EH mig-50 schematic
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Posted: 28 November 2018 03:56 AM

Hi,
I own a Sovtek mig-50 which I bought brand new back in 1998. It has lasted well over the years, the only major was having to get the input transformer rewound about 10 years ago when it burnt out. There’s now a few more issues. The amp often makes a bang when the standby is turned off, it has a constant hum and some of the pot’s need replacing. I’ve decided I want to give it a complete overhaul and make it as close as possible to the new Electro-Harmonix mig-50.
I’ve looked online for schematics and I can only find two; one by Bruce Collins of Mission Amps and the other by Elan Portnoy of New Sensor Corp. If you look close, there are a number of differences in component values between the two and many of the components in my amp are different to both. Which schematic is the same as the new EH mig-50, Bruce Collins or Elan Portnoy? And if neither is, then is there a specific EH mig-50 schematic available?

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Posted: 03 December 2018 08:52 AM | Link to this reply (#1)

Hi,

Falling on your topic while looking for informations about original Mig 50 schematics.
Though my english can be pretty bad, trying to help you may help me too. wink

These seem to be two reliable sources. Bruce Collins has a pretty good reputation, and it looks like Elan Portnoy used to be Director of Engineering at Sovtek around 1995. At least he said he was : https://www.harpamps.com/micKtubes/Sovtek-Midget.html

Let’s beleive both of them were genuine and told what they knew, I feel there are two reasons possible for these differences :

1- There are two main different MIG50. The first version without master pot (known to be very clean and difficult to make crunch, but maybe because of tubes choice), and an other one with master pot (known to be a growling beast, again, maybe because of different tubes). It could explain two different layouts, not only because these are two different beasts, but also because these two versions are some kind of top of the iceberg… there may have been several versions of these two main ones.

2- Sovtek isn’t known for having used stable sources of components. Don’t get me wrong, russian components can be very reliable (as much as they can be crappy… it depends what components). For instance during the cold war Russia was producing more reliable transistors than USA. Military grade was kind of the norm in USSR, and they produced components like crazy so it was pretty easy for Russian brands like Sovtek to get reliable components out of old Russian’s stocks. But Russian people are also known for being good at doing with what they got. It wouldn’t be surprising that if some ideal component wasn’t avaible during some weeks, but order still coming, Sovtek would have produce amps with component they had in stock. Even if values were different, cause “if it makes your windows shake while playing guitar, then it’s хорошо (good). Da !” Collins may just have correctly reported what he saw into an other MIG50.

So both layouts can be right, and your amp can be something else too, though still being a pure MIG 50…
^^’

IMO so many differences of components, while keeping people saying how good MIGs sound tells a lot about the quality of the design.

IMO again, if Portnoy may have had some reasons to not publish the real deal (copy protection), I don’t see why Collins would have fake his schematic. So I would prefer his layout anyway.


When it comes about EH giving the layout of their version of the MIG 50(no master copy), I would be very surprised (thought interested and grateful) if that would happen.


Which version of the MIG do you got ? With or without master pot ?

Edited: 03 December 2018 09:39 AM by ailurus
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Posted: 04 December 2018 06:18 AM | Link to this reply (#2)

Hi, thanks for your help. Mine is the mig-50, without the master volume. I’m pretty sure the one with the master is the midget-50H, which is a different model and schematic. I’ve had a bit of a think. I suspect Elan’s drawing is the true mig-50 plan, that was done at or for New Sensor Corp in New York as the drawing says. Sovtek in Russia, probably due to availability and/or financial issues made the amps with whatever components they could get that would do the job. Bruce’s drawing is an “as built”.

Therefore, I’m picking the new EH mig-50 would be closer too or the same as Elan’s plan rather than Bruce’s and so that’s the one I’m going to follow. This could also be why the new EH mig’s are considered better; they were actually done to spec! Here’s an example:

On the right are two linked resisters going into valves 4 and 5. Elan says each one is 750 ohm, Bruce says each are 470 ohm. My amp has 510 ohms. They both have signs of heat damage so I’m going to change them as per Elan to 750’s.
I was looking online for 750 ohm 2 watt resisters. They seem to be relatively uncommon. This may support what I said about availability in 90’s Russia…

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Posted: 05 December 2018 01:25 AM | Link to this reply (#3)

Hi !

While different your conclusion sounds pretty logic to me too.
It would be interesting to be able to compare various MIGs layouts and hear what effects that kind of changes produce in order to fine tune these amps to everyone needs.

I don’t know the impact of puting lower resistors at that particular spot in the circuit. But I know heat damage can come because of low practical wattage values. If I had to change that as you do, of course I would think about resistance value, but I would also put strong resistors, like 750v/2W ones. Because heat damages may likely show that point in the circuit is agressed, and also because high wattage values resistors tend to produce less contact noise.

I don’t know if EH MIGS are better, I haven’t heard one so far. Only thing I heard is tube sockets are on PCB, and I don’t like that at all. I’m not into buying one and lose waranty because I had to modify it so it can be as long lasting as most Sovteks.

Also I’ve found some pics of EH MIG PCB and it looks like capacitors may be Panasonics films (ECQ serie very likely) and carb films resistors. Panasonic capacitors are great, carbs resistors not so much. To me the strengh of MIGS is to not affect tone much so you can appreciate differences in guitars and pedals, and to give tons of old school balls under steroïds. I don’t beleive that carb resistors make a great difference in warmth, especially behind that kind of tube setup giving so much organic feelings. But it is pretty known carbs resistors come with 20% value variations and noises. So I would go for metal films, especially for big ones,like 1M. It sounds to me like a better choice respecting the transparent nature of MIGs while reducing unwanted noise. And once again I wouldn’t like to buy a brand new MIG and have to make such changes.

I wish I had a deffective Sovtek MIG to make all that changes. Lucky you. smile

Edited: 05 December 2018 01:29 AM by ailurus
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