Yet another L4B rescue

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

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  1. WD8OSU

    WD8OSU Ham Member QRZ Page

    A little reading material--------------

    Extending the Life of Power Grid Electron Tubes, Penta Laboratories



    “4) Degassing- After considerable time on the shelf some glass transmitting tubes have a tendency to show signs of gas. This is less likely with ceramic tubes, since Kovar is not used in the seal. Ceramic tubes may show signs of gas if overheating has occurred. At first turn-on, the internal gettering material can be activated in either tube type by operating the vacuum tube with filaments on and no other voltage for a considerable length of time. Large tubes require five or six hours of operation. Smaller tubes (due to their smaller volume) require less time. “
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    That statement does relate specifically to ceramic-metal tube designs where the anode is external.

    Unfortunately, tubes like 3-500Zs aren't like that: They're glass and the gettering material is on the anode, which remains cool when you run only the filament and doesn't do anything.
    WA7PRC likes this.
  3. WD8OSU

    WD8OSU Ham Member QRZ Page

    "the internal gettering material can be activated in either tube type by operating the vacuum tube with filaments on and no other voltage for a considerable length of time".

    So what else is needed to make it happen? The application of some HV to get it hotter as others recommended?

    I only posted this link as someone stated there was no information out there concerning the subject from a reliable source.
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The "either tube type" refers to those tubes that do not have getter material deposited on the anodes. Only "some" power tubes are made like that, and the 3-500Z or ZG is one of them.

    You can get it very hot by making a test jig that ties the grid to the anode and applying 35-40Vdc to the cathode/anode connections, in addition to the standard filament (also cathode) connections. This will require some forced-air cooling as the cathode pins get very hot with just filament current. The glass envelope really doesn't need any forced air cooling, although most designs use a heat dissipating anode cap to help keep that seal cool.

    That's all very inconvenient to try if the tube or tubes are in an amplifier. Much easier to just use the amp and load it up (into a dummy load) with enough anode current to get the anode very hot. "Normal use" of this type of tube anywhere near its power rating does that, anyway.
    WA7PRC likes this.
  5. KM1H

    KM1H Ham Member QRZ Page

    Wow, when shown to be completely wrong on that gettering statement you resort to pulling the personal attack card....So Sad and thin skinned.
    Tell us how many amps in the amateur radio power range you have designed and built?

    I doubt if anyone here cares what you do with tubes that require a small crane. You call yourself a professional but it appears to be in a very narrow range and you constantly belittle those who work PROFESSIONALLY at lower power. What I see is a power snob.

  6. KM1H

    KM1H Ham Member QRZ Page

    Running filaments only on a glass thoriated tungsten filament tube only wastes electricity no matter if you do it for days, weeks, etc.

    OTOH many ceramic metal tubes use an indirectly heated cathode and those can be regettered with just filament power since the gettering material is on the cathode. Some examples are the 4CX1000A, 8877, 3CX800A7, and many more.
    Directly heated thoriated tungsten versions include the 3CX1000A7, 3Cx 3000 (all versions) which have been used in commercial ham amps. (all versions). These have the gettering material on the inside of the anode and getter in normal operation.

    Many make the mistake of believing that a tube that has not been properly flashed in production (very common with some Chinese tubes is the same as losing the vacuum seal. Going to AIR is always fatal while excessive outgassing often can be salvaged by regettering one to many times. AM/FM glass broadcast tubes rarely if ever need regettering since their anodes run red to orange in normal use. You wont find Eimac, RCA, etc mentioning regettering in the specs since it isnt necessary for their commercial customers. OTOH, a phone call to Eimac would have brought the answer from Reid Brandon, W6MTF, who is on the Zed often and was an applications engineer until retiring.

    WA7PRC likes this.
  7. KM1H

    KM1H Ham Member QRZ Page

    Ive been told that method does a very poor job for some reason that escapes me at the moment. Plus if a 3-500 anode is red the tube sure needs air

    Running an amp at zero bias (jumper the Zener for instance)is often enough to get the anode red without any RF which CAN flash internally and cause tube and component damage. I built a jig decades ago using a Variac controlled HV in the 900-1200 VDC range which will not cause a plasma arc which can get out of hand real fast. Ive regettered up to 4-1000's.
    Vintage amps that switched to ~ 1800V for CW are fine when the outgassing is not severe or if a separate filament transformer is used then rewire the plate transformer to a Variac.
    WA7PRC likes this.
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Yeah, I mentioned that. But the 3-500Z is almost entirely radiation cooled and the reason for the "air" is to protect the glass seals at the filament pins and the anode pin. If not for those seals, it wouldn't need any air. If we all used big copper or aluminum heatsinks on those pins, we could do away with fans and blowers, I'm pretty sure.

    I think the reason the LV "gettering" (or as Bill Orr once told me, "it's not gettering, it's reconditioning!" -- and the reason they never mention it was to sell more tubes) isn't as good as HV is because the LV might get stuff hot enough to reactivate the getter material, but it won't blow out any prior arc damage; that takes HV, and sometimes a lot more HV than we actually use in amplifiers.

    I think hams and hobbyists are the only ones who ever do this stuff. BC stations and even those using power grid tubes in MRI machines and stuff commonly replace after X many hours. Every 4-1000A or 8877 I ever bought (a lot of them over the years) was a "pull" from a BC station or an MRI machine or some industrial hardware; I don't think I've ever bought a new one. The 8877 in my 6m amp is from a PlasmaTherm generator, a pull purchased in 1984 and it still works fine.

    My last 4-1000A purchase was a BC pull from KCSN-FM, replaced after 10,000 hours service or something in 1989 (the TX was pretty old) and it also still works fine. Cost me $100.
    KM1H and WA7PRC like this.
  9. WD8OSU

    WD8OSU Ham Member QRZ Page

    Great info fellows. Kind of got this thread off tack but still learning stuff.

    Last question oh wise ones, what about bottles with graphite in the mixture? Isn't that getting to what we are talking about with the 500Zs? The graphite itself when hot will do the job.

    In my real life applications years ago, we only had to mess with them if one pulled a tube from supply with a very old date code. It happened in the military where some of these tubes were on a shelf in some warehouse for a decade or more before being pulled. The glass bottles were easy as you never hit them with full power initially. We just looked at the color of the ionization to tell if it was gassy or not.
  10. W9JSW

    W9JSW Ham Member QRZ Page

    This has been an interesting walk in the weeds, I guess. I am just trying to find out how I should proceed with bringing this amp back online. Where do I start and what set of steps do I follow to hopefully bring the tubes back from their long nap without destroying them due to lack of knowledge on my part. I am not ready to put HV to the amp yet. Working with Lou on a PS design right now. I see no evidence of heat on the filament pins. Have the copper to ground the grids. Ordering some boards from Harbach - soft-start combo board and the PS board.

    Should I verify some amount of correct function prior to installing the soft start board or just go to it and then plan to follow a set of bringup steps once I have a good HV PS. Just don't know.

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