Sovtek MIG50H (detailed)

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Sovtek MIG50H (detailed)

Postby Sir Fayzalot » 27 Jul 2018, 17:28

I have a slightly better scan of the schematic than the one usually found on the net. If you want an even more detailed scan, please send me a message.
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Re: Sovtek MIG50H (detailed)

Postby Sir Fayzalot » 27 Jul 2018, 17:33

My particular amp (stock) differs from the schematic in several ways.

When there is nothing plugged into the high gain input, the screen of gets connected straight to earth without going through R10. There is another connection in green (on the enclosed schematic) which differs, as well as two extra resistors, and some with differing values.

I have also noted the correct fuse values and the AC voltages coming from the transformer.
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Last edited by Sir Fayzalot on 27 Jul 2018, 17:35, edited 1 time in total.

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Re: Sovtek MIG50H (detailed)

Postby Sir Fayzalot » 27 Jul 2018, 17:34

There are two resistors in the heater circuit, they are not on either schematic.
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Re: Sovtek MIG50H (detailed)

Postby Sir Fayzalot » 27 Jul 2018, 17:50

Here is the preamp section. Note that R52 is really a capacitor and it has a capacitor symbol on the schematic as well.

(C)R52 and C5 were originally blue Russian caps, and R4 was originally a reddish brown Russian resistor, They are not stock.
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Re: Sovtek MIG50H (detailed)

Postby Sir Fayzalot » 27 Jul 2018, 17:55

Here is the power amp section. Note that the black filter caps are not original. The amp came with Illinois Capacitors there (really).
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Re: Sovtek MIG50H (detailed)

Postby Sir Fayzalot » 27 Jul 2018, 18:02

Detail around the power valves. Note that the 15K ohm resistor is stock, not Russian and not on the schematic (I bought this amp new).
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Re: Sovtek MIG50H (detailed)

Postby Sir Fayzalot » 27 Jul 2018, 18:04

Line output. I tried to record from this years ago, without success. I may try again.
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Re: Sovtek MIG50H (detailed)

Postby Sir Fayzalot » 19 Aug 2018, 09:10

I am not getting any signal out of the line output. I tried a 4-track cassette, a computer, and no signal. I checked the cable each time by plugging my guitar straight into it, and there was a signal that way.

I checked out the resistors earlier and resoldered them. No signal and I have never gotten any signal, as I tried this many years ago as well.

Anyone have any idea as to how this is supposed to work?

I guess I'll check the connections again later when I decide to open it up again, but I have a feeling I've done this before. I guess it's just as well that it doesn't work because I need an extra hole for mods, but it still bugs me.

-------

Another thing I wanted to point out about this amp is the cathode resistors. One was fried as in the above pic, and the other measures about 90K, which bumps the plate voltage to over 300V. Right now I have a long plate valve in there, but I'll put in another of closer value so I can put whatever valve I want in there.
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Re: Sovtek MIG50H (detailed)

Postby Sir Fayzalot » 19 Aug 2018, 12:49

Sir Fayzalot wrote:Another thing I wanted to point out about this amp is the cathode resistors. One was fried as in the above pic, and the other measures about 90K, which bumps the plate voltage to over 300V. Right now I have a long plate valve in there, but I'll put in another of closer value so I can put whatever valve I want in there.

A 100K resistor is what I meant, if it's unclear.
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Re: Sovtek MIG50H (detailed)

Postby phatt » 20 Aug 2018, 12:58

Sir Fayzalot wrote:I am not getting any signal out of the line output. I tried a 4-track cassette, a computer, and no signal. I checked the cable each time by plugging my guitar straight into it, and there was a signal that way.

I checked out the resistors earlier and resoldered them. No signal and I have never gotten any signal, as I tried this many years ago as well.

Anyone have any idea as to how this is supposed to work?

I guess I'll check the connections again later when I decide to open it up again, but I have a feeling I've done this before. I guess it's just as well that it doesn't work because I need an extra hole for mods, but it still bugs me.


That line out does not make sense :scratch:
If it's standard socket then when you insert a plug then R27 would be shorted to ground = no line signal :whappen:
It maybe a factory stuff up. As I;m not able to see the other side it's a guess but worth checking. :thumbsup
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Re: Sovtek MIG50H (detailed)

Postby Sir Fayzalot » 21 Aug 2018, 19:53

phatt wrote:
Sir Fayzalot wrote:I am not getting any signal out of the line output. I tried a 4-track cassette, a computer, and no signal. I checked the cable each time by plugging my guitar straight into it, and there was a signal that way.

