Sovtek Mig-50 repair

Check out this beauty!  It's a Sovtek Mig-50 model 001 guitar head.  If you look closely, you'll notice that they misspelled "Presence".  I was told that this means that it's a rare first issue, which might explain why it doesn't work.

In Soviet Russia, guitar plays you.

This amp has been broken and in the possession of one of my bandmates for quite some time.  A month or two ago, he and his father attempted to fix it, and briefly succeeded by replacing a resistor (so I'm told).  The amp lasted for about 2 hours of play time before it just quit working again.  We're going to open it up and try to figure out what went wrong.

Removed the wooden cabinet.  This is the equivalent of a circuit in it's underwear.  Stay tuned for bare circuits

You can see the output transformer on the left, the power transformer on the right, 5881 power tubes, and the two cans are power supply capacitors.  Behind the output transformer hides 3 12AX7 preamp tubes.


Turning over the chassis revealed the problem immediately.  Two of the power resistors are scorched, and one is completely missing.  There's another less scorched resistor out of frame.  We tried for an hour or so to find a schematic for this amp online, but to no avail. This is the closest thing we could find (http://music-electronics-forum.com/attachments/8148d1266759648-mig-50-oem-full-crop-print-landscape.jpg), but the resistors in the schematic and on the board (R34, R27, etc) don't match the schematic.  I traced the components by hand to figure out which resistors had burnt out according to the schematic.  I found another Mig 50 schematic which the same power supply design which actually listed the power rating for the resistors, and hopefully this is going to work.

R34?!?  Where are you R34?

I have every value of 1/8 watt through hole resistors you can buy, but I don't have any of these mammoth 2 and 3 watt resistors.  Lucky for me, Digi Key exists, and I found all of the parts I needed, and a few spares for about $7.

Now, we play the waiting game...

Getting closer

The time has come!  I removed the two 12'' speakers from a Crate GLX212 that I purchased about 10 years ago, and secured them to the baffle.

I used #8 Brass screws and Tee nuts to secure the back plate

Connect the speakers!

Screw the baffle on.

Amplifier put in it's case.

Booyah!

So... there's a problem already.  That cabinet is enormous.  I can barely get my arms around it.  I brought it upstairs from the basement and wasn't able to get my hands on the side handles.  I had to just squeeze either side to get a hold of it.  By the time I got it up two flights of stairs, my forearms were like jelly.  I could barely play a bar cord.  I have a looper pedal that has some stuff recorded already, so I just turned on a loop to see how the amp sounds.  There's something off about it...  I think I hooked up a speaker backwards.  There's some point where ones head is directly in front of the speakers where it's very quiet, but it's quite loud a foot to the right or left.  Hmmm...

Waiting...

I've was practically peeing my pants all last week in anticipation of that sweet sweet vinyl so I can finish up the amp and cabinet enclosures...

To my dismay, the package arrived with the grill cloth and piping, but no vinyl.  Evidently my vinyl was back ordered and I have to wait another week or two.  Bllllalaaarrrhhghghgh!!! /shakes fists at sky

In the meantime, I constructed a frame for the grill, and painted the inside of the amp enclosure.  Also, I built an amp enclosure.

Amp enclosure, unpainted (in case you couldn't tell)

Since the inside of the enclosure will be partially visible, I just hit it with a few coats of black spray paint.

I also put the cabinet pieces together and stared longingly at it.

It says to me "I'm naked!  I need my vinyl covering!"

I guess all good things come to those who wait, and I'm stuck until the vinyl arrives.  I can only hope that the package is delivered by Gandalf, on his mighty steed, Shadowfax.  I heard that horse can run fast.

No more Outkast puns for my guitar cabinet

Not much time to work on the cabinet last night...  Did manage to get the edges rounded, holes for the handles cut, and test fit the hardware.

Used 1/2'' round router bit for those smooth as a baby bottom edges.

