Sovtek MIG 100U-needs TLC

[Steve Fundy]

Hello all, It's been some time since I have needed to call on the "Gods of the electrons" but I do need some advice, so here I am. I am trying to revive a Sovtek MIG100. I have very little prior experience with Sovteks. I know they're similar to Marshalls, and the technology isn't rocket science, but....
This amp was deemed to have a bad PT, and it was removed and the bells were removed. Since a replacement is rather pricey (Mercury $377) I decided to reassemble it and test the voltages. All of the voltages actually are there, though the high voltage taps are definitely off, rather low @408V. However, I decided to put it back into the chassis, reconnect and see what happens. After reassembly, I tested it on a light bulb tester, and I have a short. The short occurs when the bias circuit is attached, and goes away when I lift the yellow lead attached to the ground side of the circuit. I have checked, replaced and/or isolated all associated components, but I still cannot determine the reason for the short. I'd really like to revive this amp, which belongs my luthier friend. He's repairing one of my guitars and a trade of services is in the works. Any help greatly appreciated. Many thanks, Steve

[johnnyreece]

Some voltage measurements at different points in the bias circuit would be a good start.  How much bias voltage are you getting at the tubes?  A schematic wouldn't hurt, either. 

[Steve Fundy]

Hi, So unfortunately I haven't been able to measure any voltage on the bias circuit, as when I attach both leads from the PT it goes short. When the leads from the PT are disconnected I read 67VAC across that pair. When I attach the +Ground side of the bias pair, that's when the short occurs. I do have a schematic, and have attached it.
Thanks

[sluckey]

The short occurs when the bias circuit is attached, and goes away when I lift the yellow lead attached to the ground side of the circuit.

Your schematic shows three yellow wires on that PT. Two for the bias winding and third wire for the HV winding that connects to the junction of two series filter caps. If you connect this third yellow wire to chassis you will create a short that will destroy two of the bridge diodes.

Use your ohm meter to be absolutely sure you are connecting the correct yellow wires to the bias circuit.

[Steve Fundy]

Hi Sluckey, Thanks for the reply. Well, I am pretty certain that the 3 yellow wires are connected to the proper place, as a result of the length of the 2 yellow wires for bias circuit are longer, the same gauge and reach the 2 terminals on the board to where the bias circuit begins. The 3rd yellow wire which I believe is the CT of HV winding is of a thicker gauge, and shorter, reaching perfectly to the smaller board where the filter caps & diodes are and the terminal which connects to the 2-100uF caps.
I did isolate all 3 of those leads and did some testing with my meter. I measure 24 ohms between the 2 wires I believe to be the bias winding. From the shorter, thicker wire (likely CT Lead) I measure 15 & 36 ohms respectively to the 2 bias circuit leads. There shouldn't be any connectivity between those points, correct? Also I measured from the "CT" lead to other leads coming from the transformer and the only others that had any resistance were the two red leads from HV winding. That was as expected. This PT was "alleged" to be bad. Now that I see that resistance between "CT" and bias leads I am concluding that must be why my lamp tester shows a short when I connect that ground side of the bias circuit. What do you think?  Thanks!

[sluckey]

That PT is bad.

[SoundmasterG]

I've got an output transformer for one of these amps, and a choke, but I used the power transformer in a build, so I can't help you there. At home I believe I have pictures of a stock one of these too, so it should show where all the connections go. Won't help much if you have a bad PT but thought I'd throw it out there.


I had one of these MIG-100U's given to me years ago. I fixed it and while it worked and sounded ok, I figured I could do better, so I got a Heyboer OT and choke and made it into a Vox AC100 clone. It sounds WAY better than the Sovtek stock circuit did.


Greg

[Steve Fundy]

Thanks Sluckey. I knew it was bad, but didn't consider that the potential between the CT lead as the likely reason for the short. Thanks for pointing me in that direction. Now my friend needs to decide if he wants to pony up some serious $$ to have me try & bring it back to life.

[Willabe]

Those Mig amps are known for using too high a B+ dcv for the power tubes. Schematic shows 520+ dcv for the B+ feeding the 6L6GC's. That's really too much for those tubes.

If I were going to put a new PT in to get the amp to be usable again, I'd get a PT with a lower B+ wind. I would try and stay at 470dcv or a little lower.

[Steve Fundy]

Yes, and Thanks Willabe...I knew that. It's not mine & it's a "swap repair job" between me and my luthier friend. I had a guitar get knocked over & "lost it's head..lol" so we're swapping repairs. He wants to have me convert it to KT88s or 6550s which as I recall can better handle that higher B+ as compared to 6L6s. So, if i can get it working again, then I'll mod it as he wants me to.
Thanks

[Willabe]

He wants to have me convert it to KT88s or 6550s which as I recall can better handle that higher B+ as compared to 6L6s.

Yes, but they need/draw more heater current than 6L6's.

Can't use a PT with the same ratings, like a Mercury clone, as the original.