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Tube gurus and experts - replacing a 12ax7 with 12au7

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  • Tube gurus and experts - replacing a 12ax7 with 12au7

    I just scored an Ibanez Tube King (no starved plate, this one the provides 300 volts to the tube). I remember reading somewhere that substituting a 12au7 could possibly damage the tube due to some kind of circuit design that would really push the 12au7. I've always thought that they were interchangeable without any problem.

    Any of you electronic gurus heard of anything like that? I'm simply looking for more headroom and less gain.

  • #2
    Hmm, AFAIK the 12A?7 family are all pin compatible and differ only in the amplification factor or the tube. 12AX7 is 100% and the value drops with a 12AT7 and even further with a 12AU7. By lowering the amplification factor you lower the amount of drive on the plates which basically means when the tube is running full out, a 12AU7 isn't as full out as a 12AX7. Since the upper 85%-ish [more or less] of a tubes range [0 being off and 100 being full open] is where the distortion happens, having a tube that only goes to 80% should give you what you seek. After a point on all tubes, they overdrive instead of getting louder as you turn them up.


    • #3
      Thanks Verne, I know about the different gain factors in the 12A family but the 80% thing is very useful info. It was something about reducing the drive on the plate would increase current?/voltage that would stress the tubes plate that I recall reading. Wish I could remember the source.


      • #4
        If you want to reduce gain try the 5751 tubes. I learned SRV used them in his amp and out of all the tubes below I found it the best tube type for taming unwanted drive while retaining tone. The chart below is only suggestive. If you wanted to maintain gain levels the substitutes to the right might be best, but results will depend on the type of amp and the results you want.

        For example in the 5F6-A Bassman the stock first gain preamp tube is designated as a 12AY7 which has a gain factor of 45. Replacing it with a 12AX7 that has a gain factor of 100 dramatically changes the tone and feel of the amp so you have to experiment and see what you like. I found 5751's were what I needed in my Marshall head to clean up the drive channel that was too saturated and kept me from dialing in mild drive tones. In my Bassman I run 12AX7's and its still super clean. Experiment and see what works best for you.

        The brand of tube is going to influence both tone and gain as well.

        Tube - Gain - Acceptable Substitutes

        12AX7 - 100 - 5751 12AT7 12AY7

        5751 - 70 - 12AX7 12AT7 12AY7

        12AT7 - 60 - 5751 12AY7

        12AY7 - 45 - 5751 12AT7 12AV7

        12AV7 - 41 - 12AY7 12AU7

        12AU7 - 19 - 12AV7
        Last edited by WRGKMC; 07-15-2014, 12:21 PM.


        • #5
          I plan to use the Ibanez TK999HT as a preamp with the torpedo c.a.b. The torpedo has really excellent power amp simulations and most folks seem to think that a tube preamp pedal works best as a front end. Rather than spend money I don't have on the Kingsley's, effectrodes, etc., etc., I figured I'd give the Ibby a try.

          Most reviews say it is high gain but I want to use it as a clean front end. That's why I'm asking about the tube swap.

          Also I found the source of my confusion. Here is a quote:

          Unfortunately, it's a common misconception that 12AX7, 12AU7, 12AT7 are basically the same tube with differing levels of "gain." This misconception arises from the fact that they use the same basing. In fact, they differ from one another in some important ways. The AU7 has substantially lower plate resistance than the AX7, and will tend to draw a lot more plate current given the same operating voltages. The cathode bias arrangement usually used with these tubes will compensate to an extent, but caution is required. I worked recently on an amp where the owner had replaced one of the AX7s with an AU7 and the plate resistor opened up from overdissipation, resulting in no output.

          If you want to experiment, I recommend sticking with types 12AY7 and 5751, which are similar to the AX7 but with a somewhat lower amplification factor.

          And the source:

          Thanks again! HC is not dead yet.


          • #6
            Originally posted by ballhawk View Post
            ... I worked recently on an amp where the owner had replaced one of the AX7s with an AU7 and the plate resistor opened up from overdissipation, resulting in no output...
            That makes sense to me. The solution is to change the plate resistor to one that can handle more power.
            "In many ways, most people’s happiness, peace and love are mortgaged to the external situation. So, if the stock market goes up you are happy, if the stock market goes down you are unhappy. But the quality of life is not about what is around you. Our ability to live joyfully here does not depend on the size of the house we live in or the car that we drive. These things make your life comfortable and convenient but the essential quality of your life is how you are within yourself right now."


            • #7
              Originally posted by onelife View Post
              That makes sense to me. The solution is to change the plate resistor to one that can handle more power.
              I wouldn't know where to begin on that one. Think I'll start with 12AT7 and work from there if I'm not satisfied.


              • #8
                I remember Wyatt posting the following in this thread:

                Originally posted by Wyatt View Post
                The difference between a 12AX7 and 12AT7 is more than just gain, internal resistance has to be figured in. They are not often interchangable (like a 12AX7 and 5751 are designed to be) despite peoples assumptions.

                A 12AT7 can replace a 12AX7 at the phase inverter (actually drives the power tubes harder), but shouldn't be used as a replacement gain stage. A 12AU7 also should only be used where designed. 5751's were designed to be lower gain replacements for 12AX7's and have same internal resistance. 12AY7's are close enough to 12AX7 and 5751.


                • #9
                  Thx preacher. I've settled on 12AY and/or 5751 for the experiment.