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12AY7 or 12AU7 in V1 instead of a 12AX7? Why?

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  • 12AY7 or 12AU7 in V1 instead of a 12AX7? Why?

    What's the purpose of putting a 12AY7 or 12AU7 preamp tube in V1 and possibly V2? What does this accomplish vs. the standard 12ax7 tubes? This would be for a Twin Reverb RI.
    ♪ Cats 'n' Strats, 'cause that's how I roll

    I Surf therefore I am.
    Strat Blender Pot Modification HERE






    Originally Posted by Mike Eldred, Fender Custom Shop


    The discussion about nitro (and many things on forums like these) is largely based on folklore, innuendo, and assumption that it "sounds better". Poppycock.

  • #2
    I'd be interested in this to. I've heard that a 12AU7 will give you more clean headroom. Don't know why though.

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    • #3
      I'd be interested in this to. I've heard that a 12AU7 will give you more clean headroom. Don't know why though.
      Hopefully some tube amp guys can fill us in on the scoop.
      ♪ Cats 'n' Strats, 'cause that's how I roll

      I Surf therefore I am.
      Strat Blender Pot Modification HERE






      Originally Posted by Mike Eldred, Fender Custom Shop


      The discussion about nitro (and many things on forums like these) is largely based on folklore, innuendo, and assumption that it "sounds better". Poppycock.

      Comment


      • #4
        It's not so much that they give you more clean headroom - more that they just turn down the gain in your pre-amp. They are much lower gain tubes than the standard 12ax7.

        Try it - you'll probably like it
        Recipient of the coveted MBM Kicking Ass Award!! ...still proud of that one.

        Visit my Pink Floyd Tribute Act's website at www.canadianpinkfloyd.com







        Originally Posted by Strung_Out


        your rig is excessive and loud; I dig that.

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        • #5
          I replaced 12AX7 tubes with 12AU7 tubes in a few pedals. They gave me a smoother tone with less distortion.
          From what I recall, they're in the same family and what varies is the gain each model has.

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          • #6
            I'd suppose it will give you a cleaner tone. Personally I like all of the gain I can get from the preamp.

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            • #7
              My Mesa amp had so much gain that I couldn't turn the gain knob past about 9 o'clock.

              So, I changed tube 1 to an AU7. I got a much more adjustable amp out of this simple switch.

              I felt like the amp was running cleaner and quieter as well...YMMV

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              • #8
                The various 12A?7 tubes all have different gain ratings.

                Gain Factor
                AX=100 mu
                AT=60 mu
                AY=45 mu
                AU=19 mu

                But a lot of people look at these numbers and think..."that's all." And it's not.

                Unfortunately, there is also this:

                Plate Resistance
                AX=80K ohms
                AT=10.9K ohms
                AY=25K ohms
                AU=7.7K ohms

                While it doesn't hurt these tubes to be swapped in for a 12AX7, it's not ideal either. You want to know why these tubes sound clean? It's because running a 12AU7 or 12AT7 in a circuit designed for a 12AX7 biases them cold, damn near sterile. You aren't actually getting 60 mu out of a 12AT7 in a 12AX7 slot, and you aren't getting anything close to 45 mu from a 12AU7. Preamp tubes are cathode-biased (well, after the '50's anyway) but a big jump in plate resistance will put the tube out of bias range.

                Now there is an exception, the 5751 was designed to have a mu of 70, and a plate resistance identical to a AX, the 5751 tube was specifically designed to be a lower-gain, plug-in replacement for the 12AX7. (it is not however considered an ideal replacement for the 12AT7, despite their similar gain factors)

                Designing an amp is all about biasing the gain stage and correcting the curve (for a flat EQ) for each tube. Unfortunately, you can only do that for one exact brand of one exact tube when designing the amp. Which is why some builders (ken Fischer, Dr. Z, Bogner) are very specific about what tubes they made their amps for.

                I should add that, obviously, almost all these tubes are designed other names by other companies or for other purposes. For example a 12AX7 was available as 7025 (JAN/Military), ECC83 (England), CV4004 (France?), etc. Generally, many companies had their own designations and suffixes and prefixes to make their product stand out.

                As for the original question, tube rolling is equivalent to seeing what non-factory spec aftermarket part has what effect in hampering your high-end European sportscar to give it more "individual character." It's only go to run it's optimum best with a handful of the right parts, but people like to tailor feel to their own specific ergonomics. I'm not against this per say, but (going back to amps) its one thing to voice an amp by through off its curve correction with a tube from another brand and a whole other thing to use a tube type that the isn't working 100% wherever you put it.

