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A 12Ax7 and a 12Au7 met in a bar...

Discussion in 'Amps and Cabs [BG]' started by Dr. PhunkyPants, Jul 20, 2004.


  1. Dr. PhunkyPants

    Dr. PhunkyPants Guest

    Aug 11, 2002
    USA
    Can you seriously plug 12Ax7's into a 12Au7 socket? What happens if you do?
     
  2. jammadave

    jammadave Rudderless ship Supporting Member

    Oct 15, 2003
    Wash DC metro area
    Yes.

    You get a little more gain and sharper overdrive characteristics.
     
  3. Transverz

    Transverz believer of the Low End Theory

    May 3, 2004
    Los Angeles, CA
    I'm no amp technician but, don't they fit into the same socket?

    From what I know (which isn't much), the 12AX7 is interchangable with a 12AU7, just that the AX has more gain than the AU.

    Eh? Eh??? :eyebrow:

    :ninja:

    -T
     
  4. Transverz

    Transverz believer of the Low End Theory

    May 3, 2004
    Los Angeles, CA
    Doh, you beat me to it. Damn IE crashing everytime it takes me a little longer than 10 seconds to post.

    AM I THE ONLY ONE?! :bawl: This sucks.

    But yeah, more gain. Better for bass applications when called for.

    But can someone explain why solid state amps that use tubes in the signal use 12AX7's instead of a 12AU7? (For example, the ADA Microtube 100/200)
    ???

    -T
     
  5. DougP

    DougP

    Sep 4, 2001
    If a tube has more gain, does that make it more effecient for the rest of the electronics?
     
  6. Lockout

    Lockout

    Dec 24, 2002
    Illinois
    What do you mean by "sharper"?
     
  7. Dr. PhunkyPants

    Dr. PhunkyPants Guest

    Aug 11, 2002
    USA
    ...I wish the super-genius-expert-imatubescientistandillforgetmoreabouttubesthanyoulleverlearn TB'ers will all weigh in.
     
  8. Dr. PhunkyPants

    Dr. PhunkyPants Guest

    Aug 11, 2002
    USA
    Bingo. That's the question. www.thetubestore.com has this chart where 12Ax7's are 100 on a gain scale of 1 to 100 and 12au7's are a 19.
     
  9. Transverz

    Transverz believer of the Low End Theory

    May 3, 2004
    Los Angeles, CA
    Sorry, it read wrong. I meant that the 12AU7 is quite possibly better for bass applications because of it's lower gain (generally and theoretically speaking, of course).

    And I think that though they are drastically different in gain scale, the 12AX7 is still usable when you want an overdriven, more grindy sound. There are tons of preamps that use 12AX7's in bass applications. Plus I'm sure that they are all used differently to not get so gainy when it doesn't call for it. Even used smoothly too.

    But yeah, all in application choice and use.

    -T
     

  10. Moi? ;) In answer to your first question, yes, you certainly can, BUT whether you'd want to is another matter entirely. Generally, if a circuit calls for a 12AU7, it's for a couple of reasons, namely more gain is not needed, and the tube will be passing a larger amount of low-end, and hence current, than a "standard" 12AX7 gain stage. If you sub a 12AX7 in its place, the sound will typically be VERY distorted and thin sounding, great way to turn a bass amp into a guitar amp.
     
  11. Dr. PhunkyPants

    Dr. PhunkyPants Guest

    Aug 11, 2002
    USA
  12. Bongolation

    Bongolation

    Nov 9, 2001
    California
    No Bogus Endorsements
    Why? It's a simple answer and you already got it: A change in gain. How that will exactly work out in your particular application can't be predicted at this remove, though I would never personally put a 12AX7* in a 12AU7 preamp application. I can't see that big of a jump in gain (5X) doing anything positive. Think about it.

    You want more complexity? Here's a page from my pard Bill Machrone on his experiments with the 12A*7 family of tubes.

    Lots of scope pics if that's edifying for you.
     
  13. jammadave

    jammadave Rudderless ship Supporting Member

    Oct 15, 2003
    Wash DC metro area
    I'm sorry, by "sharper" I used the only succinct word I could think of to compare the two sounds... kinda like a Metal Zone vs a DS1 I guess. When overdriven, the AX tube will sound more like a drive/dist set on 10 gain whereas the AU will sound more like a 5 or less on the gain knob...

    That whole 100 vs 19 on a scale to 100, that you mentioned, illustrates a larger difference than I thought there would be.
     

  14. That's all well and good, but it really only holds true to THAT particular Blues Junior. EVERY circuit has its own its own particular quirks and while the scope pics may be illustrative of that particular tube in that particular amp at that particular frequency, what you see may or may not happen in another. Some stages based on 12AU7's can be forced into cutoff, oscillation, and all kinds of bad noises if you sub a 12AX7. Don't be so hard on the guy; it's a legitimate question.

    BTW, if the thread's title actually happened, there would be a 12DW7 nine months later. ;) :D :bag:
     
  15. Bongolation

    Bongolation

    Nov 9, 2001
    California
    No Bogus Endorsements
    As I said (go back and read it), I couldn't see anything good coming of a fivefold increase in preamp tube gain. It's a no-brainer. Don't do it.

