3-500Z Home Brew problem almost solved and I need ideas.

Discussion in 'Amateur Radio Amplifiers' started by KM6CND, Jul 21, 2018.

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  1. KM6CND

    KM6CND Ham Member QRZ Page

    Thanks Pete

    Here's what I do:
    1) Tune antenna to desired frequency with my Rig Expert AA-54
    2) Tune the radio to the antenna
    3) Check SWR in CW mode

    All this is done with the amp on bypass

    4) Amp off bypass and key up. I turn the top "Tune" dial till the forward power peaks. I do the same for the Load monitoring the meter till it peaks.

    Correct? Not correct?
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page


    I wouldn't do the DYMO labels thing (ugly, and can leave sticky residue when removed), but do make a small "tuning chart" for dial settings vs. frequency -- and remember that can all change if you change antennas.

    But assuming you don't change antennas, the chart can be used to pre-set the dials and then just tune up at full power -- you'll be "very close," and the tuning should take just a few seconds.

    Remember the dial settings (tuning indications) will change not just with the band, but with frequency within each band. On a "wide" band like 80 meters, you'd likely need 3-4 pre-sets, maybe one for CW, one for "low phone," one for "mid-phone" and one for "hi phone" or something like that. On 40m, maybe 2-3 pre-sets; on 20m, probably 2-3; etc. If you always use the same part of each band and never work CW or RTTY, and just stick with a narrow section of each phone band, just one pre-set per band would probably work.

    3-500Zs don't need much cooling. The only parts that really need cooling are the filament pins (under the socket) and the plate pin seal (on top), but that one usually has a dissipating heatsink as the connector so only needs a small trickle of air. The big glass envelopes themselves don't seem to benefit from cooling as far as I can tell. But power supply components like filter capacitors and bleeder resistors can usually benefit from some air movement.
  3. KM6CND

    KM6CND Ham Member QRZ Page

    I do like the way the builder used these small fans for cooling, rather than some seriously over-sized ones that just blow air all over the place. It's also not seen in any photos, but the cover is punched aluminium screen so the top and sides are in open air.
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    If you're going to use the amp, "tuning the radio to the antenna" normally wouldn't accomplish anything.

    With the amp activated and drive applied, the TUNE control would normally be adjusted for a forward power peak and plate current dip, which should occur at the same tuning point. The LOAD control should also be adjusted for a forward power peak (and this can impact the TUNE setting, these two are slightly interactive), but over 50+ years of doing this, I've found by far the most important thing to watch for final adjustments is GRID CURRENT. You can tune the LOAD for "maximum output power" and at exactly that point, grid current can be excessive because the amp may still be underloaded. Advancing the LOAD (reducing LOAD capacitance) will reduce grid current -- sometimes a lot -- while having very little impact on output power.

    For a 3-500Z amp, I keep grid current <150 mA per tube (150 mA for one tube, 300 mA for two tubes), and make sure it stays there by proper LOADing. That helps assure both good linearity and also long tube life.

    With some tube types (3CX800A7s, 887X series, 8122s and many other power tetrodes...) GRID current (or with tetrodes, both grid and screen current) are way more important than plate current; and tuning up by watching only output power (or output power and plate current, both) can destroy tubes very quickly.
    K2XT, AF7XT and W1BR like this.
  5. AF7XT

    AF7XT Ham Member QRZ Page

    NCL-2000 ...

    5. Press the PLATE-OFF switch to PLATE. The red PLATE lamp will light and the multimeter should read 2000 volts.

    6. Press the CW-SSB switch to SSB. The multimeter should now read 3000 volts. No plate current will be indicated on the plate meter since the output tubes are cut off when the exciter is in the receive mode.

    7. Press the PLATE-OFF switch to OFF, and put exciter in transmit mode. Adjust exciter for zero output in transmit mode, preferably by depressing push-to-talk switch with microphone gain backed clown, or with the NCX-3, by placing the function switch in the CW position and depressing the key with the carrier completely balanced out.

    8. Return exciter to receive mode, and press the NCL-2000 PLATE-OFF switch to PLATE with the CW-SSB switch still in the SSB position. Put exciter in transmit mode with zero output as above. You should hear the NCL-2000 antenna transfer relay click, and idling plate current should now be indicated on the plate meter.

