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Sovtek MIG 60 - diagnosing problem

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  • Sovtek MIG 60 - diagnosing problem

    My Sovtek MIG 60 head recently started doing this thing where it gradually loses volume as i play it until it eventually ceases to make sound altogether. Before going out, the tone resembles that of a tube amp that just had its power switch flipped off (saggy, saturated, etc).

    I would love to fix this myself if possible. Any idea what the problem could be?

    Click image for larger version

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    Last edited by DavidPretzelrod; 01-21-2021, 08:04 PM.

  • #2
    Do you have a schematic that you know matches your amp? For some reason I was thinking the Sovtek amps can be kind of quirky and variable.

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    • #3
      https://music-electronics-forum.com/forum/schematic-requests/45648-links-to-sovtek-schematics?t=44503
      It's All Over Now

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      • #4
        Originally posted by DavidPretzelrod View Post
        My Sovtek MIG 60 head recently started doing this thing where it gradually loses volume as i play it until it eventually ceases to make sound altogether. Before going out, the tone resembles that of a tube amp that just had its power switch flipped off (saggy, saturated, etc).

        I would love to fix this myself if possible. Any idea what the problem could be?
        Sometimes a flaky old tube will do that. Also, a resistor, especially in the power supply, that heats up and changes value (upwards) over the course of time. Would be good to 1) pull & push back in all pre tubes, look for corroded pins & either clean the pins or replace any tube that has corroded pins. 2) replace one tube at a time, see if that gets you anywhere. 3) monitor supply voltages as amp heats up over the course of time, see whether supply voltages, especially the lower ones sag as time goes by. Do you really need to be playing for the gain to drop, or will it happen after the amp warms up some.

        There's also the possibility that one or more of the capacitors in the power supply is cause for concern. A filter cap that leaks current heats itself, it will become uncomfortably warm to the touch. Would be good to have a cheap "laser" thermometer so you don't have to risk shock. An excellent tool to have around the shop, you don't have to risk burning fingers or attracting a shock.

        Worth saving. Sovteks were cheap "sleepers" for a long time. Now the prices are heading upwards a bit as some folks recognize they are actually pretty good amps, and the prices on the classic amps of the past have gone clean out of sight.

        Like the raked brick on that wall too, kool old stuff you don't often see these days.

        This isn't the future I signed up for.

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        • #5
          I agree with Leo. Most likely a failing tube. As he says, try tube substitution first. It's probably just one tube, so no need to buy a set.
          "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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          • #6
            Or one tube losing its heater current, like a loose socket pin.

            Or a bad connection to the heater supply, and the tube heaters all fade to dark.

            I found the Sovteks to be well made and reliable, when they came out I bought four of them. A 30, a 50, and two 60s. In fact I bought them to grow some of my own projects in. The 60 was basically the 50 altered for EL34.
            Education is what you're left with after you have forgotten what you have learned.

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            • #7
              You said the sound goes out altogether, is that the guitar sound or all sound? Can you hear any noise at all? If you twiddle knobs do you hear anything change? If you twist a knob and hear something come out of the speaker that control is presumably after the break in the signal chain.

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              • #8
                Originally posted by DavidPretzelrod View Post
                My Sovtek MIG 60 head recently started doing this thing where it gradually loses volume as i play it until it eventually ceases to make sound altogether. Before going out, the tone resembles that of a tube amp that just had its power switch flipped off (saggy, saturated, etc).

                I would love to fix this myself if possible. Any idea what the problem could be?

                Click image for larger version

Name:	fSovtek_Mig_60__600_Athens_5d9e5d29b8bab.jpg
Views:	573
Size:	53.2 KB
ID:	923147Thanks
                i fixed one of these,problems were several solder joints on the power supply board.
                Take care because this amp has 600V on plates.
                But it can be anything,potentiometers are really cheap in these amps,

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                • #9
                  Would like to jump in here as I have a mig60 that's exhibiting the same exact symptoms and could possibly help the OP fix his! It looks like Enzo is right (for this one at least) about the heater supply. Preamp tubes v1 and v2 go dark after flipping standby, I replaced the rectifier connected to the heater and R46, the 5.6k resistor that separates both filament lines (it had drifted DOWN to 1.5k) , sound came back and everything was functioning well, filaments would now stay lit.

