High Gid Current

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

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  1. NE7X

    NE7X Ham Member QRZ Page

    The band shaft that has the three fiber cams on it came loose and the door knobs were not being placed into circuit. This is what was causing no RF output. Once I get this tighten and working, I will post another update.

    Thomas NE7X...
  2. W9GB

    W9GB Ham Member QRZ Page

    Time for Tom to put on his Rube Goldberg hat to reassemble that band switch :)

    The Officual Rube Goldberg web site


    Attached Files:

  3. W1QJ

    W1QJ Ham Member QRZ Page

    OH Darn, those darn lumped capacitors I talked about were not getting switched in correctly. Never liked that system. Glad you got it fixed.
  4. NE7X

    NE7X Ham Member QRZ Page

    I don't know if the original high Ig issue is resolved or not. I need to get this band switch issue resolved first to get RF output again. I purchased this amp from the guy in "pieces." He was troubleshooting the high Ig issue and could not get it resolved, and then sold the amp to me disambled, as is. The price was to cheap to pass up. I am currenting removing and re-assembling the band switch, where the three cams switch in the different door knob caps. I did test the 3-500Z tubes in another amp, so I know the tubes are good. Watch this thread in the next 24 hours, I should have the amp back together and power up.

    Thomas NE7X...
  5. W8JI

    W8JI Ham Member QRZ Page


    The blocking cap, if open, will result in high grid current and no RF output. It will NOT blow a fuse.

    If you have normal RF output and high grid current, you have a blown grid meter shunt, or you installed the wrong shunt, or there is some wiring error. It cannot be the transformer.

    It is normal to blow the zener, sometime open, when the 3-500Z's arc in that amp. that is because they do not have enough resistance in the HV path after the filter caps, so surge current can be hundreds of amps when a tube arcs.

    If you have normal RF output and the grid meter pins, double check the grid shunt R118 (0.2 ohms, NOT 2 ohms!) and the grid shunt wiring. Maybe something around the zener got wired wrong.

    Also watch D106, the protection diode.

    73 Tom
  6. NE7X

    NE7X Ham Member QRZ Page

    Little history first……

    The guy who I acquired the Henry 2K Classic from, one of the 3-500Z tubes shorted and took out the 10V zener (open). He said he replaced the tubes and the zener, then noticed the Ig meter was showing full scale (pegged). He claims this was not the case before the tube shorted. He then checked and replaced all the meter shunt resistors and .01 by-pass caps with values indicated in the schematic. This did not resolve the high Ig reading. The amplifier did put out full power, however he was concerned about running it with high Ig.

    The amp was basically disassembled when I brought it home. So I reassembled it and power it up on the workbench, driving it with an IC-7000 and a dummy load. The amp worked for a short period of time, full RF power out, Ig meter indicated full scale, and then it started arching and popping and went dead.

    I discovered two items:

    1) He removed the band switch that selects different door knob caps for each band. He never tighten the screws to the chassis, thus there was high RF voltage arching to ground. The door knobs thus were open circuit.

    2) The roller inductor was oiled and lubed with some heavy (and dirty) grease. There was arching between where the roller slides on the shaft and where the shaft fit into the end bushing and connected to the LOADING cap.

    I totally disassembled and removed “both” the band selector switch and roller inductor from the chassis, then totally disassembled the switch and roller inductor. I then cleaned all the pieces in de-greaser, buffed out all the contact points to clean bare metal with a wire brush Dremel tool, and then used silver polish to clean the roller coil. I re-assembled the roller inductor and band switch, then mounted and connected everything back inside the chassis.

    Now when I power up the amp and apply RF, I have full 1200 watts out with 100 watts drive and there is NO ARCHING anywhere, even on any of the roller inductor contacts. The amp is working solid….. However the Ig meter still shows full scale and I noticed one of the two 3-500Z tubes seems to be a bit more red on the plate (Grid?) then the other tube. I would assume both tubes (being balanced) would show equal redness on the plates.

    What Tom W8JI suggested will be my next step. I am going to connect up an external current meter and see if this external meter shows the same Ig as the internal meter.

    Two questions:

    On the roller inductor, should I add lube to all the moving electrical mechanical points or leave it dry? If lube, what kind?

    The tube plates are showing red. How can I tell if this red is coming from the plates or grids? How red is red before I should be concerned?

    Thomas NE7X…
  7. AC0H

    AC0H Ham Member QRZ Page

    Here's a pair running at full tilt boogie in a Ten-Tec Centurion amp.
    The light at the bottom are the filaments. You can see a faint red glow on the plates.

    Attached Files:

  8. NE7X

    NE7X Ham Member QRZ Page

    I installed a jack on the rear of the amplifier in series with the BIAS zener wire. I then inserted two different VOMs set to MA into the circuit. Both VOMs show show the exact same current readings which matches the meter reading on the amplifier.

    Key, no drive (idle) = 300 ma Ig
    Key, 50 watts drive = 650 ma Ig

    This tells me the meter circuit in the amplifier is showing the correct value. Full meter scale is 400ma. At 650ma, no wonder it pinned full scale.

    Any suggestions why the Ig is high? What can I check ? What is normal idle cuirrent for a pair of 3-500Z?

    Thomas NE7X...
  9. W1QJ

    W1QJ Ham Member QRZ Page

    First of all you should have 0 grid current at idle with no drive. if you are reading grid current in idle with no drive you have to have a bad tube, perhaps one with a filament to grid short. There should be absolutely no grid current with no drive. Pull one tube at a time and key the amp into transmit with no drive. if one tube is good and the other is bad, the bad one will read grid current in idle and the other would not. Simple dedective work.
  10. NE7X

    NE7X Ham Member QRZ Page

    Wtih both tubes installed, there is ~300ma of Ig, keyed, no drive. With only tube #1 installed, 90ma Ig keyed, no drive. With only tube #2 installed, 90ma Ig keyed, no drive. I then put both tubes in my other amplifier, keyed no drive, no Ig. I installed the two tubes from the other amp, keyed, no drive, 300ma Ig.

    This tells me the tubes are not the casue.

    Thomas NE7X...
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