    Bill's page is a good overview of more or less typical results coming from the fad of replacing 12AX7s with lower gain preamp tubes, a much more common question - here and elsewhere.
     
  16. For several years these sorts of discussions have been showing up...Basically I think this illustrates that "a little knowledge can be a dangerous thing". In this case, somebody hears a vague description that a 12AU7 can be plugged into a 12AX7 socket and change the amp's tone. This means they're interchangeable, right, so let's try it!! (After all, the engineers that designed the amp surely couldn't have been more knowledgeable than some nincompoop off the Internet...) Kinda like the idea of putting a Variac on your tube amp (Eddie Van Halen was supposed to have done it!! I'll do it too and sound just like Eddie!! Note to the reader: do NOT plug your tube amp into a Variac, your heater filaments will not be the correct voltage and you could ruin your tubes quickly)

    Sure, some amps can be "enhanced", but a great deal of thought, research, and experience is in order before trying these modifications. In other words, I am trying to discourage people from blindly jumping on any modification that they hear ("Hey, you can cut a 4x10 cabinet's impedance in half--or double it-- just by moving a wire"....duh, sure, you're bypassing TWO SPEAKERS...)

    I'll get off my soapbox now. :D

    Oh, I commend the original poster for asking about this....A little knowledge may be dangerous, but more knowledge is POWER.
     
  17. notanaggie

    notanaggie Guest

    Sep 30, 2003
    Yes, they "fit".

    No, probably nothing horrible will happen to the tube or the socket if you do that.

    Maybe some strange sounds will happen. Possibly oscillation.....that would not be good, if it is loud, or above audio so it frys output tubes by diddling the bias, etc without you noticing right away that the tubes are bright red.

    The 12AX7 has a much higher "plate resistance" than the 12AU7. So the circuit for a 12AU7 will typically have much lower resistances in it. That means the gain won't go crazy even with a 12AX7, most likely.

    It is possible for a 12AX7 to have LESS gain than a 12AU7 in certain circuits.

    If nothing else will satisfy your curiosity, go ahead and do it. I suspect there is an 85% chance that nothing bad will happen. You never know, you may find a new "hotrod" technique, that will take the internet amp scene by storm.....
     
  18. Bongolation

    Bongolation

    Nov 9, 2001
    California
    No Bogus Endorsements
    Well, a couple of points here: Yes, it's stupid to blunder around inside your amp (or axe) for no good reason, particularly with those big electrolytic caps just looking for their chance to send the unwary on an ambulance ride.

    Anyone who knows me is aware that for over twenty (20) years on the Internet I've been advising owners against modifying gear, as it usually results in substantial collateral damage and devaluation. I've also advised them against having most so-called "techs" do this work for the same reason.

    Changing a preamp tube is usually benign. Assuming you don't break anything or kill yourself, you probably won't hurt much in a simple tube swap to a LOWER gain tube preamp. Do it enough and you'll loosen the socket's grip on the pins, though, which will cause problems, especially on unserviceable PCB tube sockets that require a lot of grief and disassembly to replace (as is the case on the SWR).

    On the other hand, there are an absolutely huge number of engineering screwups and cut corners in production amps and an engineer with some time can straighten these out. Machrone devised a retrofix for the egregious reverb misdesign in the greenboard versions of the extremely popular Fender "Blues Junior" amp. Fender vehemently denied the problem until they quietly redesigned the circuit in the later whiteboard versions to correct it! :D

    Machrone's mods - which are conservative - have stood up under a couple of years of peer review, and implementation in a very large number of these amps. He's not a nincompoop.

    Still, my own rule of thumb is if it's not broke, don't fix it and if the design's unsatisfactory, don't buy it in the first place.

    That's as good advice as anyone's likely to get this week! :p
     
  19. Passinwind

    Passinwind I Know Nothing Supporting Member

    ...I wish the super-genius-expert-imatubescientistandillforgetmoreabouttubesthanyoul leverlearn TB'ers will all weigh in.

    Will an opinionated, wiseass, know-it-all tube amp builder do?:cool:

    Here're possibly the worst cases: you sub a 12AX into a cathode follower 12AU circuit, either a finals driver like in a Fender Super Twin Reverb, a reverb tank send circuit, a buffered preamp output, or a tone stack buffer a la Ampeg. The 12AX either chokes on the current demand, shifts the tone stack frequency, overloads the heck out of the tone stack's makeup gain stage, or fails to drive the subsequent power amp stage fully, or maybe creates grid blocking in the power tubes on account of excess gain. You might like any of these effects, but generally, most people wouldn't, IMNSHO.

    However, in a gain stage application, the cathode and plate resistor values typically used in a 12 AU application will oftentimes not allow the 12AX to fully expoit the gain "advantage" it potentially has, in my experience. I would recommend that if you do this sort of sub, you scope the stage and be prepared to put some snubber caps in there somewhere (grid to ground, plate to ground, or plate to grid) to suppress possible oscillations.

    Going the other way is often a cool thing, however. Just be sure the plate resistors aren't super-wimpy, or they can fry.


    ------------Charlie Escher
     
  20. Dr. PhunkyPants

    Dr. PhunkyPants Guest

    Aug 11, 2002
    USA
    Awesome. Thanks for the info. Conclusion...this is not something I plan to undertake anytime soon.