AL-80B taking out finals....

Discussion in 'Amateur Radio Amplifiers' started by AE1P, Jul 3, 2020.

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  1. AE1P

    AE1P XML Subscriber QRZ Page

    Update:

    Got the new RF-Parts 3-500zg tube today... I had already soldered in new
    10 ohm resistor and 100pf mica caps...
    Plugged in new tube...very carefully tightened anode cap...
    With dummy loads on both RF input and RF output, powered on...
    tube glowed a nice orange glow...no bangs or pops...so far, so good.

    Next step...get exciter in line... got the trusty ol TS-820 tuned up
    on 40m thru dummy load...turned carrier down to 20 watts... and
    got ready to key the amp... crossing fingers and hoping not to fry a 3rd
    transceiver...with plate and load set for 40m put AL-80b out of standby,
    and keyed the TS-820...peaked the load and plate and was getting about 300w
    out...slowly increased carrier until 60 watts, and dipped the load and plate on
    amp and got 800w... not pushing it any harder...

    With 60w drive from the TS-820, had a grid current on new tube of 140ma, and a
    plate current of 375ma... again, 800w out thru Palstar dummy load.
    So, was it the mica caps? or bad tube? or both together..??
    (definitely had a flaky mica cap)...
    Thanks to all who helped me troubleshoot...
    73,
    Neil
    AE1P
     
  2. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Did you measure any capacitor resistances while they were out of circuit?

    BTW, I think you meant 100 ohm resistor, or did you actually replace the 100 ohm resistor with a 10 ohm?

    Of all the AL-8X series I have worked on I have never come across one that had parasitic oscillations or one that damaged exciters, nor have I ever seen an SB-220 do the same, even with their convoluted OEM grid circuits.


    Pheel
     
  3. AE1P

    AE1P XML Subscriber QRZ Page

     
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Sounds like it's fixed!

    800W output at 375mA Ip (assuming B+ is 3kV) is remarkably efficient. Mine can't do that. It can make 800W output okay, but at 400-450mA Ip.

    You may have a supertube, or possibly the Ip reading is a bit off. In any case, it works!:)
     
    AE1P likes this.
  5. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    I second that.

    For comparison I recently put a new RF Parts 3-500ZG tube in my old AL-80A and with 990 Watts input (3kVX330 mA) it outputs 400 Watts on 40m for an efficiency of 40%. Now this is also assuming the meters are correct and was tube test only as I don't run that power level on AM. That is with 27 Watts carrier in so the power gain is about 12 dB, also assuming the meters are correct. ;)

    Pheel
     
    AE1P likes this.
  6. AE1P

    AE1P XML Subscriber QRZ Page

    "You may have a supertube"

    Works for me!!! I'll be sure to leave good feedback for RF-Parts... :)

    73
    Neil
    AE1P
     
    K9ASE likes this.
  7. W1QJ

    W1QJ Ham Member QRZ Page

    It was probably that faulty parasitic suppressor, the amp was taking off and if certain conditions prevail it could take out the finals in the radio. That was obviously what happened to you. There really is no other explanation and it sure fits.
     
    AE1P likes this.
  8. N3DT

    N3DT Ham Member QRZ Page

    I feel gypped. I got a Matchlett tube recently ($225 delivered?) and although I get better gain, the Ig seems to run higher. I used to run about 65W in with 700± out and 425 Ip, maybe 130 Ig. The new tube will drive out to 700± W with 55W in but the Ig is running closer to 140. Of course the higher bands have somewhat less PO. I don't like to push the 80B past about 700W, at least in continuous mode. SSB/CW is a bit easier, but why bother for the 0.5dB? Measuring with LP100A and good match to antennas. I find the PO meter on the 80B reads a bit low on some bands, like 100W.

    Glad you got it fixed. Now relax.
     
    AE1P likes this.
  9. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    My previous tube in the AL-80 was from Alpha; new tube is RF Parts 3-500ZG.

    The old tube peaked power on 40 meters; new tube peaks power on 80m.

    There must have been some differences in input and output capacitances between the tubes.


    Pheel
     
    AE1P likes this.

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