    9. Adjust the rear panel BIAS ADJUST control for 225-250 ma. idling plate current.

    10. Return exciter to receive mode, press CW-SSB switch to CW, and press PLATE-OFF switch to OFF. Check for proper straight through operation of the driver transceiver or transmitter-receiver. This completes the preliminary checks and adjustment of the NCL-2000.





    1. Set Controls as in paragraph 10 above.

    2. The exciter, whether transceiver or transmitter-receiver combination, should now operate conventionally as a low power station when either the NCL-2000 primary power or plate power switch is off, except when the MULTIMETER switch is in the EXCITER TUNE position or when no separate antenna transfer relay is used with a transmitter-receiver combination as noted in paragraph 2.10.

    3. Place MULTIMETER switch in EXCITER TUNE position (PLATE-OFF switch still OFF).

    Place the exciter in the transmit mode. You should hear the NCL--2000 antenna transfer relay click, indicating that the exciter output has been switched from the antenna to the 100 watt input resistor in the NCL-2000 which will now act as a dummy load providing optimum exciter adjustment.

    4. Place the exciter in the transmit mode and adjust it for normal CW loading and output as indicated on the NCL-2000 multimeter, which is reading 0-50 ma. grid current in the EXCITER TUNE position. At least 15 ma. should be indicated. If not, check for proper exciter operation or, if necessary, adjust the tap on the NCL2000 input resistor per paragraph 5.7.

    5. Adjust the exciter so that CW output may be varied continuously from zero to full output by insertion of more or less carrier in the CW mode in equipment such as the NCX-3, or by whatever means is available. It may be found easiest in some equipment to detune an. early transmitter stage.

    6. Note carefully that the NCL-2000 screen current will be your most valuable tuning and loading indication during the following steps. The object of the tune-up procedure, as you will see, will be to strike the proper balance between plate current and screen current for maximum output. Excessive screen current (above 25 ma.) indicates insufficient loading, and in addition, the screen current will act as an extremely accurate indication of plate circuit tuning by reading maximum at resonance. The NCL-2000 is so designed that maximum efficient output may be accurately determined in every instance by proper adjustment of plate and screen.

    The above posted because I have not contributed to any sort of thread drift in , wait , 20 min or more ....
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I've tuned up many a tetrode amp by watching ONLY screen current and power output.

    Most of these external anode tubes can take tons of plate current and I've actually never seen one fail due to excessive plate dissipation, provided the fins were clean and the blower was working properly. I've seen hundreds fail from excessive grid or screen dissipation, and watching for a plate current dip will not do anything to protect the tubes.:p

    Even 8560As in paging transmitters (when they were common years ago) ran "unattended" with nobody watching them or any meters for years, and would go for years and years at full power, full carrier, until "something" caused the grid or screen current to soar; then they lasted another five seconds and were history. I've rebuilt several of those (they make great 2m FM repeater amps) that were many years old and the tubes were completely shot, but always due to a grid failure. They could run out of resonance for years at substantially more plate current than "rated," and as long as they didn't get full of dust or have a blower failure...they were fine.:) I suspect 8122s are in the same category.
  7. KM6CND

    KM6CND Ham Member QRZ Page

    Thank you all for helping me figure this out. In review, I may been the "Fool on the Hill", but the problem was a large chain of small issues that together, caused the amp to fail.

    The more we learn, the more we can pass on to others, who we hope, will not graduate from the School of Hard Knocks!


    The final touch was to find this USB, LED lamp at the 99 cent store. I can finally see the meters when the Shack is dark :)
  8. AI3V

    AI3V Ham Member QRZ Page

    You would ignore plate current in a tetrode?

    All screen grid current is telling you is the instantaneous plate voltage is lower than the screen, and now the screen is the plate.

    If your only watching screen current, better hope the anode circuit never goes open.....

    AC0OB likes this.
  9. AI3V

    AI3V Ham Member QRZ Page

    No matter how many times you post this Steve, it'll still be wrong.

    The number one killer of tubes is heat.

    Heat that comes from not knowing how to tune a amplifier for correct plate current.

    If your plate current is correct,

    And your output power is correct,

    Because you checked drive vs output power, and you are in the linear region of your tube.

    Then the only possible reason grid current would be to high is if your tube was defective.

    As it's been explained to you several times, the reverse, tuning for a specific grid current, does not tell you the above.

    I'm sorry you have been tuning by the poke-and-hope method for 50 years, it's never to late to do things correctly.

    AC0OB likes this.
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    And I'm sorry you're wrong.

    I also don't care, so keep on believing whatever you wish.:)

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