4-400s and an al 80a

Discussion in 'Amateur Radio Amplifiers' started by KB4MNG, Jun 13, 2020.

ad: L-HROutlet
ad: l-rl
ad: L-MFJ
ad: abrind-2
ad: Left-3
ad: Left-2
ad: Subscribe
  1. KB4MNG

    KB4MNG Ham Member QRZ Page

    I had a Johnson thunderbolt several years ago and I acquired spare 4-400s. Can these be direct drop ins for the al 80 a? I understand they are very close to the 3-500. Would you risk trying an old tube in an amp? Can it damage the amp if the tube is open or shorted?

  2. W1BR

    W1BR Premium Subscriber QRZ Page

    I have tested some old 4-400A transmitter tubes that I acquired back in the 1960's while working as a transmitter engineer at WHYN in a SB-220. There are a few problems with
    the swap.

    1. The 4-400 may sit higher; make sure there is clearance to the top cover.
    2. They do not fit the socket opening fully in a SB-220. Not sure about the Ameritron desgin.
    3. Power gain will be a bit lower, you will lose a few hundred watts.
    4. All of the grid pins must be connected together since the 4-400 is a tetrode.

    Other than that, unless the tubes are gassy or shorted, they will work. They are rugged tubes.

    AF7XT likes this.
  3. KU3X

    KU3X Ham Member QRZ Page

    I had an SB 1000 which is close to a clone of the AL 80A. I installed a 4-400 in it and no problem. Like Pete said above, less gain. Also the tuned inputs may have to be tweaked? I just made a test run to see how it worked and it did. I did not mess with the tuned inputs since it was only a test run. Since it was a long time ago and my memory is not all that good, I think I got around 500 watts out with 100 watts of drive.
    Barry, KU3X
  4. W1QJ

    W1QJ Ham Member QRZ Page

    Also throw a short across the zener diode with a piece of wire. That will run the 4-400 in zero bias which it needs to be.
    KU3X likes this.
  5. W2VW

    W2VW XML Subscriber QRZ Page

    Or one can short the Zener by operating with a 1988 datecode 3-500Z.
    KB1MM, K2XT and KD2ACO like this.
  6. W1QJ

    W1QJ Ham Member QRZ Page

    What difference does a 1988 date code make?
  7. W2VW

    W2VW XML Subscriber QRZ Page

    Seems to be when production runs were in trouble.
  8. W1QJ

    W1QJ Ham Member QRZ Page

    The lady 5 years Eimac made the 3-500 90-95 was probably “the” worst!
    KD2ACO likes this.
  9. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    I remember that Carl @KM1H had a succinct timeline for what happened, but if I remember correctly, they moved the factory around the late 80's and couldn't get the pin seals to work properly after that and then gave up on glass power tubes as they became a loss leader.

    Possibly something about the Kovar and glass formulation for the seal areas...

    Last edited: Jun 15, 2020
  10. KM1H

    KM1H Ham Member QRZ Page

    Correct Chris, the move from SC to SLC was to save money and the bad tubes were 88-89. I was an authorized reseller then and customers were having seals leak just sitting for a year or less.

    Their next play to cut cost was by just spot welding the anode cap to the anode. I have one of those in the collection like the Leaning Tower of Pizza:D I actually took that tour while in the USN.....at least it was stable.
    AC0OB likes this.

Share This Page