Bewildered, but not shocked!...........[tube biasing question]

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by michael.e, Dec 4, 2003.

  1. michael.e

    michael.e Supporting Member

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    Hey all,

    I have a JoMama Keleman Reverb 45. This is such a GREAT AMP! This amp can take anything from 6v6 to KT-66's to EL-34's and therein lies my question to you all. I have been swapping out tubes and re-biasing properly, no problem there. I am bewildered about my GT KT-66's. When I bought them, they were a matched set, I would set the bias at 35 for the higher reading tube and the other one would read out at 33. This is a difference of .002 and that is what GT calls a "matched set". It all sounds great so no problem there. I have been switching out with a set of EL 34's and of course their reading of 40- allows much more current flow [I assume]. When I switch back to the KT's and reset my bias one tube is reading out at .005 different than the other tube. This happened once before and the tubes worked themselves out [I guess] and settled back down to a "matched" difference of .002, that was a couple of months ago. I had since gone back to the EL's for a few weeks and again went back to the KT's tonite. The KT's are not "settling" out after a couple of hours now as I am still getting a reading of .005 difference. I do handle the tubes properly etc. etc. so I know that it is not from misuse or mishandling. First off, is a difference of .005 going to do some damage to my amp components? It sounds just fine, but I do not want to be quietly creating a problem for myself here. Can you offer some feedback, positive or er...."negative":D? Sorry.....Thanks in advance. M.E.
     
  2. VaughnC

    VaughnC Supporting Member

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    Tubes, especially the one's they make these days are unpredictable. Some "matched pairs" will settle in evenly and some won't. However, I wouldn't be concerned with a 5ma difference....as long as the hottest one isn't exceeding its power rating. I've even used tubes with 10ma difference and noticed no difference in sound....and that's the ultimate goal, not what a meter says. I usually set my bias by ear anyway and just use a meter to make sure the tubes limits aren't exceeded.
     
  3. 59model

    59model Member

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    If you aren`t putting the tubes back in the same sockets you may be adding the OPT difference to that of the tubes. Try swapping the position of the 2 power tubes and see if that helps.
     
  4. JamesPeters

    JamesPeters Member

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    I went through five quads of GT KT-66HPs about six months ago, trying to get them to work in a Soldano SLO--all five quads were pooched. Two quads had at least one tube which red-plated under even somewhat "cold" bias conditions (30mA or so), and the other three quads were very mismatched for current draw. 5mA out was as good as I could get.

    Needless to say, I took advantage of the return policy (on each quad at a time, and eventually altogether) and got some other brand of 6L6 tubes instead. No offense to GT as a whole--but after that it's going to be hard to trust the KT-66HP tubes for a while, anyway.
     
  5. michael.e

    michael.e Supporting Member

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    Hey there James,

    What type of 6l6's did you get anyway? I am on the search for a beautiful, rich clean tone out of this amp. What would suggest? M.E.
     
  6. JamesPeters

    JamesPeters Member

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    Those were 6L6WXT+, one of the Sovtek tubes I can actually appreciate. :) They're pretty good in the SLO but I'm not sure I'd recommend them to "wake up" your amp in the way you describe. I've also used them in some Fender amps and they sound pretty good...but you might want to try JJ 6L6 tubes instead.

    I can't tell you exactly how they're going to sound in your amp though. FWIW, if the amp already isn't a "beautiful, rich clean" amp in general, the tubes probably won't make much difference in that regard. You'll hear differences between tubes, but I'm not sure what's going to float your boat the best.

    Good luck!
     

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