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12AX7 vs. 5751?

Discussion in 'Amps [BG]' started by steamthief, Jul 12, 2009.

  1. steamthief

    steamthief

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    I'm a little confused on how the gain of these tubes relates to headroom and volume. I get that a 5751 has about 70% of the gain of a 12AX7. Would that mean a 5751 would only put out 70% of the volume in the same circuit? The more non-technical the explanation, the better. :atoz:
  2. B-string

    B-string Supporting Member

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    If you have a amp that distorts easy with your instrument or distorts harshly, a low gain tube (like the 5751) will help you out (the signal is not increased as high from the instrument).
  3. majortoby

    majortoby

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    I don't know, it may drop the gain enough to a point where at max volume, the ouput stage is just clipping less, without losing too much volume. IME, it seems that many tube amps just start distorting more and stop getting appreciably louder as you approach max volume.
  4. Wolffgang

    Wolffgang Supporting Member

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    I'm going to give this a bump because I've been wondering about this myself.

    A little background: I've experimented with tube rolling with my Streamliner to great success (finally settled on a GE long plate 12ax7 in v1, if I recall. One of the most common recommendations is a JAN Philips 5751 in V1, and I can certainly see why, it sounded great.

    My question is what am I actually doing by swapping out the 12ax7? I also liked it with a 12au7 in V1, but the drop in volume was noticeable. By using a 5751, are you basically just chopping out that last 30% of the gain that might be considered too distorted to sound good by some, or are you actually getting more clean headroom? Like, does it just seem like more because you can turn the gain knob more, or is it more?
  5. B-string

    B-string Supporting Member

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    It just seems like more, the "gain" control has to be turned up higher before the gain stage saturates (starts distorting). One advantage is finer control over the gain before saturation so you can seemingly get more clean output before distortion is "noticed". I have used a 12AU7 as the first tube in one channel of a V4-B for an "active bass input".
    The degree of gain change and how well it will work depends on how the circuit was designed for both gain factor and plate voltage as it falls in the plate load curve.
  6. Wolffgang

    Wolffgang Supporting Member

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    Brilliant answer, thanks a lot :)
  7. B-string

    B-string Supporting Member

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    You're welcome :)
  8. Chainsaw Willie

    Chainsaw Willie

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    I have a Mesa Boogie guitar amp that was hard to keep below the "coffee can full of bees" fizzy distortion level. Swapping in some 5751s gave me a wider range of gain knob setting before things got ugly - without making the amp any quieter at the tone I wanted.

    Of course the real solution was probably sell that amp to someone striving for that buzzy tone and use the money to buy some other amp not designed to sound like an overdriven op amp.
  9. chaosMK

    chaosMK

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    Disclosures:
    Hi-fi into an old tube amp
    I have some JAN 5751's and have tried to use them in various applications. Always ended up swapping 'em out, just something about the voice seemed kind of flat. I tend to prefer a JJ 83 or 803 for taming things down a bit.
  10. Wolffgang

    Wolffgang Supporting Member

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    Haha I love that description, if I'd ever thought of it it's probably exactly how I would have described my Streamliner with the stock 12ax7
  11. Mr. Foxen

    Mr. Foxen

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    Amp tinkerer at Ampstack
    Setting the bias correctly would probably help with that.
  12. Chainsaw Willie

    Chainsaw Willie

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    ...having it modified so the bias can be set correctly.

    The set of tubes I have were selected so their idle current would be good with the truly-fixed bias of the amp. A backwards method, but it worked. Seems funny how much they charge for an amp without a bias adjust pot. Would that increase production cost by $3?
  13. B-string

    B-string Supporting Member

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    But that would cut into sales of their hand selected output tubes ;).
  14. Chainsaw Willie

    Chainsaw Willie

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    That's right! I forgot that it is supposed to make you buy Mesa (re-)branded tubes!

    Some day I really gotta open that amp again and see if there is room to squeeze a pot in there. It has been years since I pulled the chassis. I seem to remember they had packed too much stuff in too small of a box.
  15. Mr. Foxen

    Mr. Foxen

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    More of a fix than a mod. Flawed from factory is still flawed. Like saying a jammed truss rod is stuck so you don't need to adjust your action when you change strings.

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