Sovtek Mig 60

[plexi50]

I have another tone fade out problem. This Mig 60 sounds great for about 2 minutes or more and then the volume drops down to near zero. It is very faint after this happens. I checked all the voltages while this happens and all voltages are present on the preamp plates,cathodes,screen and all other vital voltages needed to operate including the bias which is good at -32VDC

If i wack the OT with a heavy screwdriver the volume will come back to full but after 2 minutes or so is repeated again with a fade out. I know these amplifiers had known OT problems. But if it were BAD i wouldnt think it would function as well as it does and then fade out. It sounds like a bad solder joint or something but i have been over all of them. Plus when this happens like i said all the amps voltages are still present and solid. All resistors read there good values. I have changed the cathode bypass cap

Any other tips?

[Tiny_Daddy]

I think the only thing to do is sub in a known good transformer.

[HotBluePlates]

Tips:
Signal generator, listening amp, oscope (for looking for oscillation). I know I'm repeating myself from the other thread, but these are the things that will track the issue in the least time.

You wouldn't let a doctor send you to surgery to get an artifical heart if he didn't do some in-depth testing, right?

[plexi50]

Nope. Your 100% right HBP. I know a man can not do his job if he dosent have the right tools.  I know i will be glad when i can complete my bench with a signal generator

No nickle no phone call

[Geezer]

However, (in lieu of the signal gen) subbing in a known good OT will at least show whether that's the culprit.
I'm sure you have something around the shop you could use.....

[kagliostro]

If you have not an oscilloscope is one thing

but if you miss only the signal generator you can try to use a registered track in a CD

or a signal generated from a PC
(that can also be used as an oscilloscope with programs like virtins - www.virtins.com)

Kagliostro

[Ritchie200]

Hey Plexi,

Did you happen to chop stick around on the board?  The Migs were not exactly known for their build quality...  I would think that you would eventually see other issues if the tranny was shorted.  I'm wondering if your screwdriver impact was jarring something else.  Just a thought...  Good luck!

Jim

[Merlin]

Another thing which can cause this is a dirty/rusty fuse holder. It's probably not, but it's an easy thing to check.

[plexi50]

Hey Plexi,

Did you happen to chop stick around on the board?  The Migs were not exactly known for their build quality...  I would think that you would eventually see other issues if the tranny was shorted.  I'm wondering if your screwdriver impact was jarring something else.  Just a thought...  Good luck!

Jim

Merlin your 100% on that. I had that issue a month ago with an old Gibson Atlas amp. I removed the fuse and at that time noticed it was spring tight but wiggling it told me what was wrong. It those little things that get ya every time. So much to remeber time after time. You would think by now i would have written down a chain of command and eliminate one suspect part at a time. I do for the most part but the fuse thing can slip your mind at times. Always expecting the worst case but most of the time it's minor me error

Jim there are so many chopsick dots in the chassis it sort of looks cool by now
I did everything i know of except marinade the chassis
I replaced the OT with a Magnetics Components JCM 800 OT / Man they are cheap but look pretty well made / $59.00+ship

The Mig 60 has been on for 5 hours so far and not a sign of tone dropout. I played it for about 30 minutes and it sounds great
I like this amp much better than the Mig 50 / The Original OT would make a good paper weight or conversation piece

 

[SoundmasterG]

Those Mig 60's run really high plate voltages and sound a bit fuzzy, but are ok. Just make sure to keep good power tubes in it as not every EL34 type can handle the 600+ voltages these have for B+. My brother had one when they first came out, and it ate a couple Sovtek EL34G's (the tubes it came with, no surprise there, those were shitty tubes). They got so hot a hole got sucked in the glass from the inside! I never messed around with it to tame the fuzziness though. We sent the amp to New Sensor under warranty, and when it came back, he traded it for a Silvertone 1484 that he still has. The MIG 50H Midget was probably the best made of the Sovteks, and sounded somewhat like a JCM 800. The MIG 50 was more like a Bassman and had spotty build quality like the MIG 60. The MIG100 came in a U version and an H version, with the H sounding like a JCM 800 and the U sounding pretty blah. There was even a bass amp pair too, though I've only heard one of those. (It was ok but nothing special)

Greg

[plexi50]

Yeah i noticed that 600 VDC. I know there isnt much desire on the tube makers part to build great quality tube as in the Good Ole days. You what they say: If you want some thing done right you have to do it yourself. But im not about to attempt to learn how to build tubes. I have gone far enough as it is and my brain is almost full. I have to download whats in there now before it gets corrupt and loose it all. 

youtube had that cool video on how a guy in Europe made his own tubes last year. Im sure it's still on the site


D*m they have the Original Mullard equipment to manufacture the best of the best. But i assume that there is no one left alive to teach enough willing people that would be passionate about what they do nor understand the importance of pride and excellence. JJ though does make a nice $14.95 tall plate large halo getter ECC803s. Still that tube is a little sterile in different amps. This may not be the tube though but the amp. Long live Old School /

[Geezer]

What does the script writing (in red) say (between the "Model mig-60" & the "lead tube amplifier")??

Cuduno?
Cujumo? 

:icon_scratch:

[plexi50]

What does the script writing (in red) say (between the "Model mig-60" & the "lead tube amplifier")??

Cuduno?
Cujumo? 

:icon_scratch:

I dont know Geezer. It's messing my eyes up looking at it. I think it say IDUNO.  (I Dont Know) in Russian

[VMS]

You guys are close, but I think it says:

Сибирь

Which means Siberia in Russian.

[The_Gaz]

About the 'quality' of the Mig series: I've seen the inside of at least  3 Mig 50s, and 5 Mig 100s. They all looked different - from cold joints and brittle leads, to clean and sturdy. The rumor is that they are called "Migs" because they used surplus military parts, namely from Mig fighter jets. It still amazes me that a hardwired amp was ever so cheap. I also had a Red Bear, another Russian amp paid for by Gibson. That amp sounded like crap until I realized almost all the components in the tonestack were incorrect. I changed them out, and it almost sounded exactly like a 2204. That amp was very cleanly built, however I'm sure some of those got a bad rap for using surplus parts whose values were way off.