Yet another L4B rescue

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

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  1. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    That sounds like fun.
    IMO, a hipot tester would be worth building to use 833s and 811As. I'd gather used ones for cheap and test them out.
    W9JSW likes this.
  2. WA7PRC

    WA7PRC Ham Member QRZ Page

    The picture is of value only if you know why the date matters.
  3. KM1H

    KM1H Ham Member QRZ Page

    Bad seals let outside air in and destroys the tube. Please dont mix up the two distinct and separate issues.

    Sitting without use for years can cause the anode to outgass the gasses from production which were gettered at the time, nothing lasts forever, even some Eimacs.

    HV CAN cause gas release if the anode gets too hot such as bright orange to yellow to white.

    I gave all a hi pot before using and reported that many didnt pass. Rich, AG6K (SK), provided the 13 KV sign transformer and pair of .1 uF 10 KV oil caps in the mid 90's at no cost. Before that I could only test to 8 KV

    The correct term is outgassing when there is a blue glow inside the tube between anode and glass of a previously fine tube. A purple glow between filament and grid indicates a seal leak or insufficient pumping down and there could also be a blue glow in a new tube which were quite common in early Chinese when their were three manufacturers, only one for the past 20 years or so. I still have a pair of 1973 date code Eimacs, the ones without the added stiffening collar which was added to prevent air leakage from damaged seals by improper handling; that still pass all tests. A 1969 Eimac pair with graphic anodes built by Philips until Eimac got their act together, are regettered every 7-8 years or so in the LK-500ZC or Hunter 2000C. Another pair from an early 90's ISM machine are almost a browned opaque but still run 90% of power at 3500V and no Hi Pot leakage.

    W9JSW likes this.
  4. KM1H

    KM1H Ham Member QRZ Page

    At least 3 x the combined DC and AM modulation combined voltage peaks, 3KV maximum DC. If just SSB/CW a minimum of 2X. The typical new Eimac hi-potted at 11-12KV, Chinese are up to ~ 9-10 KV now.

    W9JSW likes this.
  5. W9JSW

    W9JSW Ham Member QRZ Page

    So one of my beer sign transformers should work good at 10kv. I have a 50uA meter. Should I plan to wire the transformer secondary in series thru a 200M ohm HV resistor and the meter movement to insure that a dead short pulls no more than 50 uA of current at 10Kv?
  6. W9JSW

    W9JSW Ham Member QRZ Page

    In a moment of clarity, I have determined that I am in over my head. I am not going to mess with 10kv on my bench. I will kill myself. I am going to send them off to be tested.

    If they test good, I will proceed. If not, will have to assess whether to spend $400 on a new set or go back to my original AM transmitter plan and part out the portions of the L4B I don't need.
    WB1E likes this.
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I think it would be better to just drink the beer.:) It always tastes better if consumed by the light of a beer sign.
    WA7PRC and W9JSW like this.
  8. KM1H

    KM1H Ham Member QRZ Page

    Actually it is quite safe if you follow the link and paste a DANGER HV sign at eyeball level. The belt of very current limited 10 KV is less than an old B&W TV and most bench scopes. Neither will injure a reasonably healthy person.

    WA7PRC likes this.
  9. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    If you had to choose between a single stage type (oil burner ignition tranny) or a Crockroft-Walton style power supply, would you have a preference?

  10. W9JSW

    W9JSW Ham Member QRZ Page

    Someone is going to test the tubes for me.

    Interesting is this note from Merrit at RFParts "My initial thinking is that are very old tubes with many hard hours on them. Both Eimac and RF Parts use temperature sensitive red paint markings on their tubes. They have apparently burned off yours. Replacing them with new would be the best choice." He did offer to test them for filaments and HiPot.

    I guess we will see.

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