                  At this point I let it run and was monitoring R46 and noticed it was getting fairly hot, too hot to hold my finger on it.... Is this a normal symptom, considering the goofy DC elevation off the B+? I want to make sure the repair would hold and not toast R46 again, or perhaps I am missing something more obvious....

                  Measurements;

                  Heaters on V1 and V2 = 2.1 VDC at each pin
                  5 VDC at R46 (5.6K.)
                  Both 100k to heaters measure well.


                  Cheers

                  http://www.guitarnucleus.net/resourc...vtek-MIG60.pdf

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                  • #10
                    R46 should be 5.1R, not 5.6k??!

                    And resistors don't drift down.
                    - Own Opinions Only -

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                    • #11
                      Does the standby switch really turn the heaters off? Did someone really pay Ed Pennpacker to draw that?
                      What is the purpose of the 100Rs and the 5.1R in the heater circuit ? I can't wrap my head around the way that is drawn.
                      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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                      • #12
                        This is the schematic that I have.
                        With penciled in changes?!
                        Attached Files

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                        • #13
                          Originally posted by g1 View Post
                          Does the standby switch really turn the heaters off? Did someone really pay Ed Pennpacker to draw that?
                          What is the purpose of the 100Rs and the 5.1R in the heater circuit ? I can't wrap my head around the way that is drawn.
                          The 5.1R is part of the pi-filter for the DC heater supply and drops some voltage.
                          R48 (100R) is wired directly across the DC heater supply output and acts as a base load, maybe necessary to bring the voltage down by a few percent.
                          R47 (100R) is used to elevate the heater supply. As the heater supply is floating, there shouldn't be a significant current (no voltage drop) in R47, so its value is not critical.

                          - Own Opinions Only -

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                          • #14
                            Originally posted by Helmholtz View Post
                            R46 should be 5.1R, not 5.6k??!

                            And resistors don't drift down.
                            Whoops! Notation error on my part. Definately R not K. And upon closer inspection the installed resistor was factory, but printed on the side was 1ROJ so probably not a drift but an incorrect value... since it was a dropper was it possible that this was toasting our preamp tubes via the heater??

                            Does the standby switch really turn the heaters off?
                            Before I replaced those components the heaters would dim down to nothing soon after the standby was switched... I figured it was a strange symptom of a stranger design but it's possible It was on but just incredibly dim? It certainly seemed like it faded down with the standby was switched.

                            SO we know now that, even though it's functioning we are well below our desired 12VDC elevated voltage, with 2VDC measured. I'll replace the 1ROJ resistor into the dropper position so see if that bumps it up, but are we looking for a failed electrolytic in the heater, probably c24 c25? Are these acting as filters for the DC supply, and could that cause a failure over time (1- 2 hours or so?)


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                            • #15
                              Originally posted by Mr_bibbles View Post
                              SO we know now that, even though it's functioning we are well below our desired 12VDC elevated voltage, with 2VDC measured. I'll replace the 1ROJ resistor into the dropper position so see if that bumps it up, but are we looking for a failed electrolytic in the heater, probably c24 c25? Are these acting as filters for the DC supply, and could that cause a failure over time (1- 2 hours or so?)
                              If the 1R was in the place of the 5R, they probably found it was dropping too much voltage and adjusted the value.
                              Yes, the heater filter caps (C24 & C25) could be killing the DC heaters, I've seen it happen in other amps. But also check for any bad solder if the bridge is mounted to a circuit board (and any other solder connections in the DC heater circuit).

                              Originally posted by Enzo
                              I have a sign in my shop that says, "Never think up reasons not to check something."


                              Comment

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