12BZ7 info?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Mike K, Dec 23, 2004.

  1. Mike K

    Mike K Silver Supporting Member

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    I was given this 12BZ7 tube and told that it could be used in V1 for more gain. Has anybody had any experience with this valve?

    Happy Holidays
     
  2. TheAmpNerd

    TheAmpNerd Member

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    My buddy swears by them. Though you might try it
    in the PI slot. That should gain things up quite a bit.

    Try it in both spots then let us know what you found out.

    Don't worry it won't fry your amp either.
     
  3. Blue Strat

    Blue Strat Member

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    Not sure where you're "gain" spec is coming from, but the specs say they have the same gain as a 12AX7....u=100.
     
  4. Wakarusa

    Wakarusa Member

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    Same amplification factor (100) but half the plate resistance.
     
  5. TheAmpNerd

    TheAmpNerd Member

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    Guys,

    I'm not talking about the specs. Stick the tube in the amp.
    It doesn't behave like a 12AT7 at all.

    Hmmm, so it has the same gain as 12AX7 and 1/2 the plate resistance, the result in an amp is more gain,
    sooner break up.

    I think it has twice the plate dissapation too.

    Try it and see.
     
  6. Wakarusa

    Wakarusa Member

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    12AX7 : 1.2watts
    12BZ7 : 1.7watts
     
  7. TheAmpNerd

    TheAmpNerd Member

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    Thanks guys.

    I knew it was more, just not sure how much.
    Not that much more than a 12AX7 either.

    Also note this tube is in a taller package then
    the other 12A?7 tube types so you can't use
    a standard tube shield on it.
     
  8. Jon Silberman

    Jon Silberman 10Q Jerry & Dickey Gold Supporting Member

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    OTm, would it possible for you to do the same evaluation as above also for the 12AT7?

    Also, when the thread finishes playing out, I plan to ask the Admin's to archive it - it's highly informative on these topics.
     
  9. Wakarusa

    Wakarusa Member

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    Of course:

    Assuming we still have 100K plate and are driving into a 500K input impedence.

    12AT7: rp = 10.9K-ohm; mu = 60

    AV = (60*83.3K)/(83.3K+ 10.9K) = 53.06


    There is, of course, a dirty secret behind some of this. Plate resistance (Rp) is a function of transconductance (Gm) which varies with plate voltage (Ep). Generally, Rp is inversely proportional to Ep and the relationship is non-linear.

    The values for Rp given so far have been for Ep of 250V. For Ep = 100V, Rp is closer to 15K

    (just when you thought it would be easy :))

    However, you can still use the equations for basic comparisons of these tubes as long as we keep in mind that the actual gain may be a bit different in your particular amp.
     
  10. TheAmpNerd

    TheAmpNerd Member

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    Yes,

    In fact I have wondered about that. Now I'm also
    wondering how we go about determining the
    next circuit load (Raa)?

    Techus Maximus....I can read the fine print
    right above the arrow pointing to an old
    tele man fooling around with an amp
    that Wile E. Coyote is checking out.

    beep beep....
     
  11. Wakarusa

    Wakarusa Member

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    Oops. You are correct. No more egg-nog before I post.

    (I was thinkin' of... umm... nevermind. I'll just take my lumps :jo)
     
  12. Jon Silberman

    Jon Silberman 10Q Jerry & Dickey Gold Supporting Member

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    Calling Old Tele man!

    When you wrote on the Weber board:

    "...if you sent the idle by CURRENT, then you can use this equation:

    Ip.q = (70%*Ppd)/Vp.q

    where:
    Ip.q = Plate quiescent (idle) current, amps
    Vp.q = Plate quiescent (idle) potential, volts
    Pp.d = Tube PLATE dissipation rating, watts

    Pp.d for JJ 6V6S is way "under" rated at 14W (the original NOS 6V6GTA spec), and probably good for 16W or more (nobody knows for sure)."

    Can you please explain that last sentence in super-plain English for the technically-impaired (i.e., me)? :) IOW, does it mean the JJ is "probably good for 16W or more (nobody knows for sure)." I assume this means you can get more power out of it relative to other "true" 6V6s? And what does the ability of a plate to "dissipate" do, specifically, to cause or allow more power?

    Also:

    "... it's part of the research I've done for a book about Leo Fenders' medium- and high-power tube amps."

    What is the name and author of that book? Is it available for sale now and where? And if not yet, when is it expected out?

    Thanks! Jon S.
     
  13. TheAmpNerd

    TheAmpNerd Member

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    Old Tele man,

    How did you go about calculating the resistance
    of the next circuilt load of 500K?

    The suspense is killing me.
     
  14. TheAmpNerd

    TheAmpNerd Member

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    Suspence indeed! Sticking the answer down there
    in the post script...very funny.

    Okay, that is what I've been trying to figure out where
    that majic 500K number came from.

    Now when I need it, I'll just click my spit shined USMC
    boots together three times and utter

    "There is no plce like tone...."

    You're not too bad for a squid, thanks Tele.
     
  15. Blue Strat

    Blue Strat Member

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    Gain does not equal breakup. This is a common misconception among guitar players.

    This tube breaks up do to the plate voltage lowering effect of different internal resistance.
     
  16. Wakarusa

    Wakarusa Member

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    But in this application the extra gain in the first stage will clip the following stages earlier (all else being equal). I'm betting the original post was a reference to overall behavior of the amp, not necessarily a reference to when the first stage would clip.
     
  17. jimmisixstrings

    jimmisixstrings Member

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    I read all the posts here looking for the answer to the sound/tone question, I got a lesson. Don't get me wrong, I'm happy for all the information but did anyone actually try it in a Fender amp and can they tell me how it worked out? I'm thinking about sticking one or more in my Super Reverb, any suggestions/warnings?

    Thanks in advance,
    jimmi
     
  18. TheAmpNerd

    TheAmpNerd Member

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    Yes,

    A friend of mine tried this in his vintage deluxe amp.
    He likes it.

    Of course, he is also the same guy who drilled holes in the
    same vintage chassis for derating.
     
  19. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    Make sure the circuit can support the extra current draw, and take note that the glass envelope is 3/8" taller than a 12AX7. Also note that 12BZ7 can be vibration sensitive in a combo, and may be more susceptible to microphonics. If you're cool with all that, plug one in and check it out.

    I use 'em in my rack setup, one for the main output of my Triaxis, and one for the main input of my Mesa 20/20 power amp. The benefit in those positions is that you get that extra couple of dB volume gain, per tube, which makes for a louder output, with earlier power-tube breakup. Or if you adjust for the same volume, you drop the noise floor several dB, yielding a more clear, clean and dynamic setup.

    I've investigated them in the V1 position of my Triaxis, and found them to be tonally fairly neutral (or perhaps a tad bright), with a very clean quality, and do not recall them causing earlier breakup in Lead modes. Articulate, dynamic, tight, and snappy. By the way, I've only tried black-plate 12BZ7s (black-plate tubes seem more dynamic to me, in general), and there seemed to be no significant tonal difference between 4-5 different-construction variants.

    Hope this helps.

    - T
     
  20. Jack Briggs

    Jack Briggs Member

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    What site? Everytime I go to http://www.nebsnow.com/LordValve there is nothing but a pic of a dog, some description of services, some links buttons to the left, some of which contain nothing???????? Only an email link to the Lord himself is what I see. Show me what I'm missing!!!
     

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