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  • Tube glass breakage

    This amp has come in twice now with the glass perfectly broken right where the retainer surrounds the TUBE. Different tube position each time. My guess is that the tubes took a hit...but wanted to see what you all think may cause this...or if you've seen it before. Here's a pic.
    Attached Files

  • #2
    Wow. That looks like glass cutter work.
    Originally posted by Enzo
    I have a sign in my shop that says, "Never think up reasons not to check something."


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    • #3
      I've never seen a preamp tube cut and fracture like that before. My first guess is the spring within the aluminum tube shield is out of shape, and winding itself down around the body of the tube when e-installed.

      Take and remove the spring from the tube shield. It's cone-shaped in construction. The larger diameter is supposed to be at the inside top of the shield. If not, it will want to fall out of the shield, AND, it can wrap itself around the diameter of the tube when you re-install it. I suspect that's what you have on this particular shield.
      Logic is an organized way of going wrong with confidence

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      • #4
        Reminds me of this post here from nosaj... Marshall re-branded tube. Looks like glass cutter work as G1 put it.

        http://music-electronics-forum.com/t42949/
        When the going gets weird... The weird turn pro!

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        • #5
          That's an impressive cut. Is the shield base steel? If there was a burr on the inside edge of the base, maybe the tube moving in the socket could score a line in the glass. The break does seem to be even with the top of the base.

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          • #6
            I'd agree with your first assessment, the tubes took a hit. The edge of the bottom thing acts as a fulcrum when the tube is slammed against it.
            Education is what you're left with after you have forgotten what you have learned.

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            • #7
              Originally posted by DrGonz78 View Post
              Reminds me of this post here from nosaj... Marshall re-branded tube. Looks like glass cutter work as G1 put it.

              http://music-electronics-forum.com/t42949/
              Interesting. Looks like the same distance along the length of the glass envelope. Makes me wonder how the envelopes are constructed, or if there is some point in the manufacturing process that could score a line around the bottle like both of these.

              Originally posted by Enzo
              I have a sign in my shop that says, "Never think up reasons not to check something."


              Comment


              • #8
                One way to cut glass bottles (like to make drinking glasses from beer bottles) is to thermally stress a line on the glass, then shock it with cold. If this amp was running WITHOUT the tube shield installed, the bottom portion of the tube shield might keep the lower half of the tube a different temp from the top portion...
                --
                I build and repair guitar amps
                http://amps.monkeymatic.com

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                • #9
                  I dunno. We can rationalize one such failure, but he has had more than one similar failure in different sockets. That would require all the tubes to have this glass defect, or all the sockets to have the same lucky burr, etc. This is exceedingly rare, how many such failures have we seen even here on the internet? That leads me to look at the situation for this amp, rather than a systematic problem in either the tube industry or the sockets.
                  Education is what you're left with after you have forgotten what you have learned.

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                  • #10
                    Happens with JJ tubes and looks like they missed one essential operation: stress relieving glass.

                    Not sure about large tubes, but miniatures are usually built in sequence: first pins are embedded in a glass disk called "the button", then metallic parts get assembled on those pins, then the empty upside down bottom less "bottle" is fit touching the button base and joint is molten/soldered with hot gas jets.

                    Personal opinion: when cooling, that molten glass ring creates *great* stress on the cold side of the bottle, so it should need an extra stage of gently heating the whole glass envelope, either in an oven or with some extra gas jets, and let cool somewhat slowly.
                    I suspect that stage is being skipped.
                    Only reason I can find for that perfect cut, which I have seen many times.

                    Look at this Tesla video, showing (at 0:45) exactly that: soldering the button base to the glass envelope, that clearly must create stress, itīs the exact same process to cut bottles to make mugs, etc.
                    Juan Manuel Fahey

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                    • #11
                      I would observe that the glass weld between button and glass tube is down at the button, whereas the failure of the tubes in this thread are up at the waist of the tube where the shield ends.
                      Education is what you're left with after you have forgotten what you have learned.

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                      • #12
                        It seems to be an old amp. In the past, there were some tube sockets that were narrower than the current standard ones. Many ECC83/12AX7 at that time were also narrower than current JJ and others.
                        You could cut out a small piece of 1/4" heat shrink tube with the appropriate diameter to avoid glass/metal contact in that area.

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                        • #13
                          I saw a tube break like that once in a guys Vox.No tube sheild,clean cut just like that.It was the damnedest thing I ever saw.I couldnt even guess what caused it.And it was a JJ

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                          • #14
                            Originally posted by Enzo View Post
                            I would observe that the glass weld between button and glass tube is down at the button, whereas the failure of the tubes in this thread are up at the waist of the tube where the shield ends.
                            True.
                            But the stress line might be, not exactly at the soldering line, but somewhere higher, at some "frontier" between the red hot base and the rest of the cold envelope.
                            In fact, I would expect the stress line being somewhat higher, because the base of the envelope does get molten to join the button and those are jets, not Laser beams, so I expect the hot area to reach a few mm beyond the soldering line anyway, hot gas jet reaches glass surface and then gets deflected in all directions, including upwards.

                            Or JJ might "stress relieve" with a somewhat wider flame, but not the full envelope, just a band above soldering line.

                            Just speculating, of course.
                            But the problem has been reported, and always with JJ.

                            FWIW I do electrically weld now and then, not an expert of course, and among other problems I have found I remember cast iron cracks when so welded, not at the soldering line itself but a few mm away, precisely because itīs a very inelastic material and cracks on stress instead of just stretching/deforming..

                            Only solution I know (there may be others of course) is using a special "for cast iron" electrode, made out of a specially elastic alloy which absorbs stress as if it were putty (relative to cast iron hardness of course).
                            Juan Manuel Fahey

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                            • #15
                              Thanks for the input guys. I have to admit...there were no shields on the two sockets that it happened to. So...higher likelihood of the tube being pressed against the shield base AND higher possibility of the base getting hotter than the top of the tube. And then there's the combo of both those possibilities.

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