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Thread: KT66 questions

  1. #1
    EFK is offline
    Senior Member
    Join Date
    May 2008

    KT66 questions

    I have a cathode bias amp which originally came with 6l6gc. It runs B+ on the tube at idle at approx. 480-490 volts (depending on the local power company) and has 1.5K screen resistors. The original common cathode resistor was 330 ohm, so this thing was biased pretty hot. Now, despite it maybe being contrary to tube survival, I like to use KT66 in this amp - been using Valve Art although I've also used GT chinese. I raised the common cathode resistor to 500 ohms and after doing all the math have about 21 w dissipation at idle. My OT measured 8,777 ohms primary impedance at 16 ohm load which I think is pretty good for KT66? (Let me know if I'm wrong). Anyway, I just took a look at some of the old KT66 data sheets and it states that I should be using 500 ohm cathode resistors *individually*, this supposedly being crucial. My question is - why? Why is common cathode resistor bad? Am I running it too hot? I've been running these Valve art tubes in there w/ an attenuator w/ the amp up at about 4 o'clock on the volume continuously, 1-2 hours per day for the past 8-10 months - no problems at all. ??? Also, should I have some other resistor/s involved on the socket somewhere? Just curious about all of this as I think KT66 is the king mother of tubes (in this amp anyway) and could never go back to 6l6....

  2. #2
    Supporting Member
    Join Date
    May 2006
    Staffordshire UK
    It's preferable for each tube to have their own cathode bias resistor, then they can each find their own operating point - they may well self balance. Whereas sharing, they have to reach a compromise, and the currents in each leg of the OT primary are more likely to be unbalanced, unless the tubes are well matched.
    But if it works fine then don't worry about it.

  3. #3
    EFK is offline
    Senior Member
    Join Date
    May 2008
    I see - if it ain't broke don't fix it, right? Well they seem to be holding up well so I suspect you're right.

    Are these supposed to like the same OT primary impedance as 6L6GC? I've got 2194 at 4 ohm tap, 4389 at 8 and 8779 at 16. I use the 16 into a 16 ohm load but I've been wondering if this is considered a match for the impedance the tubes want to see?

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