Help! a power tube turned blue and hot!

Discussion in 'The Workbench' started by Force235, Sep 11, 2019.

  1. Force235

    Force235 Well-Known Member

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    During practice last night, my 6100 started having a problem, a few low pops, and then the power went to about half. I quickly stopped, and looked over the amp, I noticed it felt hot, I checked the tubes and 3 were glowing brighter than normal, one seemed less bright. I shut the rig down and changed the setting to low power (50 watt, from 100 watt), still in Pentode mode. I double-checked all the cable connections. I fired it back up, and it was still playing lower, but sounded fine otherwise, that only lasted about 2-3 minutes, as I was watching the amp, I looked at the tubes, through the vent hole, and the tube that was glowing lower, was now a nice bright blue glow. I immediately shut the amp down - the tubes are TAD premium select 24 hr burn in, EL34s, only about 6 months old.

    1. Do you think this is just a bad tube?
    2. Do you think any damage was caused to the amp, because of the bad tube?
     
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  2. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor. free Mars!
    You need to check for a bias failure, a screen grid resistor failure, a bad tube or an impedance mismatch.
    The tube glowing blue is pretty normal.
    Inspect the tube sockets for signs of burns / arcing between pins. This is caused by speaker load mismatch.
     
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  3. Force235

    Force235 Well-Known Member

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    Ok, will check, I believe the impedance was ok, it's the cab I use all the time, but maybe something went wrong.

    I've never had a tube glow blue before, I wouldn't think that is normal.
     
  4. marshalltsl

    marshalltsl Member

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    Check the HT's fuses, maybe one is blow and the amp have one bad tube.
     
  5. Force235

    Force235 Well-Known Member

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    Will do...
     
  6. Exotic

    Exotic Active Member

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    oh oh bummer ,get a matched set of 2 power tubes install and bias .don't take my word for it though ,that's just my gut .
    try jj's next time? or groove tubes? might be an air leak?
     
  7. Matthews Guitars

    Matthews Guitars Active Member

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    It's almost a complete certainty that that tube has failed. The brighter blue glow would mean it is gassy. It probably cracked from the heat generated when it shorted.

    It may have red plated and you didn't even notice.

    Try spare tubes. If you don't have spare tubes, have the amp checked out by your local (qualified) amp tech, who will.
     
  8. Force235

    Force235 Well-Known Member

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    Thanks everyone, I went to pull the one tube out this morning, and found that the tube next to it also seemed burnt up, see image below. I pulled out both tubes.

    Question: Should I get a matched pair from TubeDepot (TAD) of the same rating "24" as written on the label on the tube?

     
  9. marshalltsl

    marshalltsl Member

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    If the amp has a bad tube I recomended replace all set for a new one. You will need test the bias and ajust It properly.

    I have had rusian EL34 that it glowed blue light without issues.
     
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  10. Lance Chambers

    Lance Chambers Well-Known Member

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    Glowing blue is normal for an aged tube. Purple color needs replacing.

    FYI! Just because a tube red plates doesn't mean it is bad.:agreed:
     
    Last edited: Sep 11, 2019
  11. Lance Chambers

    Lance Chambers Well-Known Member

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  12. Force235

    Force235 Well-Known Member

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  13. Force235

    Force235 Well-Known Member

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    I just ordered some
    Electro-Harmonix 6CA7 EH Power Vacuum Tube

    24-Hour Burn-In: $1.50 x 4
    Matching (up to 9 tubes): $0.00 x 4

    (4) @ $27.45 = $109.80
     
  14. EADGBE

    EADGBE Active Member

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    It could be a leaky power tube. They sometimes turn blue when there is a gas leak. Even the best tubes can fail.
     
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  15. Lance Chambers

    Lance Chambers Well-Known Member

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  16. neikeel

    neikeel Well-Known Member

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    Not uncommon for tube to die, would expect a few hundred hours of use first. Last one to die on me was a JJ KT77 in my 72 DR103 (and no not a speaker mismatch or duff lead!!). Same thing, lots of chirruping and popping coinciding with blue flashes. Power down and check tubes, one was shorting intermittently.
    Important to check:
    Sockets for arcing, carbon deposits between pin 2 and 3.
    Next check integrity of the screen resistors, check board for burns etc.
    Before putting the new quad in check pin 3 anode voltage on each socket, check pin 5 for -45v or thereabouts and your bias pot varies this up and down.
    Finally drop in your new tubes and rebias in usual way keeping under the 70% at idle maximum.
     
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  17. john hammond

    john hammond Well-Known Member

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    im not sure these tubes are bad, or that there is a problem at all.
    blue glow can be normal, its normal around here anyway.
    id just check bias voltage ( pin 5 ), and bias current ( whatever way you bias your amp )
    then carry on as normal.
    when tubes overcook, a really good way to tell is they get stinky.
    if there was no smell whatsoever, i doubt you have a problem based on what you reported and those images.
    youll know if theyre worn out because they just sound weak.

    i think youll find the one that wasnt glowing blue has drifted in value, its running cold.
    if thats the case, ignore it and carry on as usual.
     
  18. Force235

    Force235 Well-Known Member

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    ** Update: I checked the amp for internal damage, arcing in the sockets etc, no visible issues. As luck would have it, I had a pair of vintage Teslas, that came with the amp in 1993, they were recently tested and bias noted, which was close to the #24 noted on the Sino ELs. After installing the Tesla pair, a light came on, on the back of the amp, noting that there was a problem with valves 8 and 11 (red LED indicator), which is fused with a T500L 250v fuse - the fuse was blown. I replaced the fuse, fired up the amp, and watched intently for problems (back of amp facing me), I practiced for about 30 minutes, with no problems, the amp sounded great, everything worked properly! Whew...

    I'd like to thank Marshall for adding that valve fuse, there is one for 8-11, and a fuse for 9-10, I think that probably saved me a transformer!
     
  19. Matthews Guitars

    Matthews Guitars Active Member

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    The blackening on the inside of the glass around the plate means that this tube ran red hot for long enough that it qualifies as a bad tube now. No way should you run it. It's been roasted. Toss it, buy replacements, don't waste any sweat over it. They're not that expensive.

    When the paint changes color, the tube is now cooked. When the glass starts to blacken around the plates, it's cooked well done. Toss it.
     
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  20. john hammond

    john hammond Well-Known Member

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    your post doesn't make any sense to me, mathews guitars.

    if the tubes were redplating ( doubtful based on all the evidence..they look like theyve been driven hard, which i like seeing ) ..don't you think the guy should be checking why?

    if i thought the thing had been redplating, i'd be wasting sweat as opposed to just throwing a new set of tubes in.

    Should the guy do some analysis..voltage on pin 5 perhaps?
     

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