Harsh edges

Smooooth edges, like yacht rock

Better be sure to draw the proper outline for the handles.  I did it right on the first try, which rarely happens.

Finally, the cabinet and I turned the lights down, and I dolled her up with some corner hardware, a side handle, and just for fun, a top handle (which will eventually be on the amplifier enclosure)

I'm a pretty girl!

This must be the feeling that young ladies get when they dress up their dolls...

Well, silly me.... I'm not getting my vinyl and grill cloth delivered until next week, so I'm kind of at a stand still.  Maybe this weekend I can get proper enclosure ready for the amplifier, but other than that, I'm stuck until the vinyl arrives.

The Cab Below

Yo ho!  Had an interesting time tonight working on the cabinet.  I know my speaker needs a circular hole of 11'' diameter... so I pounded a nail into the center, and tied a string (5.5'' length) around the nail and to a pencil.  I don't have a proper compass... oh well.  I drilled a 1/2'' hole near the edge of the cutout region, and then cut the circle out freehand with my brand jigsaw... the one that I bought JUST for this project.  Hooray for new tools!

Gonna cut me a hole... gonna put a speaker in it

Black spray paint for the baffle. 

I learned from searching forums that you should probably paint your baffle black... otherwise the light wood will show throw the speaker grill.

In order to secure the backplate and baffle to the enclosure, I added battens (I still am not sure that's what they're called) of 3/4x3/4 all the way around the interior for both sides.

Add caption

Then I still had time to cut a hole for the jack plate in the back panel  I cut the hole a tad too big... but it's still going to be workable.  I'll remember to cut the holes for the handles a wee bit too small; you can always make the hole bigger!

That jack plate, by the way, is awesome.  You hook up two 8 ohm speakers to it, and by using different inputs, you can get either a 4, 8, or 16 ohm impedance... awesome.

I also stopped by ax man to pick up that awesome red vinyl... but they were sold out!  Oh well.. that's what I get for not purchasing right away from ax man.

I ended up purchasing some similar vinyl from Mojotone.com, as well as some piping and grill cloth.

ETA for handles/corners, tomorrow night.  ETA for the mojotone stuff, two days.

Awwww yeah!

Speakerboxxx

Allrighty... I got the frame built, glued, and screwed.  Since my table saw is el busted, I used a circular saw (and a straight edge) to cut a speaker baffle and a back plate out of my 3/4'' birch ply.  Things are starting to take shape, and I'm totally stoked.

Yeah, I'm a speaker box and I'm tough, natch.

Speaker baffle with speakers outlines penciled in

I think my next step here is to cut out the speaker holes... I'm thinking I can draw the cut out hole, and then it with a jigsaw.  It doesn't have to be pretty, since it's going to be covered by a speaker grill.  Any tips on cutting large (11'' diameter) holes in wood?  I was considering fabricating a jig for my router... but that might just be more work than it's worth.

I will also need to add some battens to support the baffle and back panel, cut a hole for the jack plate in the back panel, and cut holes for the handles in the side panels.  Then I'll need to round all of the edges for that special feel.

This amp will also be covered in vinyl... I saw some awesome vintage looking red vinyl at Ax-Man, so hopefully they still have it in stock... maybe I can even find some grill cloth!

Speakerboxx/The cab below

After much ado, I've finally gotten around to assembling the cabinet.  I'm going to make a 2x12 cabinet with some speakers out of a crate GLX 212.  Crate doesn't want to release the Thiele Small speaker parameters, so I'm just gonna make a cab, pop 'em in, and see how it sounds, because I'm flying by the seat of my pants, and have only a small clue as to what I'm doing.

Construction begins with 3/4'' birch plywood.  Unfortunately, while I was ripping the pieces down to size, my table saw decided to lock up, and start spewing thick clouds of smoke into the basement, setting off the fire alarm, and terrifying the cat.  Luckily, I have access to another (nicer) table saw, and was able to get the pieces cut to size.

