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Sovtek Mig 100 U speaker output issues

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  • #16
    Alright, I've finally been able to replicate something in my testing. I took the 8 ohm jack out of it's chassis hole and insulated it in a plastic bag and put the 16 ohm jack and the line out jack in their own insulating plastic bag. I noted that the plastic sleeve on the 8 ohm jack had failed and was cut around its base and was probable allowing the jack to ground to the chassis which was messing up the 4 ohm jack and sucking it's volume down? With only the 4 ohm jack connected and the other jacks in plastic insulating bags, the amp is now sounding like it's 100 watts of volume again. However, both the 8 ohm and 16 ohm jacks when tested in isolation like the 4 ohm jack was, are still producing low volume, which when turning up the volume all the way produces bad hum and kinda crazy distortion as well. I am going to unsolder the 8 ohm, 16 ohm and line out jacks and test for any issues. Jack replacements with insulating plastic washers are probable what the doctor would order. I'm wondering if the OT might have received some kind of damage to it's 8 ohm and 16 ohm winding's from the grounding to the chassis issue or even the phase inverter tube? So the moral of the story for the day is, don't over tighten the nuts on your jacks. I'll let your know what happens when I track down the right replacement parts. Thanks everyone for your help. 66 Kicks, thanks for the schematic.

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    • #17
      Try this, unsolder all those speaker jacks. Protect all the bare wire ends. Use clip wires and connect the common and 4 ohm tap wires to a speaker. You don't need any line out jacks or NFB. Does that sound OK? Now clip the speaker to the 8 ohm tap wire and common, sound OK? Now 16 ohm tap and common, OK? That will tell you if the signal is coming out the transformer or not.

      it is way more likely you damaged the jacks than the transformer. The transformer is a hunk of iron with heavy copper wire wrapped around it. it is pretty tough.
      Education is what you're left with after you have forgotten what you have learned.

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      • #18
        You shouldn't try playing through the light bulb current limiter. That will limit the output power of the amp and the more volume you try for the more it will limit it. If nothing is shorting out and the amp isn't drawing a lot of current at idle, then you can disconnect it from the current limiter and run it through a speaker and the amp will go to its full power potential since all the voltages will be what they were designed to be.

        I have a MIG 100 U here that I fixed several years ago. I decided it sound shitty though once I had it all fixed and I gutted it, kept the PT and got a Heyboer choke and OT for a Vox AC100 and made the amp into a clone but added a second higher gain channel to it also. I still have the original choke, OT, and board here if you run into any problems I can take a look at the board and parts, and I am into selling what I have also if you end up needing the parts. The MIG 100 H is very similar and both amps are similar to Marshall designs.

        If I recall correctly, the original jacks were plastic jacks like what Marshall uses, but they had the 'metal' looking screw on the front of them. They are not like a switchcraft jack however. I would have to go look into the box to be sure but I can do that tomorrow if you want. Any jack with a plastic 'sleeve' as you mention is not going to be a metal jack like a Switchcraft.

        I am with Enzo here.....it is very unlikely that you blew out the OT. Make sure to run the amp with a load and you should be just fine. Despite many shortcomings in those amps, the Russian OT's are pretty overbuilt.

        Greg

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        • #19
          As a side note, when you damage an OT because of lack of connection to a speaker, the insulation breakdown/internal short happens in the primary , where hundreds and even over a thousand volts can be found (specially if unloaded), never on the secondary where we are talking tens of Volts.

          So if primary were damaged, all output taps would sound ugly, but since at least the 4 ohms one does not, then the OT is fine.
          Also suspect poor jacks wiring.
          Juan Manuel Fahey

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          • #20
            One or more of those jacks is damaged. If they were all fine, the insulator breakage would only cause a ground loop/hum issue.
            You will probably find one (or more) of them has resistance between tip and sleeve.
            Replacing them with plastic type should solve your issues. From the sounds of 66Kicks post, that's the type they would have been originally anyway.
            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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            • #21
              Jesse,

              Are the speaker out jacks on your MIG 100U specifically labeled 4, 8 and 16 Ohms?

              I ask because I found the attached photo which shows a 100U model that had two of the three speaker jacks connected to the 4 Ohm tap and the third jack connected to the 8 Ohm tap. (I didn't date code the amp but I took the photo in 2000 when the MIG-100U passed through the shop so I know the amp was manufactured in 2000 or earlier.)
              Click image for larger version

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              No one has come up with a 100U schematic yet but I found the attached schematic which shows a version of a MIG 100* that used an OT with just a 4 and 8 Ohm secondary. The schematic file is missing the right edge so we can't see the jack wiring or the complete model designation in the title block.
              This information may not solve your specific problem but it may help clarify the wiring if it does turn out that you don't have a 16 Ohm tap.
              Otherwise, it's just some interesting historical information that reinforces the fact that musical instrument amp manufacturers are not strict when it comes to configuration control.
              Sovtek-Mig100-schematic Partial.pdf
              Last edited by Tom Phillips; 08-24-2015, 11:13 PM.

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              • #22
                Originally posted by g1 View Post
                Replacing them with plastic type should solve your issues. From the sounds of 66Kicks post, that's the type they would have been originally anyway.
                I didn't mean to imply that they had plastic Cliff Jacks originally. Every stock Mig 100 U that I've seen had those rough metal Switchcraft knock-offs. They had imitations of Switchcraft's #11, #12A, and #12B in those amps and they would use any one of those when they needed a #11. They had a different solder lug order, which made it easy to connect to the wrong lug if you're used to Switchcraft. It was also easy to spin the wafers when backing them up with a screwdriver between the lugs.

