12AX7 as a phase inverter tube?

Discussion in 'Amps and Cabs' started by Grand Pappy, Mar 3, 2020.

  1. Grand Pappy

    Grand Pappy Member

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    I purchased an inexpensive 15W Tube amp for my 12YO Grandson
    It is of Chinese manufacture w/ 2 ea. 6V6 power tubes is a 12AX7 in the phase inverter socket
    It has a solid state preamp.
    Does it matter what brand / value of 12A** tube you use in the phase inverter?
    Will a Mullard 12AX7 give a better tone the the stock Chinese branded 12AX7?
    Will a different value, like a 12AY7, 12AU7, 12AT7 be better in the phase inverter position work better
    Does any of it matter?
     
  2. rmconner80

    rmconner80 Cantankerous Luddite Silver Supporting Member

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    I've found that in some amps different 12AX7s make noticeable differences in what comes out of the amp. In other amps not so much. In my experience it can come down to the individual examples just as much as the brand / construction. In other words, it is possible to find a bigger difference within groups (two Chinese manufactured 12Ax7s) than between groups (between the Mullard labeled and the Chinese).

    The component values in the circuit will dictate the operational points of the tube. Currently the circuit is presumably optimized for the 12AX7. So while you can likely swap a 12AY7, 12AT7, etc. for a 12AX7 with no ill effects, it probably will produce less than optimal results from an operational standpoint.
     
  3. matt1969

    matt1969 Member

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    Different types might make a difference in overall gain but it won't really matter too much what brand you use. A NOS Mullard 12AX7 might sound different, but not necessarily better than a newer Chinese. That's a matter of opinion. Who knows, a NOS Mullard 12AX7 could sound like total crap.
     
  4. Grand Pappy

    Grand Pappy Member

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    Thanks guys
    I just don't want to put lipstick on a pig by replacing the stock Chinese tube w/ the more expensive Mullard, if it's not going to improve tone, or performance
     
  5. sunkidd

    sunkidd Member

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    but, if the phase inverter calls for a 12AT7 on the tube chart of a Super Reverb,
    what would happen to the sound if replaced with a 12AX7?
     
  6. rawkguitarist

    rawkguitarist Member

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    Man... I'm not a big tube roller. I wouldn't worry about putting a different 12AX7 in there. If you want it to clean up you could go with a lower gain 12 - like a 12AT7. Depending on the circuit any change might not make a huge difference.

    NOW - a *humongous* tonal change would be a speaker. The speaker and cab are half the tone of an amp in my experience. For instance my Carol-Ann Tucana 3 sounded terrible through an old Fender 2x10 closed back cab. That amp is magical through a good 1 or 2 celestion 12's.
     
  7. doc

    doc Supporting Member

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    It depends on the specifics of that amp's circuit design. In some amps the phase inverter generates some distortion or otherwise makes a noticeable contribution to the tone of the amp. In others its run in a way that keeps coloration to a minimum. I'm guessing a change might be noticeable but mild.

    sunkidd - more drive and crunch.
     
  8. Grand Pappy

    Grand Pappy Member

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    He is running it thru my Avatar 1x12 cab loaded w/ a Celestion Green Back
    Sounds ok for what it is.
     
  9. ProfRhino

    ProfRhino Member

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    A Sovtek LPS would probably be a solid step up, and they are dirt cheap. :idea

    there are Mullards, and there are Russian imposters.
    absolutely not even close ...
    the Russians are an alternative to the LPS, (almost) identical, but 3 times the price ! :p

    and obviously I completely disagree about what has been said about real Mullards !
    whether you want to invest that much in a cheap amp is a different story.
    ymmv,
    Rhino
     
  10. ProfRhino

    ProfRhino Member

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    going from a 12AX7 PI to an AT is worth a try, the outcome will depend.
    the other way round is not recommended without clearance from a competent tech.
    Rhino
     
  11. Sirloin

    Sirloin Supporting Member

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  12. Grand Pappy

    Grand Pappy Member

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    it is a Pulse 15 watt head. Got it dirt cheap
     
  13. Laurence

    Laurence Silver Supporting Member

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    More drive in the PI and to the power section, but not entirely pleasant depending on what you want to hear.
     
  14. Promit

    Promit Member

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    These two are the answers you needed. Odds are this Chinese amp has cloned some existing well known circuit, and it will be much easier to help if you can identify the original design this one is derived from.
     
  15. easyed

    easyed Silver Supporting Member

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    Did I miss something? Is the PI long-tail pair (LTP) or Cathodyne? 12AX7's are common in Cathodyne PI's; for LTP PI's the 12AT7 is more common, but I've come across several that use 12AX7'.
     
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  16. oneblackened

    oneblackened Member

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    Less headroom.

    Depends on the amp's heritage really - Marshall based amps almost all use 12AX7s for LTPs, as well as quite a few Fenders and Fender based amps (see: Mesa Mark series).
     
  17. Grand Pappy

    Grand Pappy Member

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    Amp came w/ a 12AX7 in the phase inverter position. I would guess it is Fender based because of the 2 ea. 6V6 power tubes?
     
  18. darkwing

    darkwing Member

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    Can't speak to the 6v6 / 12ax7 setup, but, I do have a Marshall 4203 that uses EL34 / 12ax7 PI (solid state pre) and there's a very noticeable tonal difference swapping different 12ax7s (Ruby, new Mullard , various NOS, etc) I've found some really open the clarity and give more sparkle versus others that sound blanketed.
     
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