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Bad JJ 6L6GC?

Discussion in 'Amp Tech Center' started by Diverted, Jan 19, 2021.

  1. Diverted

    Diverted Tele-Meister

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    A friend of mine just gave me an AB763 amp to look at. He said "one of the tubes was sparking." The amp (I built it) is about six months old and the JJs are new. It's a Super Reverb circuit running with a solid state rectifier.

    Took it home, pulled the tubes and checked on my tester (Hickok TV-D10/U mutual conductance tester.
    The tube in question passed all shorts tests, but when I hit the button to measure transconductance the needle bottomed out and the tube glowed lavender.

    With tubes out, I went through the amp and all seemed OK. Put a fresh set of JJs in, brought it up slow on the variac and the numbers on the 6L6s are fine. They are consistent with the plate, screen grid, control grid and cathode voltages specified on the schematic. Maybe 10-15 volts high overall, but otherwise consistent.
    Currently the tubes are running about 478 on the plates, with dissipation around 40ma on one and 38ma on the socket that had the bad tube. So they're biased right around 60 percent or so. Don't want to bias them any colder for a few reasons ... tone and the fact that the plate voltage will rise are the chief ones.
    At 60 percent and 478 plate voltage though amp has been running smooth, stable and fine for hours.


    What would cause that internal glow? Might I just have gotten a bad JJ? I've read about JJs handling 500VDC on the plates no problem, as long as they're not biased hot. I think my biasing is pretty conservative at 60 percent so I'm wondering if it was just a tube that decided to quit on its own.

    Any thoughts?
     
  2. tubelectron

    tubelectron Tele-Afflicted

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    Probably yes. You troubleshooted the problem the right way... :cool:

    While servicing amps, I found several times JJ power tubes (6L6, EL34, 7591) defective brand new, right out of the box, or after a very short time : interelectrode leaks or even shorts, defective vacuum, gassy glass containers (this may be your case here)...

    [​IMG]

    -tbln.
     
  3. Diverted

    Diverted Tele-Meister

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    Thanks. I should clarify that the tubes were new with the amp, so they've got five-six months on them.
    It definitely looked like gas from what I've seen of it, and was wondering about that given there were no internal shorts showing. That sucks. So here's my follow-up: What do you use for new production 6L6GCs?
     
    Last edited: Jan 19, 2021
  4. DADGAD

    DADGAD Friend of Leo's

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    In at least 20 years of installing new JJ 6L6 tubes in customer amps, I’ve experienced exactly one failure. I’ve replaced countless Groove Tubes, Sovtek and Chinese 6L6 tubes. I’m sticking with JJ.

    Does your friend let the amp cool before he moves it? Perhaps it got banged around while hot.
     
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  5. Diverted

    Diverted Tele-Meister

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    No idea on that one.
    I guess time will tell the tale with the new tubes.
     
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  6. schmee

    schmee Doctor of Teleocity

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    Sounds like a bad tube. It happens. I too trust JJ power tubes as well as anything.
     
  7. Jon Snell

    Jon Snell Tele-Holic

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    Back in the 60s when valves were used in television, radio and well known amplifiers, I had a direct account with Mullard, Philips and Mazda.
    In 1970 I received a letter stating that their valves no longer had a guarantee as such and as the failure rate was around 6%, pretty good for valves, I was offered a 10% discount in order to cover any loss and save the trouble of administration costs.
    Mullard being one of the better manufacturers had a batch failure rate of 6% so to get the odd failure today, is to be expected.
     
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  8. Musekatcher

    Musekatcher Friend of Leo's

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    6%? That's 1 of every 16 or 17 products? That's super high for any kind of production rates, for about anything. Especially for DOA, after being stress tested by lot at the factory. I'd think something on the order of .6 or really .06 would be more likely for a MTBF, from that era. Unless they had a lot failure discovered after shipment and issued a recall.
     
  9. mkdaws32

    mkdaws32 Friend of Leo's Gold Supporter

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    I got a bad JJ EL84 new in box. I’m not a tube amp tech and my amp is self-biasing (TubeMeister 18). New JJ pair were properly matched. With any gain and master volume over half I started getting a strange “ping” on note attack. Switched back to original unlabelled Chinese tubes and the problem disappeared again. I wouldn’t think this low wattage amp would even come close to pushing those tubes too hard.
     
  10. Wally

    Wally Telefied Ad Free Member

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