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choosing a 12ax7 preamp tube

Discussion in 'Amps and Cabs [BG]' started by nil, Dec 6, 2001.

  1. What 12ax7's break up more easily?
    I'm after a bit more grit in the pre-amp side, so i'm looking for something that will get dirty real quick!
  2. This months' Vintage Guitar Trader magazine had an article about the various flavors of 12AX7 tubes... I don't have access to it, but the author noted the various amounts of amplification from each tube model. A very wide variety.
  3. Is there a site around that gives any info like this?
  4. Donne, you da man! Thanks for that!
  5. Davemell0


    Nov 19, 2001
    SW Virginia
  6. :D
  7. jasonbraatz


    Oct 18, 2000
    Oakland, CA
    yeah i caught that one too. :)

    i like Ei tubes the most - but i'm not sure of their breakup characteristics. They sound better than any tube i've had in my stuff. problem is - they have a huge initial failure rate, so you really should get them from lord valve so you know the ones you get will work.

    i can't wait until i get a complete Ei setup for my IOD. mmmmmmmmmmmmm.

  8. Tapp


    Aug 29, 2001
    USA, Mississippi
    To be honest Nil, I don't think that you will be able to tell a great deal of difference in "breakup" between most 12AX7's because they all have a mu (amplification factor) of 100. Now, a 5751 (lower breakup) and a 12AX7 will give you some difference. I would e-mail Lord Valve (search for NBS electronics) and ask him if there are "hotter" 12AX7's he has measured.

    Most people look at the different 12AX7's for "warmth and harmonic" characteristics and microphonics. I happen to love old Mullard's (who doesn't ha ha) for bass. I am also a fan of Ei's but careful picking must be made there.

  9. So, is the 5751 substitution for the 12AX7 useful for bassplayers? Anybody tried it?

    I'm not looking for breakup or grit, just something a little warmer (while maintaining clarity) than what my ('93) swr studio 220 is giving me now. At "Joe's Lore" he keeps talking about lush mids, which is fine if the bass is good.
  10. CS


    Dec 11, 1999
    I like the EH's they seem to have more gain and more bass response than two old Mullards I salvaged from a dead amp. If you want warmth jangle and any other nonsensical term try Mullards.
    The Sovtexk WXT are high gain but I prefer the EH (yes I know they are made by Sovtek). TheEI's are rumoured to have micrphonic problems.
  11. My Eden came with a 7025 tube in it. I recently changed it to a Sovtek 12AX7LPS and I've been pleased with it so far. Only about $10 a tube.
  12. mthoople


    Nov 1, 2001
    Denver, CO
    I'm using a Sovtek 5751 (picked up from Lord Valve) in my IOD . I have quite a few tubes, and tried most of them in the IOD before deciding on the 5751. Here were my impressions in the IOD:

    Telefunken Smooth-plate 12AX7: nice, clear sound, but low output. This tube is not NOS, just old.

    GT-7025 (old smooth-plate Ei): good output, nice overtones. My second favorite in this test.

    JJ/Tesla 12AX7: I was using this before I tried the 5751, since I like keeping a backup available which can be impractical and costly with NOS tubes. Very even, flat sound.

    Ruby STR-7025: This was the tube I used in my Grand Prix. Very high output. Sounded great in the GP, made the IOD a bit bright.

    Sovtek 5751: Obviously my favorite in this test. Slightly lower output than the Tesla and GT, but (surprisingly) more than the Telefunken. Warmest, "tubiest" sound of the lot, which is what I was looking for.

    These were my opinions in the IOD, with the power tube output sent straight to a stage monitor. The drive control was set just before overdriving the signal (the actual setting varied very little when changing tubes). The tubes were NOT allowed to warm up properly, since I wanted to listen to them back-to-back.

    I originally bought the 5751's for a guitar amp to lower the gain for a better blues tone. They made the guitar amp sound warmer as well.

  13. I use a Long Black Plate Sylvania JAN 5751 in my SWR SM 400-S, and sometimes a 12AX7-A LP NOS tube pulled when new from a early eighties Fender guitar amp, which I bought a the matched set of 3 from a Mesa Engineering Employee.

    The new/old Fender tubes break-up nicely and smoothly and are very quite with very low microphonics.
  14. Tapp, I'm not getting enough moxie out of my F-1X, and am wondering if the tube is the culprit.

    I got the F-1X off eBay and the old tube was badly microphonic. It was a 12AX7A/702 according to the lable. Make is "TNT - The New Tube".

    I installed a replacement 12AX7A bought new at the local music store. It performs nicely, but I'm just not getting the amount of oomph I should be getting (I think).

    Do you have anything in print I can add to my reference data about the various MU ratings of the tubes, and which will interchange?
  15. MU Spew!

    In most cases, you can use any tube which shares the same basing. ("Basing" refers to how the internal elements of the tube are connected to the pins on the bottom.) For instance, in place of a 12AX7, you could use any of these: 7025, ECC83, 12AU7, 12AT7, 5751, 6201, 12AY7, 6072A, 5814A, ECC82, ECC83, 12BH7, and a lot of others! Each would have a different effect on the tone, or the overdrive characteristics, or the gain...depending on what the tube in that particular socket does in the circuit.

    Check this out for Preamp tube spec.'s:


  16. Tapp


    Aug 29, 2001
    USA, Mississippi
    Bgavin, I have some references but I'll need to dig some. Basically the 12AX7/7025/ECC83 has the highest amplification factor of 100. This is not really a gain measurement but a ratio of certain tube parameters that when used as a voltage amplifier gives an idea of gain. From memory in order from highest to lowest would be:
    12AU7(going from memory here on the AU7and AY7)

    Most of the time all of these are interchangeable yet some will draw more current and be better suited in specific circuit locations.

    If I were you, I would find a store or friend who has several different 12AX7's you could try in the Alembic and see if you hear a volume/punch difference. You may very well have a weak "new" tube if the store's source doesn't give a good test. I highly recommend LordValve (NBS Electronics) since he does a series of useful tests before sending out a tube. He will stand behind what he sells too.

  17. JimM


    Jan 13, 2000
    Northern California
    Would a 12AT7 give you the break-up you're looking for?
    I read on a guitar amp forum that some guitarists use them in place of 12AX7s to tame the volume of a Hotrod Deluxe.Usually when they speak of lowering volume they usually mean reducing headroom and thereby causing breakup to occur at a lower level.
  18. After last weekend's bass fest, it is apparent that my RB5 is significantly reduced in output compared to the other passive Fenders, and miles less than the active Wals and Alembics.

    I put a 12AX7A into my F-1X to replace the microphonic same type that came out of it. A friend sent me NOS of a 12AX7, and a GT-7025. I have not yet put either into the F-1X due to time constraints and de-racking hassles.

    If there is no MU difference between the 12AX7 and 12AX7A tubes, why bother swapping them out?
  19. Tapp


    Aug 29, 2001
    USA, Mississippi
    You're right there is no mu difference between the 2 but this is not an exact science and sometimes you may get a tube that is weaker possibly due to less electron emissions, structure design of the plates, etc. Let me preface by saying I am far from a tube design engineer, I've just got some experience plugging and chugging.

    Did you try the Alembic with the other "hotter" output basses? If replacing the tubes doesn't help the output (or checking other basses too) then it may be a voltage issue. Don't want to be negative this soon though.


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