Sovtek Mig 50 donar

[studiodunn]

I was just given this amp from a friend I do work for and was looking for project opinions.
I originally was planning on just giving the amp an overhaul, but after removing the filter caps and pots I noticed the screen resistors were burnt......one dead, lots of crappy wiring and the traces will lift if looked at wrong.
All this leads me to wanting to just clean it out and start fresh.

So what says the knowledge base here? You've got a great skeleton, a PT putting out 540v and not allot of real estate......go!

Wrong area........sorry folks. Mods please move this to where is belongs.

[pompeiisneaks]

Moved, the schematic doesn't seem to show what the output tubes are, but you've got a PT and OT that can handle 3 preamp tubes and a pair of something (EL34?). If it's EL34's go for a plexi or some similar variant, if it's 6L6 or 6V6 type, do a fender, or a tweedle dee deluxe or something similar, the sky's the limit!

~Phil

[tubeswell]

If the PT delivers over 500V B+, you won't want to run 6L6s (as its best to keep them under the 500V mark). KT77s or EL34s or 807s (different pin-out plus plate cap plugs needed) would work, or if you can find some good NOS 7027A (which is like a high-voltage 6L6). KT77s are high voltage tubes like EL34s, but need about twice the reflected load that EL34s do. KT77s and EL34s also need lower screen voltages and/or typically larger Rg2 (1k to 2k2, instead of 470R)

[Ten Over]

This amp used 5881 tubes that were Russian military spec. and not the same as the 6L6's that we are used to.

R7.1, R40 and R41 frequently burn. I kind of suspect it is caused by using the wrong tubes.

These are pretty cool, really tough amps that are just loud as hell and yours looks in pretty good shape to me. The circuit boards are real PCB's (as opposed to Chinese PCB's) and I've never had any trouble with the traces. They seem to be a little bit rare and they have some demand for them.

I notice you have the varistors on the power tube sockets instead of the diodes that some MIG 50's have. I have never gotten my hands on one that had varistors and I was wondering if you could get the numbers off of them and tell me.

The schematic you posted is wrong. That schematic got out there in general circulation and I don't think it is ever going away. If I can get a correct one onto this computer, I will try to upload it here for you.

[Ten Over]

First attempt at uploading a file.

[Stevem]

Gotta love those weird Ass Russky filters ,no?

[studiodunn]

Gotta love those weird Ass Russky filters ,no?

Kind of like spark plugs!

[studiodunn]

This amp used 5881 tubes that were Russian military spec. and not the same as the 6L6's that we are used to.

R7.1, R40 and R41 frequently burn. I kind of suspect it is caused by using the wrong tubes.

These are pretty cool, really tough amps that are just loud as hell and yours looks in pretty good shape to me. The circuit boards are real PCB's (as opposed to Chinese PCB's) and I've never had any trouble with the traces. They seem to be a little bit rare and they have some demand for them.

I notice you have the varistors on the power tube sockets instead of the diodes that some MIG 50's have. I have never gotten my hands on one that had varistors and I was wondering if you could get the numbers off of them and tell me.

The schematic you posted is wrong. That schematic got out there in general circulation and I don't think it is ever going away. If I can get a correct one onto this computer, I will try to upload it here for you.

Thanks for the help.
Yes the amp had 6l6wXT tubes in it when I got it.

pic of the varistors.

IMG_1788 (2).JPG

That said, with the high voltage and 5881's, I think I'll dump the low input and try squeezing all I can from the amp. I attached a "fill in the blanks" version of the schematic for any interested in sharing ideas. Can I just take
R23 to pin 7 and start adjusting CF values to suit?

Sovtek MIG 50-007_config001.gif

thanks again

[studiodunn]

How does this look for a start?

Sovtek MIG 50-007_KK.jpg

[Stevem]

That power Tranny may not output the needed extra 1 amp of filament current to power EL34 heaters !

[studiodunn]

That power Tranny may not output the needed extra 1 amp of filament current to power EL34 heaters !

I was going to stick with 5881s.

[studiodunn]

So, I got the amp back together and it fired up..............no smoke! Big deal for a hack like myself.

Now on to the modding
PT is putting out 460v / 565V on the screens.

How do I go about testing the PT to determine whether or not I can convert to el34s?

[pompeiisneaks]

You'd need to check if the transformer itself can handle the current requirements. See if you can get datasheets on the transformer. Look up the code on it and see if you can find anything.

~Phil

[studiodunn]

You'd need to check if the transformer itself can handle the current requirements. See if you can get datasheets on the transformer. Look up the code on it and see if you can find anything.

~Phil

No way to measure that?

[pompeiisneaks]

You'd need to check if the transformer itself can handle the current requirements. See if you can get datasheets on the transformer. Look up the code on it and see if you can find anything.

~Phil

No way to measure that?

Not easily, it's based upon the construction of the transformer, size of the gap, size/gauge of the windings etc.

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