12AX7 vs 7025 ???

Discussion in 'Amp Tech Center' started by Bartholomew3, Oct 5, 2019.

  1. Bartholomew3

    Bartholomew3 Friend of Leo's

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    Question:

    Since the 7025 is a silent 12AX7 is there any reason not to just use 7025 instead ? Have a few that I saved from 1970 when I was on the road switching tubes every couple of years...still test and sound great. RCA, Telefunken, other Russian & Japanese brands.

    Why does my twin reverb use both of those tubes in different places if they are the same thing?

    I can't hear any difference --- is that my ears ?
     
    Last edited: Oct 5, 2019
  2. moosie

    moosie Doctor of Teleocity Ad Free Member

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  3. brokenbones

    brokenbones Tele-Meister

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    I notice the difference in those two tubes in medium gain circuits. If your Twin stays clean with 12ax7's, go ahead and wrap the 7025 tubes up and put them away some place safe.
     
  4. marshman

    marshman Friend of Leo's

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    I never understood if there were actual differences between 12AX7s and 7025s and the design/engineering level, or did they just test a couple thousand 12AX7s and label the quietest, high-gainiest one’s as 7025s.
     
  5. kbold

    kbold Tele-Holic

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    ^^^ Pretty much this.
    12AX7 selection basically comes down to 2 things: noise and sound.
    Noise: If noise is an issue, it may be the tube or another part of the circuitry. Tube rolling and experimentation is in order.
    5751's having lower gain would lower the noise floor. If you're obsessed with removing any noise, try a 12AD7 (good luck finding one though). 12AD7's create about as much noise as a vacuum (relatively speaking).
    Sound: A tube that will sound great in one amp may not in another amp. Tube rolling and experimentation is in order.

    Also, be aware that if you're checking opinions online, many of these opinions are based on hi-fi use rather than use in guitar amps.
    Typically in a guitar amp you want the first third of the gain pot adjustment to be clean, the second third to be thick/compressed, and the last third to be distortion. (Well, that's how I like my gain adjustment.) Different 12AX7's will give different results: too much vs not enough distortion, glassy vs creamy, etc.
     
  6. avspecialist

    avspecialist TDPRI Member

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    Basically in the older days, the 7025 was a selected 12ax7 for noise and generally considered an industrial version or the 12ax7.
     
  7. EsquireBoy

    EsquireBoy Tele-Holic

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    According to what others answered, I would guess that price is the reason?
     
  8. avspecialist

    avspecialist TDPRI Member

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    I haven’t seen a real price difference, but most current production 12ax7 I believe are the same specs now as 7025, from what I have read.
     
  9. PCollen

    PCollen Friend of Leo's

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    The 7025 was a more robust version of the 12AX7, often used in military applications (aircraft comm, radar, etc ) . I have read that eventually the specs of the 7025 were adopted for 12AX7's, and that the 7025 nomenclature eventually dissapeared for new production in late 60's.
     
    Last edited: Oct 6, 2019
  10. AndyPanda

    AndyPanda Tele-Meister

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    back in the day (I'm old) the tube guys (who were old back then) always told me that 7025 had a different internal suspension of the parts to keep them quieter.

    but I would buy 7025 and 12ax7 and still have to go thru them all to find the quiet ones. I got plenty of 7025 tubes that were noisier than 12ax7 tubes. The surplus guy I bought from let me buy a dozen tubes and bring back all the noisy ones after I had picked out the quiet ones.
     
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  11. Middleman

    Middleman Friend of Leo's

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    If my amp is down and needs a replacement, either will do.
     
  12. schmee

    schmee Poster Extraordinaire

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    Either is fine. I think with what is left of the old tubes now, any given 7025 may not be any quieter than any given 12AX7. I can't seem to hear any difference in any I have.
     
  13. telemnemonics

    telemnemonics Doctor of Teleocity Ad Free Member

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    If buying old tubes from unreliable sellers I'll pay a dollar or two more for a 7025, because when it was sold it was theoretically tested and sorted as being quieter.
    But once in hand we have to determine for ourselves if it suits our needs.
    It does seem that at one time they made the 7025 a little different in response to noise complaints and as the tech improved.
    But for the most part they are the same tube but sorted for low noise, which may not still be true by the time we plug them in.

    And a better quality 12ax7 will be quieter than a poorer quality 7025.

    I still reserve my quiet strong preamp tubes for early audio stages and would not put a nice strong quiet 7025 or 12ax7 in a trem or reverb driver/ return stage.

    Also worth noting is that among theoretical amplification factor of 100 12ax7/7025, there is variation and not all amplify the same amount.
    One of the big tube sellers tested all the new production and gave amplification factor test results.
    In some cases a 5751 had a higher amplification factor than another 12ax7 or 7025.
    We know that lowering the gain or amplification factor lowers noise, like the design of the 5751.
    It is possible that a random tube that sounds more quiet is just lower gain.
    If listening for noise and microphonics turn up all the way and note how loud as well as how noisy or microphonic. Might be hard to tell volume difference, but if the amp is one that only distorts at highest volume, the lower gain quieter tube may not be the one you prefer in that amp.
    Buy & try is the last word, or if you have them use them in as critical a stage as they excel in, or move them downstream in the circuit until they perform well.

    Also note that long plate designs are more prone to microphonics, yet vintage long plate 12ax7 samples may be quieter than lower quality short plate designs. Many say only use the short plate type in 1st gain stage or high gain amps, but they are worth testing if you have excellent vintage tubes already.
     
    Last edited: Oct 6, 2019
  14. Bartholomew3

    Bartholomew3 Friend of Leo's

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    Also have 12AX7B - Chinese tubes in a TSA15H Ibanez tube screamer head. Super quiet, even better than any 7025 for noise.

    My old pre-amp tubes from the Sixties are more musical, sound a bit better but the Chinese 12AX7B are more silent regarding floor idle noise at any volume.
     
  15. muscmp

    muscmp Tele-Afflicted

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    i have a bunch of each and all i can do is roll them until my ears like the sound.

    play music!
     
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