal to some tubes. The 3CX800 has cathode material painted on in strips or stripes. The control grid is aligned so the grid wires are positioned between the stripes of cathode material. This causes the electrons to largely miss the gold plated grid, and just the electrostatic field between grid wires controls electron flow. Few of the electrons hit the grid.

    If they didn't quite get the grid perfectly aligned or if the grid/cathode alignment shifts slightly with heat, it has a profound effect on grid current. The normal shift, since the grid is centered when cold, is to move the grid toward the electron stream. This would increase current. It is possible to have one move the other way if slightly misaligned.

    I have the above effect with many 3CX800 tubes, so it is not uncommon.

    The second common cause is heating of tank components or heating of something affecting load on the amplifier like something heating in the antenna system. As a loading capacitor heats and if capacitance increases, grid current will increase. If load capacitance decreases or the plate comes out of resonance from capacitance drift, grid current will decrease.

    There are other less common causes like exciter power drift or high voltage loss, but the two causes above are most common.

    No matter what the cause, watch that grid current. If your amp foolishly does not have a positive grid trip circuit to shut it off from excessive grid current, I would install one. I'm not sure about yours, but some amps saved a few bucks but put the PA tube at risk. That was OK when 3CX800's cost $180 wholesale, but not today.

    I've blown up 3CX800's in about 45 seconds or so accumulated time when running excessive grid current, and the damage is cumulative over time!! Be careful because a little extra current here and there will eat away at that gold plating on the grid and eventually poison the tube.

    73 Tom
     
  5. W5DM

    W5DM Ham Member QRZ Page

    My thanks to all who replied. An off-forum communication suggested the possibilty of the transceiver's output rising as the heating caused by holding the key down shifted some component values in the transceiver.
    Haven't yet decided on a course of action but, based on some information on one of W8JI's web pages, am leaning towards getting an amplifier which uses a tube with a more robust grid and also requires a higher driving power.
    73 Paul W5DM
     
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    That would certainly be an unusual explanation and cause. With a solid state rig, I've never seen that in my life, having used virtually everything ever manufactured commercially.

    With a tube rig: Maybe. Can't say I've seen this, but I can see how it could happen.

    What transceiver are you using?
     
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