Sovtek Mig100U - Help!

[abeck591]

Hey guys, new to the board!

I have a Sovtek Mig100U, that died mid-practice yesterday. Was working great, when I put down my guitar for a moment, and when I came back to it, it was dead. No smell, no smoke, no sound. Tried different speaker cables, and a different cab, but nothing worked.

I got it back on my bench, and immediately tried to ohm out the speaker outs. It has a line out, two 4 ohms, and one 8 ohm out. The outputs seem to be measuring what I would expect from a 2 ohm output, clearly not right. I seem to have about 45 ohms between the center tap of the OT and the anodes of the tube sockets. My question, and concern, is that would indicate the OT is dead, right?

I've done a good amount of repair work on modern power amps and vintage recording consoles, but don't have a ton of experience with diagnosing busted OT's, I was under the impression that was very uncommon. I just can't think of what else this might be!

Thanks for whatever help you can give!

[nickb]

Hey guys, new to the board!

The outputs seem to be measuring what I would expect from a 2 ohm output, clearly not right. I seem to have about 45 ohms between the center tap of the OT and the anodes of the tube sockets. My question, and concern, is that would indicate the OT is dead, right?

The DC resistance of the secondary i.e. outputs would be very low certainly less than you can measure with any accuracy with the average DVM. So what did you actually measure?

45 ohms on the primary plates to CT is fine.

Is the indicator lit? Is the fuse blown? Can you see the heaters light up? Have you measured any voltages? Start with the basics before suspecting particular components.

[Justin Thomas]

Since you've got some electronic experience, check out RG Keen's tube amp debugging page:

Geofex.com

Tube Amp debug page in top left corner. And, it will confirm your suspicions that the OT is probably the LAST thing wrong with it...

Justin

[The Dude]

^^^^ Agree with the above. As you properly stated, OT failure is very uncommon. Check for a blown fuse, first. If the amp is blowing fuses, my first suspect would be an output tube. If you think that might be the case, remove the tubes and see if the amp will hold a fuse.

BTW, Welcome to the place!

[gbono]

Tubes light up? Schematic? Was the amp in standby?

[abeck591]

First off, thanks a ton for the speedy replies, and I realize I left out a fair amount of my fact finding.

Indicator was fine, both fuses are OK, my recollection is the heaters were on. I will be grabbing my dummy load tonight, but testing voltages is next on my list. I just removed the speaker jacks after reading another thread on this forum, and noticed one of the plastic nuts was cracked. One odd thing, and I think uploading some pictures may make this easier, but I think the output wiring for the 8 ohm jack might be backwards. The 'blue' wire coming from the OT goes to the sleeve of the 8 ohm jack, while the 'green' wire from the OT goes to the tip. The 4 ohm jack's sleeves are all ganged together to a ground point on the chassis. There is then a wire from the sleeve of the 4 ohm jacks to the sleeve of the 8 ohm jack. I can post pictures in a little bit if that was confusing. Seems wrong to me, though? Theres also a a large white wire and a small white wire coming off the OT, going to the tip of the 8 ohm jack. Will report back with a schematic, voltages, ect.

[g1]

It was working then it quit. So it's a faulty component or connection. It's not something miswired or it wouldn't have worked before.

[abeck591]

G1,
Even though I found the broken washer?

Am I way to concerned that all the speaker jacks are measuring the same (if not accurate) ohmage?

[The Dude]

......Am I way to concerned that all the speaker jacks are measuring the same (if not accurate) ohmage?

Short answer: Yes. As nickb pointed out, "The DC resistance of the secondary i.e. outputs would be very low certainly less than you can measure with any accuracy with the average DVM."

Any schematics I have of the Migs don't show any chassis isolated outputs, so IMO, the broken washer is meaningless. Speaker ground = chassis ground, so I don't see a broken plastic washer causing a problem.

[abeck591]

Update: plate voltage was 500v even. Seems a little high to me? What're peoples thoughts about 6L6s that hot? The 6L6s seemed a bit dim to me, honestly.

BUT, I did find a broken trace off one of the 1000uf caps going to the v1 heater. I'm guessing thats ultimately what gave. I'm still sort of curious about the output wiring, can anyone lend any info on that?

[Justin Thomas]

I bet it came with Sovtek 5881WXTs. They won't blink at that voltage. Keep those in it, and I bet you'll never have a problem. They go in Ampegs @550-590 all day long for years.

I worked on one of these a while back. Liked it better than the 78 JMP I had in at the same time.

Justin

[Leo_Gnardo]

Update: plate voltage was 500v even. Seems a little high to me? What're peoples thoughts about 6L6s that hot?

Typical for Sovtek amps. No worries as long as the bias current isn't sky high. Somehow they seem to run well with Sovtek wafer-base 5881's. 30 to 35 mA is plenty, I usually clock 'em at just under 50W per pair at clip. Good tough amps, still at "sleeper" prices.

[Enzo]

Hi, get a grip. 500v is fine, the circuit as delivered is fine, as you reported, this amp has been working. So your jacks are not wired wrong.

As was said above, you cannot measure the IMPEDANCE or a transformer with the resistance scale of a meter. The resistance of the secondary is VERY low. REsistance is not impedance.

If you can SEE the heaters glowing, then the amp should make sound if everything else is OK. IN other words, dim heaters is not why your amp stopped working.

[gbono]

Is there a signal at the line out jack on the back of the head?