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High gain tube for Phase Inverter?

Discussion in 'Amps and Cabs [BG]' started by BlacknWhite, Dec 17, 2017.


  1. Was considering swapping out my PI tube with a higher gain tube like a AT7 or AX7.... What I was hoping to achieve was more gain/signal sent to the power section to get more break up at higher volumes. Will this make a noticeable difference and is there any downside to doing this?
     
  2. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    I'm not a tube guy, but I've studied a lot of amp circuits. The gain of a class-A amplifier stage depends on both the characteristics of the active device, and the configuration of the circuit. A phase inverter circuit is designed so that both phases have the same gain, but opposite phase, and that gain is close to unity. Also, the signal fed to the phase inverter is typically throttled by cathode feedback from the speaker output. So, I doubt that a higher gain tube will make much of a difference.
     
  3. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Frances is correct, the typical PI circuit is very low gain but more important is the lower drive impedance that a 12AU7 provides.
     
    Passinwind, BassmanPaul and aborgman like this.
  4. A 12AU7 has a higher plate dissipation than an AT or AX. Exchanging one for a higher gain tube might well lead to a shortened tube life for the replacement. The designers of your amp were pretty smart cookies so second guess them at your peril.
     
    agedhorse likes this.
  5. This is why I ask first lol. Thanks guys
     
  6. Drgonzonm

    Drgonzonm

    Sep 4, 2017
    American SW
    What brand and year of amp? Fender used three different PI designs in his early amps. CBS used long tail PIs when they took over fenders.
    And what type of power tubes are you driving.
    12ax7's are used in a lot of tube amps.
    IMO, experiment with the amp.
     
  7. chaosMK

    chaosMK

    May 26, 2005
    Albuquerque, NM
    Hi-fi into an old tube amp
    If you can use something as a clean boost in your FX loop it may achieve desired result.

    I do this with my Bass 400 by running my Sansamp BDDI in the loop at Line level. What I like is a relatively clean signal through the preamp stages of my amp, while the power tubes are pushed closer to a saturation/breakup threshold.
     
  8. If you want more gain, the PI is not the place to look for it.
    You didn't say what amp you want to push into power tube breakup.
    Some have an unused half of a dual triode already there, looking for something to do.
     
  9. Its a Traynor YBA200-2. I had it replaced as I cracked the chassis at a show and it was under warranty so they just replaced the whole thing. When my new one showed up it wasn't breaking up like the previous one. Opened it up and realized they had switched from Sovteks to JJ and EH pre's. I had another JJ AX7 laying around that's the only reason I was considering it haha. I have a set of sovtek ax7s and a 12au7 on the way
     
  10. Drgonzonm

    Drgonzonm

    Sep 4, 2017
    American SW
    I looked up some of the traynor schematics. And saw traynor used 12ax7's as PI in at least one 4 power tube amp. As I said experiment.
     
  11. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Was it the model he owns?

    I wouldn't recommend doing something unless I was darn sure that there wasn't going to be a problem.
     
    BassmanPaul likes this.
  12. Drgonzonm

    Drgonzonm

    Sep 4, 2017
    American SW
    I looked up some of the traynor schematics. And saw traynor used 12ax7's as PI in at least one 4 power tube amp. As I said experiment.
    I am in error, stick with the 12au7.
    From ampbooks, a partial discussion on the longtail PI {Having a large tail resistance limits the maximum output voltage swing, which is of particular concern for high-power amps. For this reason a smaller tail is often required, which causes the inverted phase to have substantially more amplification than the non-inverted phase.}
    The long tail PI on the YBA200, is setup different some of the older Traynor amps, the larger tail resistance on the yba200 is much greater than the YBA 3, which used a 12ax7.​
     

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