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Sovtek Mig-50 blowing rectifier diodes

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  • #1

    Sovtek Mig-50 blowing rectifier diodes

    Hello all, looking for some help regarding my Sovtek,

    For reference, it is a Mig 50 with the modification where the two channels are wired together to one jack, and one of the inputs have been replaced with a master volume. Picked it up a few months ago played great until I started noticing the volume drop slightly from old tubes so I retubed the amp with a set of Sovtek 5881's and Sovtek 12AX7. About two weeks later after light use the amp wouldn't turn on with the main fuse blown. Took it to a tech for repair and it was determined to be a blown rectifier diode. Worked fine for about a week until it did it again, and I took the amp back and the tech again replaced the diode. Played two shows this weekend, the first it ran like a champ, however, yesterday I was not so lucky and for a third time the amp died after about 25 minutes of use. Wondering if anyone can give me some troubleshooting help with this issue as the tech seems stubborn that it is the mod that is causing the problem, however I really dont want to take it out, as it's handy, and doesn't seem like that has anything to do with the issue. From reading around, to me it sounds like either a bad filter cap, or a bad power tube even though they are new. I love this amp and really don't want to part with it, but I don't really want to keep pouring cash into it practically after every time it is played. Any advice or help with the issue is appreciated.
    Tags: fuse, rectifier, sovtek
  • #2
    Sounds like the problem started after you installed the new 5881's and you are still using the same ones? It would certainly be worth trying another pair, even the old ones.
    Just because a tube is new does not mean it is good. Some are bad right out of the box.
    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
      A preamp mod (that only affects the ac signal path) absolutely will not cause an amp to blow rectifiers repeatedly. How do you know that this new failure is a rectifier diode? If the new tubes were installed without biasing and the new tubes are working under poor operating conditions then it could be a power tube failure causing the fuse to blow. Based solely on what you've said I would suggest that you find a new tech to look over the circuit and bias the amp properly. Possibly replacing tubes as needed.
      "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
        The two previous failures have been blown rectifier diodes, and the replacements have been a temporary fix, so based on the failure symptoms its the same this time around. It lasts around an hour of play time so to me it sounds like something heating up and failing (another suspect to a tube failing/being bad out of the box). When I retubed I did bias it myself using a bias probe and based it off of a suggested bias. I have the original tubes so I will definitely try swapping them out with the diode and see if that fixes it. I'd think the tech would have checked the tubes but I can't be sure. I'll see if I can source a diode this week and try to replace it myself, and switch out the tubes. I don't really have the money to take it to a tech again.


        Thanks

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        • #5
          Originally posted by BugVomit View Post
          I have the original tubes so I will definitely try swapping them out with the diode and see if that fixes it.
          Is it the same diode that blows each time? Have you tried replacing all the diodes in the rectifier? Is there a proper fuse in the amp? How old are the filter caps? What was the suggested bias?
          "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

          • #6
            The diode blown is a square bridge diode, looks something like this
            Fuses are proper, I bought a 10 packof 3.15a slow blow ceramics as it says on the back. Not sure of the age of the filter caps, I purchased the amp in November, not sure when it was serviced beforehand. I believe I biased the amp at 70% dissipation around 36mA per suggestion on Eurotubes

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            • #7
              3.5 amps is a lot for a fifty watt amp. Doesn't matter though. The fuse is there to prevent a fire. Not save the rectifier. Not knowing the exact spec of the rectifier it's impossible to say that the fuse should go before the rectifier (as is the case with the common 1n4007 quad found in similar amps). Your filters are likely bad considering the age of the amp. That's a place to start anyway. If your going to replace (or have replaced) the power supply caps, be sure to replace the bias supply and preamp cathode bypass caps as well. It only makes sense to do this while the amp is being serviced anyway considering it's age. A bias problem could cause excessive current (and your problem) bad filters can as well. After replacing ALL the electrolytic caps you'll want to hook the amp to a signal and see what kind of current it's drawing in use. If all seems normal the amp is probably fixed.

              There is also the possibility that the replacement rectifiers have been under spec. If the tech repairing your amp doesn't know well enough to replace the old caps then who knows if the correct rectifier has been installed.?.
              "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

              • #8
                Does it die while you are playing or while you are switching it on?

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