Significance of PC: and TC: on TAD 6L6's

Discussion in 'Amps and Cabs' started by mbruffey, Aug 20, 2005.


  1. mbruffey

    mbruffey Member

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    Hi All!

    Another of my infamous questions . . . .

    I have a couple of questions about the numeric indicators PC: and TC: on TAD tubes. I realize the numbers assigned to these will vary from brand to brand b/c equipment used to make measurements varies. So here, I'm only asking about the TAD's. (I also understand the numbers will have different meaning for different style tubes, even by same company.)

    First, what are the highest and lowest numbers for each you have seen on the TAD6L6GC? On the TAD6L6WGC?

    Second, assuming fixed bias, I would expect a lower PC: would draw less current at idle than a higher PC: for the same tube, making less work for the transformer. Am I right?

    Third, what about TC:? If I'm on track about PC:, then does a lower TC: also equate to less load on the transformer?

    OK, what am I after? I'm still gathering information to help me determine if I can use a lower PC: and TC: rating to calm down my (cathode biased) amp.

    Thanks again for all helpful comments on my other posts. I really appreciate the group here!

    Mark

    Thanks,

    Mark
     
  2. Blue Strat

    Blue Strat Member

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    PC (plate current) will affect the idle current of your amp whether fixed or cathode biased, though to a lesser degree in a cathode biased amp.

    TC( Transconductance) won't have much if any influence at all.

    When you say "calm down'" do you mean you want to lower the idle current?

    BTW, your transformer probably doesn't care if it was spec'd properly. The tubes DO care if they're being run beyond 100% of maximum plate dissipation.
     
  3. Blue Strat

    Blue Strat Member

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    Since he's talking about a cathode biased amp, the grid voltage is unknown but related to the cathode voltage which varies with current draw which varies with the inherent idle current (at set parameters) of the particular tube.:D
     
  4. mbruffey

    mbruffey Member

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    I'm sorry about the confusion. I'm attempting to understand/learn about my amp. I want to know how it works as well as how to use it! The effects of TC and PC on a cathode biased amp is really my focus. My apologies again.

    Can anyone answer my question about the ranges for PC and TC you might expect to find in the TAD tubes?

    Thaks,

    Mark
     
  5. mbruffey

    mbruffey Member

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    Now, am I right to reword this as:

    In a cathode biased amp:

    (1) Inherent idle tube current determines/affects current draw.
    (2) Current draw determines/affects cathode voltage.
    (3) Cathode voltage determines/affects grid voltage.

    Therefore, inherent idle tube current determines/affects BOTH cathode and grid voltage.

    Since inherent idle tube current determines/affects BOTH cathode and grid voltage. A tube with a lower inherent idle current will also cause lower the cathode and grid voltages, reducing the load on the transformer.

    Am I anywhere close?
     
  6. mbruffey

    mbruffey Member

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    hmmmmm, maybe I should just ask, "what tubes do you run in your vibroclone?" . . . .
     
  7. Blue Strat

    Blue Strat Member

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    With all due respect OTM, in 25 years of engineering, school, working with employees, customers, etc, I've learned that presenting ALL the facts can be useful for some audiences, BUT, you must know your audience and select your material appropriately.

    I'm all for "teaching the person to fish" rather than just handing him the fish, but there's a fine (?) line between giving the subject every conceivable bit of information he could possibly need and putting him to sleep. ;)

    Yes, a tube with higher inherent idle current (PC, which is arbitrary, but relative to all other tubes measured on the same test gear) will draw more idle current in ANY amp regardless of whether it's fixed or cathode (aka "self") biased.

    The cathode biased amp will keep the maximum and minimum cathode currents within a tighter range than a fixed bias amp.
     
  8. Blue Strat

    Blue Strat Member

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    PS, Mojo, or whoever you bought the tubes from, is the best source of the range of PC for those tubes. After all, I'm sure they've tested thousands of them.
     
  9. Killa-B

    Killa-B Member

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    I've enjoyed this thread tremendously, thanks Old Tele and many thanks to you also, Mike.
     
  10. mbruffey

    mbruffey Member

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    According to one source, 29 and 57 are the respective low and high numbers for PC:
     
  11. mbruffey

    mbruffey Member

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    I don't mind the technical responses, even though I'm a neophyte. The information I needed and recvd was that PC/TC factors in with bias. If I understand correctly, the circuit will exhibit fewer/slower electron flow as TC increases relative to PC. Looks like I want tubes with PC near the bottom of the heap and TC near the top. At least that's what I'm going to try . . . . S-T-O-P me if I'm wrong! QUICK!
     
  12. mbruffey

    mbruffey Member

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    Oh, I forgot to tell you, this amp has a Cathodyne PI, too, and no negative Feedback.
     
  13. mbruffey

    mbruffey Member

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    According to TAD in Germany, the normal range on PC: (plate current) for the 6L6WGC and the 6L6GC is 25-60 ma. I have not recvd and answer about TC: (transconductance) yet. Mark
     
  14. mbruffey

    mbruffey Member

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    The TC: for 6L6 TAD's will APPEAR as 500-700 on the tubes; however, because of the setup of the equipment, the figure needs to have an additional "0" added to the end, yielding an actual range of 5000 to 7000 mhos.

    THIS explains why on my TAD 6L6WGC's the TC: is reported as only 613 on the printed carton (and on the tube itself)! It's actually 6130 mhos, on whatever special equipment TAD is using.

    Hope this info is helpful to those who may care.

    Mark
     
  15. tonedaddy

    tonedaddy Member

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    What if you're working on /13's?
    :D
     

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