Genalex KT-66's Running a little Hot

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by 767400, Mar 20, 2008.

  1. 767400

    767400 Member

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    I just purchased a pair of Genalex KT66 and I set the bias on my Marshall Bluesbreaker RI to its lowest lever prior to installation. When I turned the amp on one tube was 45.6 and the other 42.3. Again, this is at its lowest bias settingl. My valves in there before (another set of GEC KT66) was 40 and 38.9 at a mid range.

    Should this be a concern, or just that the tubes are running hot and will burn out quicker?

    Thanks for any help.
     
  2. rockon1

    rockon1 Member

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    What plate voltage are you running? If the old tubes were set properly then the new ones running a little hotter will just wear out a bit faster. Of coarse this depends on how hot it was running at 40 ma. To know that you need to know the plate voltage. Bob
     
  3. stratman_el84

    stratman_el84 Member

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    Generally speaking, this is fairly normal as tubes vary widely, even more so today than in the 'glory days', when tubes were much more consistent.

    Seeing that this is a reissue, it might be worth it to mod the bias circuit to be able to deliver a higher negative bias voltage.

    This can usually be done by changing resistor values in the voltage divider preceding the bias pot. Usually, dropping the series resistors' value a bit will work. If doing this yourself, the usual safety warnings apply about lethal voltages present, discharging filter caps before work is attempted, one-handed measurement/adjustment, etc etc.

    Good luck!
    Cheers!

    Strat
     
  4. 767400

    767400 Member

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    Thank you very much for the information. My biggest problem is lack of knowledge. I'm comfortable biasing but not checking plate voltage. I'm not sure I even have the right equipment to check plate voltage. A friend did mention changing the resistors, but again that is beyond my pay grade. So I might take it in and have that done.

    At least it's at 45 on the hottest valve. 40 is where its recommended, but I tried the amp out today and it sounds great so I'm happy.

    Here is a picture of the valves. My GEC are clear glass and I was curious what if any the Smoked glass valves had. The jury is still out on that one.


    [​IMG]
     
  5. BluesForDan

    BluesForDan Member

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    those tubes are too expensive and hard to find to allow them to be toasted, for want of a minor bias circuit adjustment. take the amp in, and don't use those tubes until the amp is worked on.
     
  6. mcdes

    mcdes Member of no importance

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    i think that depends if they are reissues or original NOS genalex valves?

    :rolleyes:
     
  7. 767400

    767400 Member

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    These are originals. I wish I knew the date code NF 4. My question is if I had another pair of KT66 in the amp and they were running at a mid bias of 40, and these valves are set on the lowest bias reading 45 what would be the big concern? I could see if they were reading 50 or 60 ma.

    Again my knowledge is very slim to none so I will take any information you can provide.
     
  8. slowfingers

    slowfingers Member

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    Nf 4 should be year 1956 according to my notes.
    You want to change your bias resistor so your bias pot has adjustment
    between 35 Ma up. Nice looking valves.
    slow.
     
  9. rockon1

    rockon1 Member

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    Crap-those are originals?!? Well its your cash but I'd want to know those tubes were running at proper idle. A nice,not too hot, 60% idle. Bob
     
  10. Blue Strat

    Blue Strat Member

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    It's safe to assume 450V for the plate voltage. It won't be off by more than 5% in either direction.

    Using 450, the idle dissipation is:
    450 X .045 = 20.25 watts

    20.25/30 = 67.5%

    I'd have that bias resistor changed.
     
  11. 767400

    767400 Member

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    Thanks guys for all the help. I guess the consensus is get the bias resistor changed, so that is my next move. Thank you also to Slowfingers for the Date on the Valves. I was very curious about that, and I could not find any information on the subject. And thanks Mike for the math to help me understand.

    I've had in this amp Sovtek 5881s, GT KT66, and two different NOS KT66 and in my opinion the GEC and Genalex valves blow the others away. And if you compare them to a $300 Distortion pedal they are not that much more expensive, and really enhance tone quality. As always in my humble opinion.

    Thanks again for all the help.
     
  12. Blue Strat

    Blue Strat Member

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    ...and then there's the 10,000 hour life expectancy which is about 20:1 over new production KT66s.
     
  13. TheAmpNerd

    TheAmpNerd Member

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    If I recall, the GECKT66s are only supposed to have 400v max
    on the plates.

    I had one pop in my Marantz 8B, and subsequent plume of
    smoke followed.

    I repaired the amp myself ; ) What beautiful workmanship in
    these old Marantz amps. In the glory days of American Radio
    and quality control. (Think vintage Hiwatt, only Marnatz
    did it first).

    These tubes sound great and very musical too. : )


     
  14. 767400

    767400 Member

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    I have a new resistor in place and my Genalex KT-66's are smoothly running along at 40~37 ma. Thanks again for all the suggestions.
     
  15. Blue Strat

    Blue Strat Member

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    When in doubt, Google "KT66 data". http://tdsl.duncanamps.com/show.php?des=KT66

    This shows 500Vplate which I don't believe to be an absolute max.
     
  16. muddy

    muddy Member

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    i discovered something pretty clever (i think so) a coupla months ago. for giggles, i pulled one of my 2 smokies & paired the remaining one with a valve arts (my soultone has ext bias points), & goldarn if the amp's tone reaped almost as much benefit from just the one, as opposed to using two! howzat for saving the toob life of my gec's?! go figger!


    ml
     
  17. Blue Strat

    Blue Strat Member

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    Interesting!

    Those GECs might outlast you anyway, so why not just enjoy them?:drink
     

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