Sovtek Mig-50H Tube Midget - Need Resources

Discussion in 'Amp Tech Center' started by jsnlynn76, Aug 3, 2021.

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  1. jsnlynn76

    jsnlynn76 TDPRI Member

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    I recently got a Mig-50H in a trade. Previous owner knew very little about it, had it as a project he never finished. When I popped it open, I noticed a few things.

    1) Someone replaced the heater wiring, including disconnecting the elevated heater voltage winding for V1 and replaced with standard 6.3v daisy chain.
    2) Output jacks have been changed out for standard mono jacks and there is a shielded lead that comes under the board that is connected to the board next to the bridge rectifier that is detached. It's my assumption that this is the NFB lead that connects to the tip of the 8ohm output on the schematic but I am having a hard time finding good reference pics online. The one's I can find all vary slightly and frankly the schematic I have isn't great quality either.

    Other than that, the build quality seems solid, tubes test good, all the voltages seem ok but I can't seem to trace an input signal any further than the plate on V1a. It seemed unlikely that it would be because of the heater supply but then again I'm not sure why it was designed that way in the first place so I am a little stuck and hoping someone has had a few of these on their bench and can chime in.


    NFB - board.jpg GUTSHOT.jpg heater wires.jpg
     
  2. radiocaster

    radiocaster Poster Extraordinaire

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    It won't work with simple jacks, the gainy channel is connect to the rest of the circuit through the cleaner channel jack (it switches if nothing is plugged in). That is, unless it is modded further with a switch or something. You cannot plug into both jacks at the same time and expect a sound from both inputs on a stock amp.
    sovtek-mig50h.jpg sovtek-preamp.png sovtek-power.png mig-valves.png heater-resistors.png
     
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  3. radiocaster

    radiocaster Poster Extraordinaire

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    Also have some pics I saved of stock insides.
    sovtek0inside.jpg sovtek-tube-midget-50h-487041.jpg
     
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  4. jsnlynn76

    jsnlynn76 TDPRI Member

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    Thanks for the reply, I appreciate the pics! I've actually read through your previous thread re: line out while researching. About those jacks...that could possibly explain why I have no signal on V1b, however it looks like someone has installed switchcraft-style shorting jacks on the inputs which I thought would work. I will have to investigate if there is signal on the Low tip when the high jack is engaged.

    What about the V1 heater winding? Should I disconnect the 6.3v from V1 and re-attach the dedicated winding near C19&C20?
     
  5. radiocaster

    radiocaster Poster Extraordinaire

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    I don't know, mine is pretty noisy, although it's not from the first triode, it's on the "clean" channel as well (both use V1 though), and I gave up trying to fix it.

    I did change those electrolytic caps, they were mostly bad except maybe for one or two. Mine had the biggest 4 IC and the rest Soviet, like that last pic.

    Is that a voltage switch at the top left that all the full pics have? Mine simply has a bunch of lugs with voltages next to them; to change voltage you have to desolder and resolder one wire.
     
    Last edited: Aug 3, 2021
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  6. jsnlynn76

    jsnlynn76 TDPRI Member

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    Yes, mine has the voltage switch located between the fuses like some of the others shown. Apparently there's no one way to build a Sovtek :)
     
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  7. jsnlynn76

    jsnlynn76 TDPRI Member

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    Thanks again for your help and the reference photos. I was able to get it up and running! R11 had gone bad and was open, and in addition, someone had monkeyed up the wiring on the first pot. Once those were fixed...crunch city. This is a fun little amp.
     
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