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12AU7 as cathodyne phase inverter

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  • 12AU7 as cathodyne phase inverter

    What would be the advantages of using half of a 12AU7 as the triode for a cathodyne phase inverter as opposed to a 12AX7 (disregarding the fact the other triode would probably be the preceding gain stage with a lot less gain)?

    I see on the data sheet for this tube that it lists a grid resistance of 1Mohm (with cathode bias) and that it does not list a maximum positive or negative grid voltage. The 12AX7 on the other hand lists max neg and pos Vg and nothing about grid resistance. This is leading me to believe the 12AU7 would be a better design for the Cathodyne PI function as opposed to a 12AX7. I have an amp with a cathodyne pi and have already added a 1meg grid stopper, but if I was using a 12AU7 type triode in this spot it seems as this would not be needed. The theory of adding the grid stopper on the 12AX7 was to try to limit grid current if the grid voltage swing tries to go positive.

    Strangely enough if a 12AU7 type triode is the better triode for a Cathodyne PI, there is the 12DW7 which is half 12AX7 and half 12AU7, but every cathodyne PI amp circuit I've seen (including mine!) has the wrong sections wired for the 12DW7! In my case its a PCB, if it was HW I could try it out easily, just getting a 1M grid stopper in there required some creativity. I have seen that JJ makes a non standard tube the ECC823 which is actually a reverse wired 12DW7, so I could grab one of those to check out if there is further reason to do so. (wish I could say I like JJ tubes, that's another issue tho).
    The 12Au7 triode has other differences (from 12AX7) so again I'm looking to see if there are other advantages in this particular application.

  • #2
    12AU7 can be a good choice for cathodyne (or any PI configuration), especially if you want to drive the power tube(s) hard into Class 2 mode, since it can deliver far more current than the 12AX7, but not everyone likes it because it is harder to make it to breakup/clip, so it depends on what you want to use it for.

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    • #3
      Originally posted by jazbo8 View Post
      12AU7 can be a good choice for cathodyne (or any PI configuration), especially if you want to drive the power tube(s) hard into Class 2 mode, since it can deliver far more current than the 12AX7, but not everyone likes it because it is harder to make it to breakup/clip, so it depends on what you want to use it for.
      JJ makes a 12DW7 with the AU7 and AX7 pins reversed.
      Bruce

      Mission Amps
      Denver, CO. 80022
      www.missionamps.com
      303-955-2412

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      • #4
        Originally posted by jazbo8 View Post
        12AU7 can be a good choice for cathodyne (or any PI configuration), especially if you want to drive the power tube(s) hard into Class 2 mode, since it can deliver far more current than the 12AX7, but not everyone likes it because it is harder to make it to breakup/clip, so it depends on what you want to use it for.
        Thanks jazbo, I've never liked the tone much of any amp with a cathodyne when its is overdriven, so I'm looking to see if a 12AU7 type section would 'hang in' there more than a 12AX7. Looks like I'm trying to investigate (high gain preamp->transparent pi->pwr amp). Probably hopeless but this is just a 'fun budget project', keeps my hands off the stuff that actually works well!
        I'm new here and didn't realize how much good info is here. Found another thread also indicating this triode can work well in this position, but both plate and cathode resistor values need to be changed to take full advantage.
        this place is cool!

        Bruce, thanks as well, I had just run across the JJ reverse wired 12DW7 (ECC823) before posting which started me in this whole direction. I'm going to source a few and while waiting determine circuit values to take better advantage of the AU7 triode for the PI.

        Initially I wouldn't even use the high gain channel on this amp due to the 'ratty' mess you get with a well overdriven cathodyne pi. Adding a 1M grid stopper along with some preamp mods for freq response has made the amp usable, however I'd like to see if I can move it even further along to a low budget screamer.

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        • #5
          The Wiz has covered getting good overdrive tone from the cathodyne.
          The Valve Wizard

          First use big grid stops on the output tubes (say 22 to 47K) to limit grid current. An 'AU7 section will help too but should not be absolutely necessary unless driving parallel tubes.

          If you are going to overdrive the cathodyne section itself then you need a big gridstop (say 470K) on the cathodyne triode (again to limit grid current).

          The 12AU7 triode is the default selection for cathodyne splitters in HiFi Amps but a 12AX7 should be fine for driving EL84, 6V6 and similar.

          If you look at those "weird" triode / pentode tubes ( ECF80 etc.) used in Ampegs you will find that the pentode section is used as a voltage amp and the triode section as the cathodyne AND the triode section is a low rp type similar to a 'AU7 section.

          Cheers,
          Ian

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          • #6
            Thanks GT,

            I had first added a 1M grid stopper on the PI tube and it made the amp 'workable' on the high gain channel. The PI drives a pair of 6L6's that currently have 1.5K grid stoppers. Increasing these might be the next tweak I try. Thanks!!

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            • #7
              Yes,
              The Output Tubes will be the first "things" which go into overdrive, The cathodyne itself will go into overdrive at a higher signal level again. That mean that you want to address the output tube overdrive effects first. You want to stop or at least limit the output tube grid current from loading down the cathodyne and screwing with its balance. An 'AU7 will be more tolerant of this grid current draw than an 'AX7 (so long as it is operated at muych higher current). For the 'AX7 with typical 56K anode and cathode load resistors I would try 47K grid stops on the 6L6 in place of the 1K5 which are currently fitted.

              Cheers,
              Ian

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              • #8
                Thanks Ian,

                Next chance I get to pull the chassis I will scope the PI input and outputs with a test signal setup (with dummy load) to get a baseline then I'll go with 47K on the grids for the 6l6's and check again. I think I've got this little project close to what I want it to do and hoping this will be the finishing touch. Of course it is a project amp so who knows! I did grab a reverse 12DW7 I could use but I'd rather not rework the plate/cathode resistors for that section unless I really see an issue after the grid stopper changes.

                Everything on 10!

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