Yet another L4B rescue

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

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  1. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    You can think out loud?o_O

    I'd be sure to include a big limiting resistor, like 250 Megs or something, rated at HV, in the design to assure current is limited to something very low. Maybe 500 Megs for an 8-10kV supply.
  2. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    I guess it's only me, myself, and I who hear the voices...
  3. WA7PRC

    WA7PRC Ham Member QRZ Page

    Compared to film capacitors, electrolytics are leaky. When you don't have a lot of current out of the rectifier, that can matter a lot. With your 1000 VRMS transformer, the quadrupler shown by Chris @KD2ACO will result in 1000 x 1.414 x 4 = 5657 VDC. Add more sections and you'll add 1414V per section.

    With the right multiplier (voltage) or shunt (current), a 50 uA meter can be used to measure voltage and/or current.
  4. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Indeed, so 1500v caps are a little low for this app with that tranny, and so is the voltage rating on a 5408.

    Anyway, put some ideas together and run them by us and we'll figure it out.

    It may make sense to use a lower voltage transformer and more stages... you have to juggle the variables around.
    Last edited: Apr 20, 2018
  5. W9JSW

    W9JSW Ham Member QRZ Page

    Decided to take a look at the tubes. They are clear, no markings of any kind that I can see. They look a bit hazy inside. When I get a few minutes tomorrow I will check them with a voltmeter to see if the filaments are good and maybe put 120v to the deck to see if they will light up. The guy I bought the amp from said they came with the amp from the SK. He said he regularly swaps his tubes in his other amps. Wonder if he could have dropped some duds in the amp to get it to sell. No way to know. The plate connections were disconnected, btw. A clue, maybe? I got the amp for $200 with tubes. If they were good tubes, shouldn't he have priced it higher? Feeling suspicious tonight.

    Wondering if my efforts to hipot them will be in vain. There is the option to send them to RFParts and have them test them. Then, if bad, I can have them send me some new ones. If they test bad, bet they might sell me new ones and drop the testing charge. Things to ponder. Uncomfortable about getting them damaged in shipping.

    If I put 2500V on them in a controlled manner (thru a 100M series resistor or such) are they likely to gas or do I have to go over 4kv to be thorough. Heck, given them to be no-name tubes, they may gas with a 9V battery....hihi
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I'm always suspicious of an L4B or similar amp sold "without the power supply."

    What happened to the power supply? They were sold "together." In fact, in the late 60s/early 70s I was selling them, and sold a lot of them. But always together.

    Where did the power supply go?
  7. WA7PRC

    WA7PRC Ham Member QRZ Page

    The more unknown, the more I'd be inclined to non-destructively test it.
    HV doesn't cause gas. Leaky wire-glass seals cause that. The worse the leakage, the shorter the lifespan (they can getter only so much gas). According to Carl @KM1H, when Eimac moved glass tube production to Salt Lake City, they developed bad wire-glass seals. From

    "Eimacs 86-89 had bad seals and welds all due to QC problems at the new SLC plant; the welds were due to trying to save money by using only 3 spot welds.

    I dont have the actual date code groups handy."

    FWIW, the Eimac date code looks like this:

    IIRC, on one of the amplifier email reflectors, someone (maybe Carl) suspected the early Chinese 3-500Zs may not have been pumped down as hard as the early Eimacs, and tend to go gassy sooner. A pair I bought from RF Parts lasted about six years before flashing over. The pair of Eimacs I had prior to that lasted about 30 years.
  8. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Sending the tubes to RF Parts doesn't sound like a bad idea at all, especially if you will only have a one time use for the hipot tester.
    They can ship you boxes for them that are really nice.
    WB1E likes this.
  9. W9JSW

    W9JSW Ham Member QRZ Page

    I saw that pic on another thread. That is why v decided to pull the tubes to look closer. Was surprised to see absolutely no marks.

    My intent for going to the hamfest was to look for parts for an AM transmitter. Was planning on a 813 modulated by a pair of 811s.

    I initially looked at this deck for the tank parts and meters. The guy i bought it from said it would be a crime to not get this amp back running. He seemed to be on the up and up.

    So, if these tubes are bad, maybe iI'l drop a pair of 813s in there and convert it to a AM rf deck.
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    It may have been "pumping down" issues, but that can be automated so it's really easy to do it right. I believe there may have been more to it than that.

    Any contamination on the Kovar pin seals can cause them to not seal properly.

    I've never manufactured tubes, but did manufacture hermetically sealed high voltage hybrid circuits, and unless the packages are stored in dry nitrogen, handled with gloves and assembled in a chamber inside a clean room, they all contaminate and then fail seal tests later; and our packages had either glass or ceramic (aluminum oxide) seals with the packages made in America and very well done. But occasionally we tried "imported" hybrid packages that would fail fine leak pretty often.

    My "guess" is the Chinese didn't have this down pat for some time, but got better at it.

    They're not stupid, they're just cheap. Eventually, they figure stuff out.
    WA7PRC likes this.

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