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12AX7EH CF Failure

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  • #1

    12AX7EH CF Failure

    Hi,

    I don't know whether I hit a bad batch of these or something but recently I had 4 12AX7EH fail in CF position /driving tone stack/. One of them worked several hours, one worked for a month, couple of them failed shortly after turning the HV on.
    Anode voltages were 350-390V, Voltage drop at 100k cathode resistor before failure was 160-190V. Tubes were manufactured between 01.01.07 and 01.01.08.
    I read somewhere in a forum that other people were experiencing similar problems with 12AX7EH when used as CF. I have no problems when using them as regular triode gain stage. Actually they sound very good and have plenty of gain.
    I was wondering if anyone here had similar problems?
    Tags: None
  • #2
    Look up the spec sheet on the tube and see what the max allowable heater to cathode voltage might be. I be suspicious you were exceeding it.

    Try elevating your heaters.
    Education is what you're left with after you have forgotten what you have learned.

    Comment

    • #3
      Look up the spec sheet on the tube and see what the max allowable heater to cathode voltage might be. I be suspicious you were exceeding it.

      Try elevating your heaters.
      I'm using 12AX7EH from a long time but I started having these issues only recently. When CF tube is replaced with other brands they work OK. Since it's a 12AX7 shouldn't they all have the same ratings?
      I'm running regulated DC heaters feeding opamps and other stuff so the elevation of heaters is not an option here.

      Comment

      • #4
        Is the 12AX7EH of russian origin, cos if so they may be the same as 12AX7WB, which look to have the same structure as the 6N2P, but with different heater wiring.
        This could be important as although the 12AX7 has a max h-k of 180V, the 6N2P is only 100V.
        Hopefully someone will know if this is just supposition.

        Comment

        • #5
          Since it's a 12AX7 shouldn't they all have the same ratings?
          NO.

          A 12AX7 should have a certain amount of gain, and certain operating characteristics, but that doesn;t mean ALL criteria are the same tube to tube.
          Education is what you're left with after you have forgotten what you have learned.

          Comment

          • #6
            Originally posted by GainFreak View Post
            I'm using 12AX7EH from a long time but I started having these issues only recently. When CF tube is replaced with other brands they work OK. Since it's a 12AX7 shouldn't they all have the same ratings?
            I'm running regulated DC heaters feeding opamps and other stuff so the elevation of heaters is not an option here.
            Hmmm. Lessee here.

            A guy walks into an amp tech's (doctor's) shop (office) and says, "Hey, tech (Doc) it burns this tube out (hurts) when I put this kind brand of tube in (do this.) The tech (doctor) looks up and says "Well, don't do that."

            Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

            Oh, wait! That sounds familiar, somehow.

            Comment

            • #7
              The Sovtek 12AX7LPS is known to fail in the CF position, so it doesn't surprise me that the EH 12AX7 does it too. I'd use different tubes in that spot myself, especially after 4 of them fail for you. Save the EH for different locations in the amp and they'll work fine.

              Greg

              Comment

              • #8
                The Sovtek 12AX7LPS is known to fail in the CF position, so it doesn't surprise me that the EH 12AX7 does it too. I'd use different tubes in that spot myself, especially after 4 of them fail for you. Save the EH for different locations in the amp and they'll work fine.

                This problem with EH has been reported in other forums as well. In a russian forum for example somebody claims to have relatives working in the factory and they told him they're having technological issues.
                Also the comments there come from people using these tubes in hundreds so we're not talking about couple of accidents only.
                My tubes in question come from a group buy of 100 pcs 12AX7EH directly from New Sensor. I think if we have a 4% failure so far this looks like a problem to me .
                Now I'm using 12AX7EH in non-CF positions and they sound very well with plenty of gain. I'd hate to move to other brand/s but if this issue is not solved I'll have to.

                Comment

                • #9
                  The CF position in guitar amps probably runs the heater/cathode insulation beyond any spec that any 12AX7 ever had. Even if it doesn't exceed the spec on DC conditions, it gets violated hugely on signal peaks. I'm more surprised by the fact that other tubes withstand it, than by the troubles with these particular tubes.

                  Heater/cathode insulation was always a black art of the tube era. It's no easy matter to find a substance that keeps its electrical insulating properties year after year under orange heat and a high DC bias. Most glasses and ceramics that I know of start to conduct once they get red hot, and once this happens the DC drives ions through them and destroys them.
                  "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

                  Comment

                  • #10
                    The CF position in guitar amps probably runs the heater/cathode insulation beyond any spec that any 12AX7 ever had. Even if it doesn't exceed the spec on DC conditions, it gets violated hugely on signal peaks. I'm more surprised by the fact that other tubes withstand it, than by the troubles with these particular tubes.

                    Heater/cathode insulation was always a black art of the tube era. It's no easy matter to find a substance that keeps its electrical insulating properties year after year under orange heat and a high DC bias. Most glasses and ceramics that I know of start to conduct once they get red hot, and once this happens the DC drives ions through them and destroys them.
                    I'm not familiar with tube technology but the fact is most of 12AX7s can withstand the conditions in question. I don't mind replacing them from time to time but not every month or so /or every couple of minutes.../.

                    Comment

                    • #11
                      What I was trying to tell you is that the Sovtek 12AX7LPS and 12AX7LP does not work in that position in amps. Every vendor I know of tells you not to use those tubes in that position. Given that EH is made at the same factory, I would suggest that it is certainly feasible that it will have the same issue when used in that position. All of these tubes work great in other positions in the amp. So use the EH somewhere else and use some other brand in the CF position such as JJ as they work fine there. Problem solved.

                      Greg

                      Comment

                      • #12
                        Yeah, I don't see the mystery either. This particular tube doesn't like being used as a CF, it is fine elsewhere. Most other tubes are OK with it. So don't use this brand 12AX7 as a CF, use one of the most other brands there.
                        Education is what you're left with after you have forgotten what you have learned.

                        Comment

                        • #13
                          Every vendor I know of tells you not to use those tubes in that position.
                          I didn't know that. Certainly no such statement on New Sensor's site.
                          Thanks to all for the feedback.

                          Comment

                          • #14
                            New Sensor doesn't say anything on their site except advertising their products. You also have a hard time getting accurate info and specs on some of their products if you try to get it from them. The non-use of the EH 12AX7, Sovtek 12AX7LPS and Sovtek 12AX&LP as a CF has been all over the amp forums for at least 2 years. Now you know I guess...

                            Greg

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                            • #15
                              What's "CF" mean? Center-fold? Center-field? Crummy-Fender? What?

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