I'm cutting what I think are called Dado joints to join the sides.  The table saw left some nasty ridges, so I hit 'em with some sandpaper to calm them down.

Pre sanding

so fresh and so smooth, smooth

In order to strengthen up the joint, I put what I think are called "battens" in the corners.  I'm not too familiar with carpenter lingo, so someone yell at me if I got that wrong.  Also, praise me if I got it right.

Clamp 'em down and screw 'em in!

It turns out that clamps are expensive... I'd spend hundreds of dollars on proper clamps if I wanted to get this box done all in one shot.  Instead I got a couple of 90 degree clamps and I'll do one side at a time.  I suppose I could have gotten 8 of the corner clamps, but they're $8 a piece... I guess I'll just have to take this build slow.

A ton of glue, obviously.

Time to let the joint cure, and then it's time for side #2. I'm hoping to get the rest of the sides done tomorrow night, then I can start working on the baffle and the back plate.  Wooo!

AX84 noise immunity.

Adding some noise immunity!  Here's some brand new twisted filament wires.  I re routed them so they are as far away from the signal wires as possible (green/white twisted wires)

It seemed to help a bit, but at high gain levels, there's still plenty of 60hz hum.  I was advised by another AX84 building to use a filtered AC input module, so I'm going to get a hold of one of those.

I also realized that the schematic had indicated that I need to ground the speaker outputs.

I didn't ground it exactly to point B... I just ran a wire directly from the AC input earth ground to the speaker output.  It's the lone green wire in the picture below.

I was able to solve some of the odd oscillations in the tone stack by adding three components.  This does result in some losses on the high end... hopefully it's not too noticeable.

I also found neat tone stack calculator from Duncan Amps, available for free!  It should prove useful for future designs.

Once I add a filtered AC input module, and probably a bottom plate for the chassis, this baby is going to be ready for an enclosure.

AX84 Noise issues...

Now you can see and hear noise issues from the AX84 hi octane.

After the video, I did some more probing on the tone stack, and found that in certain parts of the circuit, when I put the probe on, the oscillations would stop.  Those voltage probes have a small capacitance to them, so I added caps into the circuit and it seemed to stabilize.  I still don't know *exactly* what I'm doing... but it seems to be helping.  I'll have to post some new clips as the debugging continues.


After some online reading of other folks troubleshooting these amps, I might have an explanation for the 60hz hum.  Each tube needs a low voltage to heat the filaments.  This amp has a 6.3V AC signal to do that job (green wires on the tube sockets).  I made a mistake in my wiring... I should have twisted the filament wires together and kept them as far away from the signal wires (red wires on tube sockets) as can be...  Neither of which I did.  There's only a few wires to re route, so it should take more than 10 minutes.  Did you notice in the video how I switched out a gain stage and the 60hz stopped?  That could be because that particular gain stage has one of those AC filament wires running directly underneath the signal wire.

At least I have some possible solutions, eh?


Questions?  comments?   Ideas for reducing nose?

Email me or leave a comment!

AX84 Hi Octane build part 4: Wiring, testing

So I left off with installing all the parts in the chassis and ruining the finish in the process.  Now it's time to wire everything up.  Following the schematics and wiring diagram is easy enough, just takes a while.  First thing I noticed is that when I drilled the hole for one of the tube sockets, the chassis wasn't clamped well enough and it vibrated around, making the hole a bit too large.  The socket isn't secured and needs to be fixed to the chassis.

A little epoxy should do the trick

Then, on to finish the wiring... through the magic of internet it's all done, yay!

So, working with tubes is dangerous, the voltage rail is around 300 volts, and it can be deadly... so I'll be careful.  I ran through a "first power up" check list that I found online.  You basically take out the tubes, and turn the unit on.  If nothing blows up, you measure voltage rails and make sure they're at appropriate levels.  Everything checked out so I plugged in the the tubes and fired it up.

