Differences in power supply circuit JMP/Sovtek MIG 50

[beasleybodyshop]

So I picked up a Sovtek MIG 50 the other day for cheap in good condition. I had heard that it was essentially a JMP clone, so I dug up some schematics for it and compared them to a 1987x schematic I have (both are posted)

Aside from resistor/capacitor value changes in the preamp (topology looks the same) I noticed some differences in the bias circuit of the MIG 50 compared to the JMP - see the 1500 uF cap by the bias pot to ground? Big value difference from the two 8uf caps in a Marshall bias control. Also on the MIG i'm seeing a 22uf 25v cap and a 56K resistor to ground right off of one of the windings on the PT - the MIG has no choke and the Marshall does - would it be correct to assume the cap/resistor is subbing in for a choke? I see 3 windings on the secondary side (as opposed to the 2 sets in a Marshall, B+ and filament) What is this additional winding do?

So i guess to make this easier i will sum it up:

1. Why such a large amount of capacitance in the bias control?
2. 22uf 25v /56K resistor on secondary winding - does this replace a choke in the marshall?
3. What is the deal with the 3 secondary windings?

Any clarification would be great, just trying to wrap my head around this!

[wyatt]

It's more of a modded 5F6a Tweed Bassman blended with JMP.

I would guess that separate heater windings were to make use of the PT they had on hand. Funny thing is the HT winding is exact to the original B;50's Bassman PT, except they didn't compensate for the modern wall voltage or the SS rectifier and ended up with a B+ in the 525V range.

[John_P_WI]

On the Mig-50, the 500 ohm, 3 watt resistor replaces the choke.

The 22u and 56k elevates the heater center tap, the schematic may be missing a second resistor to use as a voltage divider???

[martin manning]

Does the amp you have match the schematic? I bet there is a resistor between the screen node and the 22u cap. The V1 and V2 filaments are elevated, but they surely didn't want to elevate them to screen voltage. Re the multiple windings it's almost like they made do with some existing transformer.

[NickC]

MIG 50 has a secondary PT tap for the power tube heaters, and another one for the preamp tube heaters. Different voltages, the PT secondary voltages and wire colors are as follows (I ended up replacing the stock PT with a MM built for modern wall voltage ..... as you'll see, the filament voltage is already high for the big bottles):

B+
Red 0V
Red 453V

Bias
Black 0V
Green/Yellow 67V

Green
White 6.3V Filament (preamp tubes)
Green

Brown 0V (heavier gauge wire)
Brown 7.3V Filament (power tubes)


Somewhere here at TAG I have a build-log for gutting and rebuilding mine with a home-brew G10 circuit board ...... because what started as a recap job escalated on account of the pads lifting off the cheesy PCB.

[beasleybodyshop]

Nick, Mine also has a MM transformer in it as well.

I think this amp would be a good candidate for a board replacement as well, Im going to try to find your post. Very interesting!

So whats the deal with the big ol cap in the bias circuit?

[beasleybodyshop]

Nick, I looked for your MIG50 post. I found it but all the photos are missing

I like the idea of doing a handwired epoxyboard. Did you copy the PCB layout ? Or did you just follow a JMP/JTM layout?

[beasleybodyshop]

Gut shot

[NickC]

Nick, I looked for your MIG50 post. I found it but all the photos are missing

I like the idea of doing a handwired epoxyboard. Did you copy the PCB layout ? Or did you just follow a JMP/JTM layout?

Sorry about the dead picture links. Apple nuked their web hosting a year or so ago. I'll post some pictures here. Hope they help (contact me if I can be of any assistance, but keep in mind I'm strictly an amateur builder).

[NickC]

Couple more piccies .....

[NickC]

.......

So whats the deal with the big ol cap in the bias circuit?

I used a 100uF 160V e-cap for the bias circuit.

[NickC]

Some more photos from my build log attached. Five more to follow, see next post.

[NickC]

Remainder of photos attached.

This build was messy, largely due to my inexperience. There were several iterations, as the project morphed between becoming more of a Blackface Bassman or Marshall JTM45, both very similar to the MIG50. I did not do a thorough job of documenting those changes.

I probably could have made the board smaller. There are some components that would probably be better mounted elsewhere, either on tube sockets or front-panel jacks (68k and 1 Meg input resistors). I mounted the 100uF 160V bias e-cap on the main board, but other bias components (diode, resistor) on a small piece of G10 mounted on (but isolated from) the PT mounting bolts. Probably would be better to consolidate the entire bias circuit in one place. At one point I had the power supply mounted on a G10 board on the chassis top (ala' Fender) but without a doghouse. Bad idea, Dangerous. Changed to chassis mounted can caps.

You'll notice that placement of the PS rectifier changed in some of the diagrams. It ended up being mounted on the far right side of the main board, near the PT.

I am sure a more experienced builder will greatly improve on my layout.

I ended up removing the flyback diodes on the big bottles, that are on the original schematic.

There appears to be a resistor apparently missing from the schematics widely available on-line (but present on the original main board). See picture of MIG50 schematic with notation. I don't know if that was an undocumented change at the Sovtek factory, or oversight in drawing the schematic, or simply something I misunderstand. In any case, without that resistor (which IS on the original main board) there were several burned resistors and lots of smoke as I grappled with trouble-shooting. At one point, I modified the power supply and added a choke ...... later removed once I traced the original board and discovered the 1 Meg R discrepancy.

I hope these notes help, rather than add more confusion. Sorry I didn't do a better job documenting the build. I still have a LOT to learn.

[beasleybodyshop]

Nick, thanks for posting this. I will put some time in on it and document my build here on TAG

[beasleybodyshop]

Nick, looks like you have more than one schematic for the MIG 50? I spot the Mission Amps one (thats the one i have) and the other one where you notated the missing resistor. Do you have a link for that schematic as well? I can't seem to find that one around. Thanks.