                I've only been shocked twice in 45 years of working with tube amps, and the second time was by a Mig 100 U. They had no bleeders on the first set of filter caps and they used a DPDT switch for standby so that the unused lugs were hot when off. I brushed my finger across an unused lug while touching the chassis with my other hand.

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                • #23
                  Originally posted by 66 Kicks View Post
                  I didn't mean to imply that they had plastic Cliff Jacks originally.
                  Sorry, I meant to say that you had mentioned they were all insulated.
                  I should have said the jacks should be replaced either with metal types and insulating shoulder washers, or plastic types.
                  By the way, welcome to the forum, and thanks for the input.
                  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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                  • #24
                    Here is a schematic that came with a Mig 100 U.
                    Attached Files

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                    • #25
                      Hey fellas, I believe I finally fixed the amp. Turns out the shoulder washers that were insulating the output jacks had deteriorated so much that they pulled apart and or just fell off on one side exposing the the jacks to the chassis when I replaced the rusted washers and nuts on the output jacks (I had no idea that's what they were used for). This created a ground loop I believe and caused the huge volume drop. After I un-soldered all the jack ground lines and tested the jacks individually, I decided to just connect the 4 ohm jack (in a plastic bag) ground line to the line input jack sleeve where the yellow OT ground lead was connected and which is where the black ground line from all the jacks had been connected, leaving the 8 ohm and 16 ohm grounds unconnected. That worked so I added the 8 ohm jack (in a plastic bag) ground line and that worked, so I then added the 16 ohm (in a plastic bag) ground line and everything worked just fine, WTH? I thought I had already done a plastic bag isolation test before, but maybe the 4 ohm was still connected to the chassis because that was kinda working. I also noted that the NFB was connected to the 16 ohm jack, which I would think was probable done to give more headroom and a more dynamic feel like Marshall used to do in the early 60" with their NFB? I think the NFB is normally connected to the 8 ohm jack for most new tube amps anymore isn't it? I just ordered 8 packs of shoulder washer pairs for around $10.00 from "Studio Sound Electronics". The OT must be o.k. (right on with the right on's)! So I gotta say I learned a bunch from all this which is what it's all about. Hopefully when the isolating washers come in and the amp is all put back together it will still work the same as the last test (I'm ready for some serious pay back at my rehearsal studio concerning the death metal guys) haha. Thanks for the help guys, appreciate it...P.S. 66 kicks, that's a great find.
                      Last edited by Jesse Pearson; 08-25-2015, 07:40 PM.

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                      • #26
                        The NFB is part of the circuit, not a thing on its own, so on any given amp design, it is tapped off wherever they wanted it. it is nothing more than the output signal, sampled and sent back to an earlier stage. The higher the tap impedance, the larger the signal voltage sample. But that feeds into the PI or nearby stage, and other resistors are involved to form a voltage divider, so the tapped voltage and that other circuit combine for a total NFB level. 8 ohm or 16 ohm can have the same final result depending on the resistor values back in the PI. Don't read more into it than that.
                        Education is what you're left with after you have forgotten what you have learned.

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                        • #27
                          Thanks Enzo, I didn't even know what the heck the NFB was until I started looking it up after reading your previous post. The OT secondary winding's on the Sovtek Mig 100 U is as follows: 4 ohm Red, 8 ohm white, 16 ohm blue, line out Yellow ground, #1 5881 pin 3 small wire purple, #3 5881 pin 3 small wire yellow. Is the line out yellow big wire the OT ground wire?
                          Last edited by Jesse Pearson; 08-25-2015, 07:18 PM.

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                          • #28
                            I'm a little confused about the last post & post #25, but I think when you say the yellow is going to the line out jack, you mean it's going to the sleeve of the line out jack, and carries on through to the circuit board ground. Is this correct?
                            If so, then yes, the big yellow is the OT common (ground) wire.
                            Saying "line out, yellow" gives the impression the yellow is going to the line out tip, which I don't believe is the case.
                            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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                            • #29
                              Cool G1, thanks for that clarification. The big yellow OT ground wire is indeed connected to the line out jack sleeve and continues to all the other speaker output jack sleeve's via a black ground wire till it reaches the power section pc board common ground point. It really pays to learn about grounds/common/reference points when trying to diagnose a problem.

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                              • #30
                                O.K. The amp is up and running. I installed extra long insulated output jacks and that took care of the last of my problems. I bought an empty 2x12 speaker cab off of Craigslist for $40.00 and rebuilt it and loaded it with Celestion 16 ohm Vintage 60 - 12" speakers wired for 120 watts. The speaker cab turned out to be an old Fender Super Sonic 60 ext cab. I used old vintage Marshall Salt and Pepper grill cloth on the cab and glued some extra on the amp head to match. I'm pretty impressed with this amp. I can get that Robin Trower "Bridge of Sighs" sound without any pedals hooked up. And did I mention loud, holy moly! These amps are great amps. I think the "Sovtek Mig 100 U" modal stands for "unmodded" clone of the Marshall JCM 800 2203. Here are some pics ...
                                Click image for larger version

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                                Last edited by Jesse Pearson; 09-25-2015, 03:33 AM.

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