Amplifier Plate Current Meter Reading Low

Discussion in 'Amateur Radio Amplifiers' started by KC0VVB, May 21, 2020.

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  1. KC0VVB

    KC0VVB QRZ Member

    I heard some snapping and popping from the RF deck before the plate meter reading dropped during tune up from 400ma to 200ma. the output has not changed still 700w pep on am

    During tune up testing a few days before the plate meter reading was 400 ma on the plate and 140 ma on the grid for 700w out pep.

    The amplifier plate curent reading now is 200ma on the plate 100ma on the grid at 2800v with 20w drive from the ranger. The peak reading watt meter shows 700w Peak on am.

    So the plate current meter is reading half of what it should. The resistance of the meter is still .01 ohm.
    none of the capacitors are shorted.

    The plate curent meter is actualy measuring cathode current between the negative rail of the power supply and the bias relay leading to the center tap of the filament transformer. The only items on this circuit are; the bias relay, various capacitors, and the diodes in the safety crowbars dp1 and dp2.

    None of the fuses blew.
    I am not seeing any fire, smoke or burned components that would lead me to the problem.
    Any ideas would be greatly appreciated.
    Erik kcovvb
  2. KC0VVB

    KC0VVB QRZ Member

  3. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    1. You could have stated the type of unit in the title.
    2. Are you measuring carrier power with no modulation or what?
    3. Does your meter and dummy load have sufficient capacity for this Amp?
    4. I don't see any meter voltage protection devices "across" any of your meters.

    1. Discharge the power supply again and blow out the dust bunnies.
    2. Check the Neutralization capacitor very closely with a bright light and a magnifying lens, unless you removed it.

    Last edited: May 23, 2020
  4. KC0VVB

    KC0VVB QRZ Member

    Meter protection devices are Dp2 and Dp1 on the power supply negative rail B- and auxilary negative rail B1- Two diodes with a bypass cap Cp1 and Cp2.

    I removed the neutralizing capacitor because im running 3-400a eimac in grounded grid.
    I am measuring Peak to Peak AM output.
    The dummy load is an mfj peak reading watt meter rated for over 1500w.
  5. KC0VVB

    KC0VVB QRZ Member

    The type of unit is a Johnson viking thunderbolt running 2 4-400a in grounded grid with a modernized solid state power supply.
    The used tubes were pulled from a Drake 4LB that also put out 700w peak into a dummy load.
  6. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    I am well aware of this Amp and I can read schematics very well thank you.

    Is there some reason you didn't ask this question over at

    What I said was: 4. I don't see any meter voltage protection devices across any of your meters.

    I was referring to devices placed directly "across" the meter terminals such as bi-polar Transient Voltage Suppressors (TVS), or back-to-back Zeners.

    If a high voltage transient occurred across B- and B1- check for a failed Cp1 or Cp2 or both. I find that some failed caps will show a light spot on one or more sides of the cap. A failed or leaky Cp2 could shunt some meter current to ground.

    The MFJ 9982 has a 50 Ohm non inductive resistor rated at 100W/10 min., 1.5kW/10 sec. so Keydowns would have to be very short.

    Use the average power metering mode for measuring non modulated carriers on short Keydowns.

    If no component failures are found then chalk it up to a fuzzball or an insect. Did you use compressed air or a vacuum to remove any foreign critters?


    Last edited: May 24, 2020
  7. KC0VVB

    KC0VVB QRZ Member

    Thank you Pheel;
    It is good to hear from a gentleman and a scholar. I appreciate your response. That assists me in narrowing down the things i would need to check. Cp1 or Cp2 don't seem to be shorted. This amp was built by me from the schematics of an engineer who designed and built one over 40 years ago K5HU. He was asking questions on QRZ to help him with the meters. I am more of an assembly line technician than an electrical engineer. So blow off the rf deck with compressed air switch to cw and see if i can get around 400ma on the plate and 120ma on the grid. Fine business. I didn't add tvs or back to back zeners across the meters because it was not in the schematic. Tom w8ji was concerned that zeners and diodes of to high a value would make the b- and b1- too hard to clamp and carry 100amps of b+ during a tube failure.
  8. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Yes, as I said, I am well aware of this modified Tbolt Amp and recall K5HU's thread and discussion at:

    If you are convinced the meter shunts are sufficient for protection then so be it, but this is your amp, not W8JI's nor K5HU's.

    You will only know if the Cp1 and Cp2 are leaky if one terminal is lifted and the resistance is measured.
    I have seen these caps fail in later AL-80's and Al-82's. These caps were specifically speced with a low voltage to fail at about > = ~ 75 volts.

    One nit on nomenclature. Dp1 and Dp2 are not crowbars in the real sense of crowbars. A crowbar circuit is usually implemented with SCR's or IGBT's to slam a circuit to ground in case of a fault. These diodes simply "conduct" any transients to ground when those transients are of the correct polarity.


  9. KC0VVB

    KC0VVB QRZ Member

    Thank you Pheel.
  10. KC0VVB

    KC0VVB QRZ Member

    I have re assembled the amp and put it through its paces once again on 80 meters. The meters are working correctly.
    I have been having an issue with the ranger that has been burning the coupling cap and the plate choke as well as pegging the plate meter
    while hooked to the amplifier.
    After some repairs to the ranger i was able to determine it is working perfectly when not hooked to the amplifier.

    After further testing with the amplifier it seemed that one of the capacitor positions of the load coupling switch is faulty the output drops to zero. Position 3 which usually corresponds to 80 meters but moving it to position 4 power comes up again. I suspect a shorted capacitor.

    I had some fire works in my home brew attenuator the resistors glowed bright red when not transmitting I may also have a failed plate coupling cap in the amplifier . I have ordered Replacement parts and will return the amplifier plate coupling cap to original.

    The output connector of the amplifier was fused to the coax conductor It seems to have taken some heat.

    I have been hearing some sizzling from under the rf deck when the amplifier is on not relay or rf related. Possibly the rf choke on the cathodes.

    I plan to do a Voltage and capacitance test on the output when my capacitance tester and capacitors for the plate arrive in the post.

    Erik KC0VVB

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