Sub a 12AT7 in the PI?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by rockon1, Jul 12, 2006.

  1. rockon1

    rockon1 Member

    Messages:
    11,454
    Joined:
    Aug 12, 2005
    Location:
    Stamford CT
    Ive read about guys substituting a 12AT7 in the PI of an amp which normally takes a 12AX7. Supposedly to drive the power amp harder(better power tube saturation) or something. I dont really understand the logic behind it if thats the case.I would think a 12AX7 would drive it harder.Any thoughts on this,thanks Bob
     
  2. tonezoneonline

    tonezoneonline Member

    Messages:
    437
    Joined:
    Dec 15, 2003
    Location:
    central Illinois
    It's easy enough to try and see if you like.It won't hurt anyhting but you are right .A 12AX7 is a higher gain tube that a 12AT7.
     
  3. Rich M

    Rich M Member

    Messages:
    422
    Joined:
    Dec 3, 2004
    Mullard CV4024's (mil 12AT7) are killer in PI's
     
  4. Blue Strat

    Blue Strat Member

    Messages:
    30,029
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
    Lot's of guys like using the Mullard CV4024s for this application.

    Forget about gain, it has nothing to do with it. Forget about drive. The circuit only asks for a certain amount of gain/drive, changing tubes has no effect. The fact that a 12AT7 is misbiased (wrong cathode bias resistor) and the tonal difference, in general, of a 12AT7 vs a 12AX7 is why it sounds different. Examining the "science" any more deeply is a waste of time, you hear what you hear...no need to justify it.
     
  5. donnyjaguar

    donnyjaguar Member

    Messages:
    4,244
    Joined:
    Nov 4, 2005
    Location:
    Canada
    FWIW, I've seen the exact same circuit used in one amp that comes from the factory with a 12AT7 and another with 12AX7. The differences of the two become more apparent when you are running no negative feedback. Theoretically the 12AX7 has the ability to drive the output stage a little harder.
     
  6. Blue Strat

    Blue Strat Member

    Messages:
    30,029
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
    Theoretically, a 12AX7 can produce a voltage gain of 100 but PIs usually have gains well below 20.
     
  7. donnyjaguar

    donnyjaguar Member

    Messages:
    4,244
    Joined:
    Nov 4, 2005
    Location:
    Canada
    Actually a gain of about 10. You'll get much more in a conventional voltage amplifier but it'll likely be somewhere in the order of x50. To net the 100 I suspect you'd have to configure a current source for the plate. I've seen amplifiers that do this but the downside is there is no local degeneration so the output is arguably less linear. The gain is also difficult to take advantage of in conventional circuits.
     
  8. Igneous

    Igneous Member

    Messages:
    1,880
    Joined:
    Aug 24, 2004
    Location:
    Mass.
    +1:BEER
     
  9. Blue Strat

    Blue Strat Member

    Messages:
    30,029
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
    Good designs never ask for the maximum or even typical spec from a component. The case is the same in solid state where transistor Betas vary widely for any given transistor.
     
  10. JohnnyL

    JohnnyL Member

    Messages:
    422
    Joined:
    Sep 5, 2002
    I did some tube swapping last night in my Rivera Knucklehead R. I had a Telefunken 12AT7 I've been waiting to try in the PI. I can clearly hear the difference, even though it is difficult to explain what the difference is.
    And speaking of hearing a difference....I installed an RCA Blackplate in channel one. A Valvo in channel two and a GE Mullard in channel three. What a lineup. All three channnel's characteristics responded well to each tube placement delivering really good tonal changes.

    Try the tube. Try lots of tubes if possible!

    Good Luck,

    JohnnyL
     
  11. donnyjaguar

    donnyjaguar Member

    Messages:
    4,244
    Joined:
    Nov 4, 2005
    Location:
    Canada
    I agree BlueStrat (my strat is brown, but I digress). However, many tube amp's run the output tubes outside of their design maxima.
     
  12. rockon1

    rockon1 Member

    Messages:
    11,454
    Joined:
    Aug 12, 2005
    Location:
    Stamford CT
    Well I did some 12AT7 PI swapping in my JSX. Tried a short Sylvania,short black plate RCA,EI,GE TM 5751,Mullard(not a CV4024),short Tung Sol. None of them sounding as good as the long gray plate RCA I had in there. To my ears most made it weaker/thinner sounding. The 12AX7 RCA stays.....
     