I checked out the resistors earlier and resoldered them. No signal and I have never gotten any signal, as I tried this many years ago as well.

Anyone have any idea as to how this is supposed to work?

I guess I'll check the connections again later when I decide to open it up again, but I have a feeling I've done this before. I guess it's just as well that it doesn't work because I need an extra hole for mods, but it still bugs me.


That line out does not make sense :scratch:
If it's standard socket then when you insert a plug then R27 would be shorted to ground = no line signal :whappen:
It maybe a factory stuff up. As I;m not able to see the other side it's a guess but worth checking. :thumbsup
Phil.

No it wouldn't, part of the signal would go to the hot connection on the jack, part of it would go to R26 and then earth/ground.

However, it doesn't work. There is a similar line out with a couple of capacitors added on the MIG-60, if you care to look that up. Apparently someone has gotten it to work but does not appreciate the sound:
https://www.thegearpage.net/board/index.php?threads/sovtek-mig-60-review.1511734/

And in this thread there is someone who allegedly likes the line out on the MIG-50H:
http://www.marshallforum.com/threads/marshall-dsl-line-out.33387/

Although I'll take it with a grain of salt, people sometimes mention the wrong amp on forums.
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Re: Sovtek MIG50H (detailed)

Postby phatt » 22 Aug 2018, 02:59

Well that socket has 6 tabs which tells me it's a stereo switching unit and the middle terminal is used as a mount for junction of Speaker Hot and R27.
Now if it was wired up wrong side then there would be No signal.

It would not be the first time a factory product went out the door with a wiring fault. :slap:
Easy to check and simple to fix. Up to you [smilie=a_whyme.gif]
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Re: Sovtek MIG50H (detailed)

Postby Sir Fayzalot » 28 Aug 2018, 11:38

phatt wrote:Well that socket has 6 tabs which tells me it's a stereo switching unit and the middle terminal is used as a mount for junction of Speaker Hot and R27.
Now if it was wired up wrong side then there would be No signal.

It would not be the first time a factory product went out the door with a wiring fault. :slap:
Easy to check and simple to fix. Up to you [smilie=a_whyme.gif]
Phil.

I see what you're saying, but I thought it was pretty much standard that the side which is active is the left side looking from outside the chassis from where you plug in a cable into the jack. The lugs on the right side are suppose to connect to the ones on left side when nothing is plugged in.

All the jacks are set up like this on this amp in any case (look at the above picture with the speaker jacks), and all the switching jacks I've had my hands on are set up like this.

However, the line out jack is faulty. This is not uncommon for this type of jack. That's the one that came with the amp, but all the switching jacks I've bought new locally have been faulty in one way or another.

I only use the kind of jacks that go into Marshalls now. I don't think I'll waste one for a line out I'm not going to use though, I have quite a few mods planned that I might even have to go with a rotary switch.
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Re: Sovtek MIG50H (detailed)

Postby Sir Fayzalot » 10 Sep 2018, 07:48

Changed the plate resistor with low resistance to one that has 100.8K ohms and now I'm getting 290V on the plate, which is what I wanted so I can use any ECC83/12AX7 without frying it.

I'm going to change some of the pots, I noticed they are stamped "MOUSER".

It's a real pain to work on this, space is tight and I've ripped some of the leads to the main board several times, changed a few as well. The amp is just as heavy as the non-master volume MIG-50 because of the heavy transformers, so I'm not sure of advantages of it being small, but it does look and sound good.

I'm thinking this amp is related, other than the bass version of the MIG-50H (MIG-500) and the 100 watt amps:
http://oecc.ru/amplifier/forte-tube-50
This isn't Sovtek, but it was also made in Saint Petersburg. It was part of a small run in the 90s, possibly made at the same time as the Sovteks. The panel and top of the cab are more Fender looking and the amp has an effects loop. The earlier amps in this series were solid-state and had the controls labeled in Russian.
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Re: Sovtek MIG50H (detailed)

Postby Sir Fayzalot » 10 Sep 2018, 11:56

Also want to point out that in the 8th post I wrote "cathode resistors" but meant "plate resistors".
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Re: Sovtek MIG50H (detailed)

Postby Sir Fayzalot » 04 Oct 2018, 14:12

Corrected power amp section, C8 was labeled as C6 on first version.
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