Yet another L4B rescue

Discussion in 'Amateur Radio Amplifiers' started by W9JSW, Apr 13, 2018.

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  1. W9JSW

    W9JSW Ham Member QRZ Page

    I bought a Drake L4B sans power supply last week. It was reported to be living it's last decade or so in a SK's garage. It is in pretty good shape overall. Just a bit of rust on the covers. Inside looks good and relatively clean. It has two unknown quality 3-500 tubes in chimneys (coleman globes, not the eimac ones). I think the guy I bought it from probably cleaned it up.

    I was told by the guy I bought it from that it needs to be powered on carefully to not damage the tubes since they have sat for so long.

    I am planning to build a power supply. I plan to use a ~3KV transformer at .8-1A, probably with a WD7S power supply board. I don't really want to build it out with a smaller transformer and a doubler circuit like the oem (unobtanium) or a substitute SB-220 transformer. I will wait for the right one to come along,

    I plan to change it to zero bias and ground the grids. I also plan to add a soft start.

    The question is, does it help at all if I use the 1kv transformer that I have now to power it up and run it at very reduced power. This transformer can do 1250V at about 280ma. Will that be enough power to allow me to condition the tubes and perhaps run it? That would be around 350W of input power capability. This will primarily be an AM amp.

    I was mainly wanting to verify most of the operational aspects of the amp as well - filament trans, blower, meters, lights, etc.

    Is this a good idea or should I just be patient and not power it up until I have the right supply.

    John W9JSW
    Last edited: Apr 13, 2018
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    You can run zero bias on 3-500Zs at such low voltage, but when you step up to "full voltage" (like 4kVdc), at zero bias the tubes will draw a lot of ZSAC (idling current), waste a lot of power and get hot.

    Above about 3kV almost everybody uses somewhere in the range of 5-9V cathode bias on GG 3-500Zs (in TX mode). Keeps it in AB2 range and doesn't create so much heat and waste so much power.

    I'd think the little transformer will get very hot and show a lot of "sag" under load at 350W, but depends on the transformer design. AM is tough on linear amplifiers.
  3. W9JSW

    W9JSW Ham Member QRZ Page

    Will this level of voltage allow me to getter the tube if i change the bias? Do I even need to do this? So much to learn.

    The guy I bought the amp from suggested running a 100K ohm from the center tab of the T/R relay to ground. I see others using a zener stud and some R's for bias. What is the recommended way to do this. Or should I just get one of harbachs's soft start that also has the bias setup?
  4. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Using a Zener or string of forward biased diodes on the filament transformer center tap are both common and well controlled ways to generate bias voltage in GG amps.
  5. WA7PRC

    WA7PRC Ham Member QRZ Page

    "Conditioning" the tubes is properly called "gettering". In the case of the 3-500Z, it entails running enough anode current to allow them to run hot enough (slightly red) to absorb any gas, and withstand HV (the datasheet sez 4KV). It's a good idea to hi-pot an unknown tube before inserting it into an amplifier. An alternative (to avoid destroying expensive parts) is some form of current limiting.
    N2EY likes this.
  6. W9JSW

    W9JSW Ham Member QRZ Page


    Thanks for the theory on the process. How would I accomplish this task with this amp? Almost everything is unknown here. That is why I was thinking along the lines of applying 240V with a tame HV value. Can I current limit the HV with a fusible link or some other sort of glitch approach?
  7. WA7PRC

    WA7PRC Ham Member QRZ Page

    The first thing I'd recommend is hi-potting the tube(s). If needed, getter the tube(s). The late Richard Measures AG6K devised a "breakdown voltage tester":

    When gettering, you want to be able to limit the HV to some value low enough to not arc. At the same time, you want to run the filament(s) at normal voltage. This typically isn't convenient in many ham amplifiers.
  8. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Tie the grid and anode together and you can warm it up at 60 volts.

    Put a 1a fuse in the HV somewhere and hipot it with a variac on the amp.
  9. KM1H

    KM1H Ham Member QRZ Page

    B+ in the 900-1200V range works well and wont sustain an arc. A variable voltage bias supply allows enough plate current to get a nice bright red glow. ALWAYS run air on the filament pins and when conditioning as the old timers called it.

    Running the HV at a low voltage is not recommended

    AH7I and K2XT like this.
  10. WA7PRC

    WA7PRC Ham Member QRZ Page

    This provides the lowest reactance path:
    Soft copper strip can be cut using shears. Hardware is #4-40.
    Jim W7RY sells a step-start PCB on that watery auction site ( After you make your purchase, he emails documentation and a link to the BOM on Mouser.

    He also sells a board for the L4/L7 that drives vacuum/reed relays:
    Your rig's T/R control output sees only about 10V @ 1mA., hot-switching is no longer a concern, the relays will never dry-switch, and they're dead quiet. He also supplies diode bias control. I like the version I bought for my Heathkit SB-220. :)
    N2EY and KD2ACO like this.

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