Plugged in a guitar, and no sound... hmmm.... After some digging around the circuit for a while I found a shielded wire where the conductor had shorted to the ground shield.  I replaced it, and then found another shielded signal wire that I had just forgot to connect... ooops.

With a now complete signal chain, I turned the amp on again, and I finally got actual noise!  Not very much though... I can only heard anything with the gains and volume knob turned all the way up.  Turns out I also forgot to connect a filament wire for one of the tubes.  Grrr...

So, I connected the last filament wire and finally I have sound!  LOUD sound!  It's only 5 watts, but you'd be surprised how loud 5 watts gets.  I messed around with the knobs for about 15 minutes, and discovered that there's some settings that hum a bit too much for comfort.  So I have some work to do fixing the hum, but it already sounds quite nice, especially on the high gain settings... mmmmm... harmonics!  I'll post some audio clips eventually...

Questions?  Comments?   E-mail me or leave a comment.

AX84 Hi Octane build Part 3: Ruining the finish

A word about finishing metal...

This is the third metal enclosure I finished.  The first two were metal stompbox effects units.  I'll start with the worst; a compressor from Build Your Own Clone.  It's a nice pedal, and he sound is great, but I tried to hand paint, and it was a miserable failure.  It's terribly ugly.  It would have looked better had I just left the aluminum bare and written the knob labels on the thing with a sharpie.  Once I saw how terrible it was, I said screw it, globbed a bunch of paint on it, and embraced the ugly.  It is a nice compressor though, so maybe I'll refinish it at a later date.

The first one (the Tube Screamer), I spent a long time on the finish.  I sprayed very thin layer of paint every 15 minutes or so, and then I let it sit over night between the primer, the color, and the clear coat.  Between the color and clear coat, I also baked the case in a toaster oven I bought at thrift store.  The baking isn't necessary, but if you bake the unit for an hour at 300-350F, the paint will almost completely cure, and when you handle it, you won't get finger prints embedded into the soft pain, or mar the surface.

I rushed through finishing the AX84 chassis.  I put the clear coat on too thick and got runs in the paint, and then I didn't let it cure enough before I installed the pots, switches, transformers, etc.  The toaster oven won't fit this chassis, and I don't want to bake it in my cooking oven, so I just left it outside over night.  I thought 9 hours of drying time would be sufficient for curing, but I was wrong.  I really dinged up the finish.  It looks pretty bad in places... and goes to show the importance of patience if you want your piece to look nice.  I'm going to build this amp into an enclosure of some sort, either a combo amp or a stand alone guitar head, so I'm not too concerned with how it looks.  A few coats of clear coat would have been fine to prevent corrosion, but I gave it color on a lark.  If nothing else, I learned how important the patience I used on the tube screamer pedal really pays off.

So here it is!

Doesn't look too bad, eh?

Parts installed

Inside

Oh no! The finish!   I killed it!

That texture there is from the a seam in my pants.  Seriously

Tightening nuts ruins your finish

That's a finger print and a few nicks.

That's probably from my heavy paint layers

Took a nice ding out of the side there... oops.

Oh well, I'm going to wallow in shame for a while, and then finish up the wiring.

AX84 Hi Octane Build Part 2: Circuit board

Part 2:  Building the circuit board.

I guess instead of getting a "real" circuit board made, these people chose to include this eyelet board, which has little metal eyes installed to solder the parts to.  I've never actually used a board like this before, and if I were going to make another amp, I'd be inclined to make a custom board so I don't have to muck around soldering all the jumper wires which you'll see later.  I built the circuit board in the time between applying coats of paint to the chassis.  Just set a timer for 30 minutes for applying new coats.

Lonely eyelet board, parts, schematic, wiring diagram.

Through the magic of the internet, I present to you, an eyelet board with all the parts installed.

The Eyelet board has friends now!

This part was the biggest pain... connecting all of the voltage rails and the ground.

Red and blue wires are the supply voltage

Black wire is ground

With the wiring complete, I just need to let the chassis cure overnight, then it's time for the final wiring.