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Amp techs/Kingsport/Johnson City Tn

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  • Amp techs/Kingsport/Johnson City Tn

    New here, played music since 6 and now going on 63 years of playing. I recently started to learn a little repair but get stumped quite easily.
    Anyone know of any good amp techs in the Kingsport/Johnson City area of Tennessee. I have a Magantone M15 with a reverb problem. I have used all my limited knowledge trying to trace it down with a scope and meters. I am that close, but no cigar.
    Thanx in advance.
    J.D.

  • #2
    If anyone is interested in anything I do, I have seventeen original tunes on youtube.
    here is a link.
    https://www.youtube.com/channel/UCMg...QUerxvNqYNJ8PQ

    Thanx

    Comment


    • #3
      Here is a link to Magnatone schematics.
      Magnatone Schematics - Tube amp Schematics (el34world.com)
      There are 4 listed for the M 15.
      Can you determine which one you have?

      Comment


      • #4
        I already have the schematic. I have checked all voltages around the 12AU7 and 7025 of the Reverb circuit. All good. I have tracked the signal with a scope. There is signal going to the tank through the rca jack, I can see it at the 1uf 450V cap connected to the plate of the 12AU7 at pin one, and at the centerpost of the rca jack. I have replaced both tubes a few times. I know the depth control is working, I can see the signal fluctuation at the 100pf cap straddling the 2.2M resistor coming from the reverb depth control . I have the original reverb tank, and a new accutronics replacement tank and another tank. The reverb was working, but if you turned the depth control up past about 10 oclock she would start to feed back. Now, with either the original tank, or the accutronics hooked up, if I smack the tank I can hear a very faint thundering. I have not replaced the rca cables, my meter shows their continuity to be good. I have checked the signal at both plates and grids of both tubes and the depth control seems to have no affect on any of them. So, I have kind of reached the limits of my knowledge and would like to get it fixed. Everything else seems to be working okay. I rebuilt the power supply, she had bad 60 cycle hum, that is cleared up.

        Comment


        • #5
          Here is a schematic if anyone is interested in taking a stab at it.
          Attached Files

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          • #6
            I grew up in Rogersville but now in Memphis. I even got my ET degree from ETSU. If I was still up that way I could help you out. I know that doesn't help you but its neat to find someone on here from my neck of the woods for a change.

            Comment


            • #7
              The majority of reverb problems have to do with the tank itself. Have you checked the transducers in the tank and the tank wires to make sure nothing is open? I usually start by checking the resistance across the wires on the amp side instead of the tank side. That way, you are checking the transducers and the wiring at the same time. If you find an open input or output, narrow it down from there- wire or transducer.
              "Yeah, well, you know, that's just, like, your opinion, man."

              Comment


              • #8
                I have a brand new accutronics replacement tank. The original tank was just working recently. I also have another tank. No matter which tank I use, no go. When any of the tanks are hooked up, when I smack them I can hear the faint rumble of the reverb. I highly doubt it is a tank. I guess you didn't read the number four response.

                Comment


                • #9
                  Originally posted by dragonwerx View Post
                  ......I guess you didn't read the number four response.
                  Oops, Sorry I missed that part.

                  The depth control, which is basically reverb level, feeds reverb only to amp channel 2. Have you verified that the channel 2 amp is working properly? With reverb off, both amp channels should be outputting approximately the same level. If the second amp isn't working or isn't working correctly, you wouldn't get much reverb.
                  Last edited by The Dude; 09-17-2021, 04:02 AM.
                  "Yeah, well, you know, that's just, like, your opinion, man."

                  Comment


                  • #10
                    Both channels are working correctly.

                    Comment


                    • #11
                      Originally posted by dragonwerx View Post
                      if I smack the tank I can hear a very faint thundering.
                      With depth control fully up, the thundering should be loud.

                      Indicates a reverb recovery problem.
                      Please post 7025 plate and cathode voltages as well as voltage at point G.

                      Also don't forget to check the reverb cables for shorts.

                      With tank disconnected you can check the recovery stage (7025) by directly feeding a signal to pin 7.
                      You might even connect a guitar here.

                      Is the cathode bypass cap (25) good?

                      - Own Opinions Only -

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                      • #12
                        Doesn't matter where the depth control is, the thunder is very faint. I know the depth control is working because I can see the change in signal where the 100pf cap and 2.2 M resistor is connected. The cathode bypass resistor has been replace and is in correctly. I checked all the voltages around both tubes and all look within ten percent of spec. Not sure if I checked point G and not sure what it should be, will do after I find it on the schematic. I did not check the voltage on the cathodes, what should I be looking for there? It is not stated on the schematic. I will double check the voltages where asked and get back here and post them. I can feed a signal into pin 7 with my signal generator, haven't done that, will do. I checked the reverb cables with a DVM and they show continuity. I was thinking of replacing the one feeding back to the 7025. I can see a signal at the center post of the cable feed into the tanks with my scope. The reverb was working, but if turned up past ten oclock it would start to feed back. Now there is nothing but the slight rumble
                        Thanx for the tips.
                        J.D.

                        Comment


                        • #13
                          Please post requested voltages. I will tell you if they make sense and where there is a problem. Cathode voltage is one of the best indicators of proper tube operation.
                          Knowing both, plate and cathode voltage can reveal if resistors have drifted.

                          Conductor continuity check of the cables won't show of there's a short between them.

                          Point G is shown twice on the schematic. Should be 200V.
                          Last edited by Helmholtz; 09-17-2021, 08:37 PM.
                          - Own Opinions Only -

                          Comment


                          • #14
                            I am a newbie at this but it looks like at point G I have 280 Volts. I checked on both sides of the 1uf @450V electrolytic cap that I replaced, one side 280V, other side 0, On both sides of the 10k resistor I have the same voltage of 280V ? Shouldn't I have less voltage on one side?
                            On the 7025
                            Pin1 plate 181V
                            Pin 6 plate 185V
                            Pin 3 cathode 1.5 V
                            Pin 8 cathode 2V

                            Thanx for the help. I have been at other sites and have gotten nowhere.

                            Comment


                            • #15
                              Tube voltages look ok. Recovery stage should be working, even though point G voltage is high.
                              There should be a voltage drop of 5V to 6V across the 10k dropper.

                              Now try the signal test.

                              Cables tested for shorts?

                              Are all of your supply nodes voltages high?
                              - Own Opinions Only -

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