Tube look like stop lights: HELP

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by 908SSP, May 4, 2005.


  1. 908SSP

    908SSP Member

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    I got this really clean 1976 Mesa Mk I. Sounds way better then I expected. Anyway I want to make it sound better I test and compare and end up replacing the Chinese 12X7 tubes with Telefunkens and the 12AT7 with a black plate GE. I test the bias with the existing Chinese coke bottle 6L6GC and all the bias are right around 20 on a Webber bias right. I replace those with a pair of Fender labeled GE gray plates and a pair of GT labeled GE both sets of tubes at least twenty years old but test good. Bias for the new/old tubes is 36/21/21/31. Everything sounds fine as a mater of opinion it sounds better then it did when I started smooth, even, tight. Then I do this vibrato on the B string 14th fret with the gain boost pulled and the first volume on 7 the second volume on 3 and the master volume on 2. I get this nice feed back sustain thing just ringing sounds grand and the two tubes on the right start to glow like cherries no stop lights...WOW. Needless to say I shut the amp off immediately. I don't believe I hurt anything in the amp but can someone maybe tell me what happened?
     
  2. RussB

    RussB low rent hobbyist Silver Supporting Member

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    Could be one power tube crapped out, and took out a grid resistor with it (that's why 2 tubes were glowing)

    Those tubes were pretty far away from being matched...
     
  3. 908SSP

    908SSP Member

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    I switched the two outside tubes for a couple of GE black plates they biased at around 20 so they are pretty even now. Anyway I can't get the tubes to light up. Maybe some kind of runaway oscillation or something due to the out of balance bias. Still running the bias at around 20 for each tube is darn cold but this is my first Boogie and they do things different. I won't modify this amp because of it's collectability but I wonder if anybody ever fits bias pots to a Mesa?
     
  4. John Phillips

    John Phillips Member

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    I'll stick my neck out on this one even though I'm supposed to be taking a break... ;)

    When you say the new tubes 'don't light up', I assume you mean that they don't glow red. If they don't light up at all, you've blown something in the amp.

    One of the two tubes that overheated is faulty. It almost certainly has a grid-to-cathode short which would kill the bias supply for that side of the amp, and make the other tube on the same side run hot too. It's not a general bias failure because all four tubes are not affected. Just because the tubes 'test OK' on a tester does not mean they are OK for a guitar amp, BTW. The stress of running at high power (far outside the conditions they were designed for or the tester works at) is what caused the short to develop.

    If you want to find out which tube, at the very slight risk of harming the amp, get the two tubes to run hot again then pull one of them. If the other one returns to normal, you've pulled the bad one. If not, it's the bad one that's still in the amp.

    If the amp now works with the outside tubes changed, you've probably already done this, effectively - it was the outside one of the two that ran hot that was the culprit. Be careful though, because sometimes these kinds of shorts are intermittent and go away after everything has cooled down, only to return next time the amp is pushed hard.

    Hope that helps!

    :)
     
  5. 908SSP

    908SSP Member

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    Thanks John
    Yea by not light up I mean like a stop light. The black plate I replaced the gray plate with biased at 20 rather the 36 you don't think that alone could make a difference? Also it ran fine until I did the feed back and that was all done with the master at the #2 on the dial. The thing sounds great now so I am leaving well enough alone.
     

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