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Vox AC15 heater fuses blowing -help!

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  • #1

    Vox AC15 heater fuses blowing -help!

    Hi all - I have a England built 1990's AC15 PCB that blows the one leg of the 4 amp sloblo heater 6.3 volt fuse. PT is a Mercury Magnetics. 5 volt to rectifier is fine, idle, all tubes out all is fine. All tubes test fine. It blows fuse with all 7 tubes in but is stable with one tube removed. Does not matter if it is a 12ax7 removed or one EL84. I am only seeing 11.3 volts AC when that last one tube is removed and it is stable.

    Is it the 6.3 winding on one side going bad?? seems unlikely. Any input would be appreciated and I am taking donations for T4 250v fuses. I am out from testing ideas.
    Tags: None
  • #2
    'I am only seeing 11.3 volts AC when that last one tube is removed'
    Surely it should be 6.3Vac?
    Is there any Vdc on the heaters?
    Pete
    My band:- http://www.youtube.com/user/RedwingBand

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    • #3
      Apart from the voltage confusion... It's unlikely a failing PT secondary should cause this. If the problem were on that side of the fuse then the fuse wouldn't blow. Something on the other side of the fuse is trying to pull too much current through it FROM the winding. So the problem could be a failing tube (even though they "test" good) or a short like a bad socket or a charred path on the board.
      "Take two placebos, works twice as well." Enzo

      "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

      "If you're not interested in opinions and the experience of others, why even start a thread?
      You can't just expect consent." Helmholtz

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      • #4
        I had an amp do something similar at a gig years ago. I wrapped the output tubes in a bar towel and threw them on the carpeted floor a few times. Put them back in and got through the rest of the night. It broke up the shorts I guess. Not a procedure I would recommend, lol. But I would recommend swapping out the output tubes. Vox amps torture el84s. If you are not using NOS tubes I would recommend JJs.

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        • #5
          Are you missing the center tap connection to ground for the heater winding?
          (You would get a 12Vac reading 'across' the heater pins)
          I do not know if the 4A fuse is slow or fast acting.
          It is not spelled out on the schematic.
          You could always connect a DVM across the fuse holder to read the AC current draw of all the heaters.
          If it is spiking above 4 amps, then you need slo-blo.
          Attached Files

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          • #6
            post#1

            Originally posted by Wrongdog View Post
            Hi all - I have a England built 1990's AC15 PCB that blows the one leg of the 4 amp sloblo heater 6.3 volt fuse.
            "Take two placebos, works twice as well." Enzo

            "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

            "If you're not interested in opinions and the experience of others, why even start a thread?
            You can't just expect consent." Helmholtz

            Comment

            • #7
              Thanks for all the reply's. I forget how I got that 11.3Vac reading but I made the time late last night to take a few readings. I removed all leads and read directly at tranny lead lugs (again, leads removed) and got 3.5Vac at each leg. Center tap ground is a good ground

              Tranny seems to be the issue...bummer for my buddy that owns amp. I wondering what ohms reading that section should have? Why would both legs read the same?? Seems like one should be different than the other if it developed a short.

              It is a custom MM PT for this very amp and heaters should not have any extra stress. I wonder if they have any warranty? Don "Toneman" Butler did the mods to this amp and I think procured the Trannies and choke from MM so I don't know about warranties etc.

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              • #8
                OH! With one tube removed amp works without blowing fuse. Sounds fine (removed tremolo tube) at the low heater voltages. Put that tube back in and it blows...

                Comment

                • #9
                  OK, you can look up the amount of current each tube heater uses. Ass them all up, and that SHOULD be what is going through the fuse. Now what is something else is already dragging a lot of current through the fuse, then each tube adds to that, and ALL of them along with the mystery draw is too much for the fuse.

                  Pull that fuse, set your meter to AC AMPS and plug the probes in the right place for that, and clip your probes to the empty fuse clips. What current draw do you read? With no tubes, still draw?

                  Unless I have the wrong scematic I see two heater fuses, but 4A slow blow. Is the fuse you are blowing supposed to be a slow blow type? Is there T next to the number, like T4A? T is for time delay, as in slow blow.
                  Education is what you're left with after you have forgotten what you have learned.

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                  • #10
                    Right on Enzo. I said above that this scenario doesn't really point to the PT secondary. I even said why. But the OP chose to ignore the electronic facts of the matter. Let him replace the PT ten times if he wants to.
                    "Take two placebos, works twice as well." Enzo

                    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                    "If you're not interested in opinions and the experience of others, why even start a thread?
                    You can't just expect consent." Helmholtz

                    Comment

                    • #11
                      Oh crap, I meant to say ADD them all up, not ass them up. That was a typo folks, not some snarky remark.
                      Education is what you're left with after you have forgotten what you have learned.

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                      • #12
                        I was trying to ignore it. I didn't even want to consider what it might mean.
                        "Take two placebos, works twice as well." Enzo

                        "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                        "If you're not interested in opinions and the experience of others, why even start a thread?
                        You can't just expect consent." Helmholtz

                        Comment

                        • #13
                          Not to harp but I suggested actually measuring the current draw across the fuse holder in post #5.
                          Oh well.
                          I think a lot of OP's just want the answer.

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                          • #14
                            Fixed - short on board. thanks all! Teeny little bad solder job at a mod where it was arcing on the one leg of the heater trace.

                            Lesson: Always check the previous work done. Even when it its your own work. I had re-heated a cold solder joint done and caused the problem....
                            Last edited by Wrongdog; 10-30-2012, 04:40 PM. Reason: add a comment

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                            • #15
                              Bummer.
                              TIF. (tech induced failure)

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