Ameritron AL-80A Plate and Grid Current Meters go to zero while tuning

Discussion in 'Amateur Radio Amplifiers' started by N1PMA, Oct 28, 2018.

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  1. N1PMA

    N1PMA Premium Subscriber QRZ Page

    Hello all. I have acquired an Ameritron AL-80a and I am just getting around to firing her up. Unit powers up fine, set the plate and load to the proper settings per the manual and begin the tuning process. Once the unit is set to operate there is proper plate and grid current readings per the manual, manual then states to "Tune the plate control for maximum grid current", during this process both the Plate and Grid Current meters drop to zero and the plate on the tube continues to glow light orange even after the unit is unkeyed. Now I have powered the unit off and then back on after a little while and gotten the unit to present the correct numbers again but this time I just left the Plate and Load settings as they were and increased drive, this drove the amp to about 750 watts with proper Plate and Grid currents and the tube plate only glowed during transmission as is normal. Now at this point I then tried to adjust Plate or load and the meters again drop to zero and the tube plate continues to glow light orange even after the unit is unkeyed.

    All connections look solid all grounds are as they should be, no smoke, no arcing, no burning smells, nothing!

    Any ideas would be massively helpful.

    Thank you,

    Craig - N1PMA
     
  2. N1PMA

    N1PMA Premium Subscriber QRZ Page

    Also, I am noticing that when this happens (Plate and Load Meters dropping to zero) my radio (Yaesu FT-920) will show a high SWR warning.
     
  3. W1QJ

    W1QJ Ham Member QRZ Page

    Your meter safety diode is probably shorted and your grid meter is tracking your plate meter and you are not really reading grid current so you are being fooled. If you key the amp into transmit and the plate and grid meter both rise the same off the zero mark, then your meter safety diode is shorted and needs to be replaced. The diode is located on the outer edge of the rectifier board could be D17 but not sure the number off hand. The amp will still operate normally but your grid meter is not reading grid current. It will read grid current once the diode is replaced. The best way to load the amp is to adjust the load and tune for maximum output as you increase drive level. Slight over coupling of the load at the end of the procedure is recommended. Wait for Bryan to tell you how do it with a scope.
     
    W1TRY likes this.
  4. W5DHE

    W5DHE Premium Subscriber QRZ Page

    The diode number is 1N5408 Specs 1000 V 3 amp. Hope that helps you. I have the same amp is a work horse for sure.

    73 de W5DHE
     
    W1TRY likes this.
  5. W1NK

    W1NK Ham Member QRZ Page

    By any chance when your meters drop to zero, does the filament of your 3-500Z go dark? I ask because several months ago my AL-80B exhibited the same symptoms of the meters dropping to zero when transmitting and plate glowing bright orange without the amp being keyed (or spontaneously glowing when the amp was sitting idle). Turned out I had a grid to filament short.

    Just a thought... Drawing on my experience

    Frank, W1NK
     
    KD2ACO likes this.
  6. WA7PRC

    WA7PRC Ham Member QRZ Page

    It's the same as you stated...
    ...except, how much over coupling isn't a guess. :p
     
  7. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I agree, it's really not.

    For those without a scope, two wattmeters, graph paper and a pencil will allow one to graph input vs. output and see where the line starts to bend and is no longer straight. It's easy to find the "1 dB compression point."

    Graph that with everything peaked for max, then graph it again with increased loading, and a little more increased loading, etc. Only takes a few minutes.

    A scope is easier and faster.:p
     
  8. WA7PRC

    WA7PRC Ham Member QRZ Page

    ...and shows what's happening in real-time. Compared to the cost of most any ham HF amplifier, a suitable scope is cheep, too. :)
     
  9. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Today more so than ever, I think.

    I've seen you post "$1 per MHz," and that seems about right. Amazing deals out there.

    I (as a hobbyist) would never buy a "new" scope.

    At work, in our engineering lab, we buy a new scope now and then (some cost $10K or more) and I always thought that was a bit silly; but for a business, it's a write-off. For me, it isn't.:p
     
  10. WA7PRC

    WA7PRC Ham Member QRZ Page

    More specifically, a meter responds to the AVERAGE level; a scope responds to the PEAK level. You want to see the peaks.
    Ja sure. I picked up my Tek 2236 (100 MHz analog dual trace) cheep in a company auction. At another company, a Tek 2465B (400 MHz analog four trace programmable) came up DOA. The company didn't want to have it fixed, and GAVE it to me. It's on my workbench, and the 2236 is in my shack. :)
     

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