Tech Q re one of a pair of power tubes...

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Kiwi, Feb 11, 2006.

  1. Kiwi

    Kiwi Silver Supporting Member

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    I thought one of my amps was not sounding quite so loud as I recalled it did, so I pulled off the back.

    One of the two KT66 tubes does not light up with a blue glow on full power, when I take it off standby. The suspect tube does glow orange with the heaters, etc.

    I swapped power tube positions, and even put the pair into a different amp - it's the tube, not the socket. No blue glow on the one tube.

    I replaced both tubes with another matched pair, and I think it's back to normal. Sounds right, and they both show the slight blue glow.

    Is that one tube shot? Is the other one good?

    Kiwi
     
  2. Dave C

    Dave C Gold Supporting Member

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    From what I understand the blue glow is due to some remaining gases in the tube (not a perfect vacuum)it's very common to see and not thought of as a problem. It's also common and also not a problem if there is no blue glow.Your old tubes may be just tired or in need of a rebias......hard to say from here. Anyway I wouldn't be too concerned either way about the blue glow. Someone with a serious tube tester could tell you if your old tubes are ok and matched or not. Hope this helps.
    Dave C
     
  3. Kiwi

    Kiwi Silver Supporting Member

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    Thanks, DaveC. I did a little more test work...

    I put the pair of KT66s into a Dr Z Rt 66. Upon switching to full power, there was now an audible hum from the speakers. Again, the suspect tube gave no blue glow, and the internal heaters seemed to be slightly less orange than on the other (normal) one.

    I played for a minute or two before pulling the pair out. Now I noticed that the suspect tube did not get very warm at all, while the "normal" one was very hot.

    Does this information help diagnose the condition? Thanks for any comments.

    K
     
  4. Dave C

    Dave C Gold Supporting Member

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    That one tube from your description sounds like it has a problem . I'd 86 it before it failed and maybe took some other component with it. Is that a Russian KT-66 ? I've had some longevity issues with them , the Chinese KT-66 has been better for me.
    Dave C
     
  5. Kiwi

    Kiwi Silver Supporting Member

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    The suspect tube is a Valve Arts from the late 1990s, which I believe is Chinese.

    FWIW, Dr Z stuffs his amps with the Groove Tubes KT-66HP, which (after three versions) he considers reliable now. I think those are Russian tubes.

    Yeah, I'm going to bin the suspect tube, guess I'll keep the single as an emergency backup or sell it cheap to a UniValve owner! Thanks.

    ==K
     
  6. brad347

    brad347 Member

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    Even a bad tube will often glow from the heater filaments.

    The blue glow you see in your tubes is NOT GASES REMAINING IN THE TUBE. This is a very common misconception! It's actually the bombardment of stray electrons on the glass envelope that gives off this blue glow, and is actually an indication of a healthy tube. If your tube once glowed and now doesn't, all things remaining equal, then it's likely dead.

    Often, blue glow INSIDE the plate structure is a sign of extremely low gas content. The NOS Mullard EL84s in one of my amps have this feature.

    What you should worry about is if there is pinkish or magenta glow, or blue, pink, or purple glow AROUND THE PINS at the base of the tube. This, along with poor performance, is indicative of a gassy tube.
     
  7. Kiwi

    Kiwi Silver Supporting Member

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    Thanks, everyone. I pronounced that tube dead at 6:21 pm Central Time on Feb-13-06.

    Anyone need an inexpensive single Valve Arts KT66, lightly used? Survivor needs good home after mate passes. Univalve owners preferred...:angel

    K
     

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