Best choice for Phase Inverter ?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Sol, Sep 6, 2006.

  1. Sol

    Sol Member

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    I've been told that the PI is the hardest working valve in an amp, and
    consequently finding the right valve for this position is important in getting the most from our amp's.

    The two amps I'm replacing preamp tubes in are : 50Watt Lead&Bass Marshall
    head.
    And: A '66 Vox AC30 Top Boost.

    I know where the PI is on the Marshall, but on the Vox I *think* it's the third along (Looking from the rear,right to left).
    Am I right on that ?

    I know they dont need to be balanced ,or anything like that, but which ,of the current production do people round here regard as the best for this position?

    Cheers,
    Sol
     
  2. Blue Strat

    Blue Strat Member

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    In new production 12AX7s, the Tung Sol Reissue is one of the best for ANY position in your amp.
     
  3. Mr. dB

    Mr. dB Member

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    On the AC30TB, if the inputs are on the left and the power switch is on the right, the PI tube is the second from the right. It's far-right on AC15s and non-TB AC30s, but the added TB tube was tacked onto the end.

    That's if you have the integral TB. If it's rear-panel add-on TB, then the PI is the one nearest the power switch.
     
  4. trdlasvegas

    trdlasvegas Member

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    Hey Mike for once I gotta agree with you. I'm having a hard time keeping them in stock. Every amp I put them in not only does the owner call back to say how great it sounds, but his buddies come in and buy some for their amps.

    New Sensor really scored on this one.

    -Tony
     
  5. Boogs

    Boogs Member

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    I use Mullard CV4024s sometimes, as they're not really expensive for NOS, and Jan Phillips 12AT7WCs are an even better deal. For new production, I bought some JJ 12AT7s as JJs are usually sturdy....we'll see how this pans out!

    BTW, is there an advantage to using 12AX7s in the PI slot rather than 12AT7s? I always thought, and I could be mistaken, that the 12AT7s are sturdier, and so ideally suited for PI applications?
     
  6. Blue Strat

    Blue Strat Member

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    :BEER

    On the other hand, since you don't carry ANY NOS tubes, it might be cool in indicate that in your posts regarding NOS tubes (in the interest of full disclosure):crazy
     
  7. Blue Strat

    Blue Strat Member

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    What OTM said. This is the reason NOT to use a 12AX7 in a reverb driver circuit designed for a 12AT7.
     
  8. conundrum

    conundrum Member

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    Here's the conversation with people that have reverb problems 9 out of 10 times.


    Customer: "My amp seems really quiet with the reverb engaged."
    Me: "Did you put a 12AX7 in the driver slot?"
    Customer: "Uh yeah, I did that cuz the internets told me it would give me more gain. They're interchangable right?"
    Me: "No."
    :rotflmao
     
  9. Hard2Hear

    Hard2Hear Member

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    Forgive me for a simple question, I'm not a tech. :)

    So should I be using a 12AT7 and not a 12AX7 in my PI position on my /13? I think this is the tube closest to the power tubes, right? Just trying to get things right while I'm doing a little tube swapping. But I put a 12AX7 in there.

    Thanks!
    (sorry to drop in on the thread, it just applied at the right time)
     
  10. Blue Strat

    Blue Strat Member

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    What tube does that socket call for? Different amps are designed for different tubes.

    Start with the CORRECT tube to see what it sounds like. Feel free to switch to another tube knowing that the amp wasn't designed to use that tube, but that it's ok to make the swap (in a PI socket).
     
  11. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    Sol -

    You may be interested in checking out how I arrived at my PI tube choice for a newly acquired amp (tweed Bandmaster clone). I wrote about it in this thread. Because of the way I use this amp, I found the choice of phase-inverter to be very influential to its tonal and dynamic character.

    - Thom
     
  12. Sol

    Sol Member

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    Thom, thankyou for that link, good stuff !

    Thanks for all your replies, and advise.What seems to come across is that
    the new Tung Sol 12AX7, is an all round rated valve for all positions in an
    amp, including the P.I. spot.

    So, is this clearly the best current production 12AX7 out there at this time?

    I say this as the JJ Tesla types also have a solid rep for reliability and good tone?

    Oh, before I forget the Sovtek 12ax7LPS, with its spirall fillament, has been mentioned as a good valve for the P.I. slot..What say you ?

    Cheers,
    Sol
     
  13. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    I believe the Sovtek 12AX7LPS can be a good PI choice, but I have not had direct experience with that tube as a PI, so I'm just going on the same rumors you are. I do like the Russian "Tung-Sol" 12AX7 for its improved clarity and smooth feel, and can say it is my favorite current-production 12AX7 - yes, better than any current JJ I've tried.

    Current-production tubes are not my forte, though, so I hope others, who've experienced the full range of current-production options, will speak up too.

    :YinYang (what's this mean, anyway: keep a balanced, but joyous perspective?)

    - T
     
  14. shizzaq

    shizzaq Supporting Member

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    Anybody got any input about which NOS 12AX7 tube works best for amps that use that tube in the PI slot?
     
  15. Blue Strat

    Blue Strat Member

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    I can't think of one that's necessarily better than another. Like most situations, it's a matter of personal preference.
     
  16. teleman55

    teleman55 Member

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    I got Fender amps that use a 12AT7 PI tube. JAN Phillips 12AT7 works great and is dirt cheap for a NOS tube. The military must have needed a bundle of them back when.
     

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