  13. elkym

    elkym Member

    Messages:
    1,323
    Joined:
    Mar 15, 2007
    Location:
    Provo, UT
    i did the same swap in my peavey VK112 (12at7 instead of the stock 12ax7). I love it... somehow softer, and maybe smoother? Also, I felt like I gained headroom... but I also swapped out my 6L6's-- so I'm not sure...
     
  14. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

    Messages:
    8,778
    Joined:
    Oct 14, 2005
    Location:
    Berkeley, CA
    Bingo! Made it "weaker/thinner" says it all - you don't like that swap. Others may have said that the swap makes it cleaner/tighter and raved about it. Clearly it is not for you, in that particular amp.

    Couple of more thoughts:
    The common early-'80s Mullard CV4024 is not like other 12AT7s - it has a fatter/thicker low-mid girth that no other 12AT7 gets close to. Even older ('60s) Mullard 12AT7, with the same "654" type code, are thinner/brighter sounding.

    In my Richter tweed Bandmaster-inspired "5E7+" amp*, I found I liked one particular 12AT7 best (a Ken-Rad), because it brought out the best clarity and chime (and less breakup). I tested it, and it was extremely weak. After going through other 12AT7 as backups, I concluded that this one could not be reproduced. Then I tried an old E180CC (aka 7062), which is like a lower-mu version of the 12AT7. Jackpot!! That particular tube is to the 12AT7 as a 5751 is to the 12AX7, and it works perfectly for my taste.

    - Thom

    *has a split-load (or cathodyne) phase-inverter, not the long-tail pair that is typical of more modern amp designs.
     
    Last edited: Dec 16, 2008
  15. ChickenLover

    ChickenLover Member

    Messages:
    1,696
    Joined:
    Sep 7, 2006
    Location:
    Florida
    Just my own lamebrain hypothesis but I really think the BF Fender PI was designed for a 12AX7. The values just scream 'designed for a 12AX7' to me. But I think someone stuck a 12AT7 in there and thought it sounded cleaner and back then cleaner was more betterer. Then CBS later changed the PI to work better with a 12AT7...and it does.

    Here's my question: You'd think that amps that get lots of power tube distortion would prefer to have a 12AT7 PI (with adjusted PI values for the 12AT7) because of the lower plate impedance. And the amps that rely on power tube distortion the most are EL84 amps. So I would have thought that an optimized 12AT7 circuit would be a great choice with EL84 amps. Yet I don't know of one (except homebrews...some Rocket brewers said they tried it and preferred it).
     
  16. tjs

    tjs Senior Member

    Messages:
    4,259
    Joined:
    Sep 8, 2004
    So what would be an example of an optimized 12AT7 PI circuit?
     
  17. Buddy Boy

    Buddy Boy Member

    Messages:
    2,640
    Joined:
    Dec 15, 2006
    Location:
    Springfield,MO
    +1 on the Mullard CV4024. Only way to go for BF Fenders.
     
  18. ekkybedmond

    ekkybedmond Member

    Messages:
    1,563
    Joined:
    Oct 26, 2008
    Instead of starting anew:

    Had a Mullard CV 4024 in PI in my JCM 900 for a while, and all the time I was gassing for a new speaker: thin, weak, sharpish sound.

    Back in went the Siemens ECC83 and bingo: crunch time again.

    So a CV4024 or any other 12AT7 is not for my/this Marshall.

    Have to agree: if it's not designed for a 12AT7, don't use one. Whatever the paper benefits might be
     
  19. Guinness Lad

    Guinness Lad Silver Supporting Member

    Messages:
    15,339
    Joined:
    Nov 11, 2004
    Location:
    On top a mountain of Chocolate Chips
    I always think, "What would Leo do?" 12at7 should give you more clean headroom.
     
  20. guitarcapo

    guitarcapo Senior Member

    Messages:
    2,337
    Joined:
    Mar 16, 2010

    This is where you lost me.
     

Share This Page