                Using a lower gain preamp tube early on means you aren't boosting the signal as much as the start of the amp circuit as normal, so less signal goes further on down through the amp, meaning there is less signal to be boosted latter and less signal to cause clipping. It's like taking to familiar cliche of a snowball rolling down hill...getting bigger and bigger...and just starting with a smaller snowball at first, and ending up with a smaller snowball in the end. It is very similar to just turning down the Volume (or Gain on a Master Volume amp), it just achieves it in a different way with slightly different results.

                IMHO, if you want a cleaner preamp, raise the B+ voltages to preamp tubes, don't swap in different tubes. You want a dirtier (browner) preamp, lower preamp B+ voltages (this will have the additional benefit of adding touch sensitivity), there are two dozen other changes to can be made in the preamp to tailor it to taste (easier to do in a old Fender than say a complicated Bogner). Though using a 5751 in any 12AX7 place is perfectly fine to try for a little lower gain, it'll perform well.

                The negative side-effect using lower resistance tubes without tweaking the circuit is you lose dynamics and touch sensitivity (the latter is something BF/SF Fenders don't have a lot of anyway) and trade off spontaneity and responsiveness for more sterile, stiffer cleans. It's a hack mod. Plus, there is, of course, Volume knobs on the guitar and amp, and on old Fenders a "padded" (attenuated) input #2 for each channel.

                (wow, long, and I'm sure the tube rollers are going to beat on me for it, but remember, I take no responsibility for typos, I'm a lousy typist)

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                • #9
                  Good points Wyatt. I'll add to my previous post that although I tried 12AU7 tubes on many pedals, I did not try them on amps and the issues you mention in your post were the main reason.

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                  • #10
                    I'd suppose it will give you a cleaner tone. Personally I like all of the gain I can get from the preamp.


                    Can you get any cleaner than a twin reverb with 12ax7's? I dont thnk so,mines got 12ax7's and is crystal clean on 10!!!

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                    • #11
                      I always hate following Wyatt up, esp on the topic of tube amps... My response always seems lame an uninformed in comparison.



                      Having said that, in both my Peavey Prowler and my AC15, replacing the 12AX7 in the V1 with a 12AU7 makes the range of gain lower overall. That is to say, before on the Prowler, I'd never take the gain past about 3 while with the 12AU7, I might take it up to 5 or 6.

                      I've never compared total output with an SPL meter, but I'm guessing that the total volume at a given gain level is going to be about the same either way.

                      I've never tried it on a clean amp like a Twin or a Deluxe. I'm guessing that it would just have a bit less output so you'd need to turn the volume up a hair more. In a crystal clean environment, there'd probably be no difference. In a noisier environment, my guess is a good 12AX7 would be better because it would let you run the volume lower and would likely result in less noise.

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                      • #12
                        Can you get any cleaner than a twin reverb with 12ax7's? I dont thnk so,mines got 12ax7's and is crystal clean on 10!!!



                        Exactly. What's the point in a Twin? So it can be cleaner than clean?
                        basement traders

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                        • #13
                          Can you get any cleaner than a twin reverb with 12ax7's? I dont thnk so,mines got 12ax7's and is crystal clean on 10!!!


                          I used a TR from the mid 60s up to the early 90s and never once ran out of clean headroom.
                          HCGB Trooper #24

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                          • #14
                            Wyatt, you do make me feel technically challenge.

                            Oh... wait.... I am!!!!


                            Thanx for another of your enlighting post!!!!!


                            Ant the Les Paul Lover!

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                            • #15

                              Having said that, in both my Peavey Prowler and my AC15, replacing the 12AX7 in the V1 with a 12AU7 makes the range of gain lower overall. That is to say, before on the Prowler, I'd never take the gain past about 3 while with the 12AU7, I might take it up to 5 or 6.

                              I've never compared total output with an SPL meter, but I'm guessing that the total volume at a given gain level is going to be about the same either way.



                              Well, it should. In theory, you should be able to get the same gain range from 0-3 before the 12Au7 as you get from 0-6 afterwards, you just have t have a more delicate touch (in reality we all know you have to get the Volume up to a certain point before the signal even starts going through the amp).

                              But the problem is you aren't even getting the tube's available 14 mu (per side), the tube is running really cold. So, if those *one* or *two* gain stages (depending on amp design, but remember the tube is a dual triode) were rebuilt for the 12AU7, you would still see 1/7th the gain at that stage and the tube would be running optimum enough to really respond when you dig in.

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