Sovtek MIG-50 amp-Fixed & Phat!!!

Hi all,

Been having some issues with my sovtek mig-50. I recentley changed from a marshal jmp-1 rack system w fx bla bla to just the sovtek as the sound was so huge in comparison.
BUT..........the pots on the sovtek have had a bit of a hard life and i think have finally given up, as my sound is no longer nice and phat, my bass pot is really loose and doesn't seem to make any difference to the bass response at all (usually this thing has too much bass)

I had the whole amp re-valved and biased a few months ago, and the tech who did it mentioned something about replacin all the pots as a few were pretty hammered, and because they were kinda odball pots, he recommended getting them all replaced with something else instead of trying to preserve the look by replacin one or two pots.

Now this was all before I discovered DIY :roll: i can fix myself?? no??
So i hunted on the net and found this schem www.heysam.com/schem/sovmig50.pdf
Now my schem understanding is ok but still pretty basic, and i seem to struggling to locate all the pots on the schem.
We have vol1,vol2 which i'm guessin are the 1MA at the inputs.
Now i can see another 1Ma and a 5k linear but thats it!! am i blind?? or are the rest not shown?

Any ideas people? I'm assuming that the pots need to be log unless otherwise stated like the 5k linear?

I know that this is valve based, and so far i've only built solid state mic pres etc, so i'm aware that this is new/dangerous territory for me and maybe left up to a technician anyway, but if i could do itmyself i'd be very happy. I will do an in depth search about the dangers of valve gear when i get a chance (1/2 hour luch breaks are a bit limiting)

I know that the power amp stage of the amp is working sweet, as i can still use my jmp-1 pre into the fx return (a mod i had done a few years ago)

Any help would be awesome, as this amp is a killer that usually eats other amps for breakfast :twisted: :twisted: (i've played against a few) and i'd like to get her back to her magical self :green: :green:

regards

Steve

There are three pots in the tone stack just after V2. Is the 5K pot a master volume?

The sovtek doesnt have a master volume. Just two input volumes (1 deep, 1 bright) bass, mid, treble and presence.

oh yeah, i missed the250ka

> Is the 5K pot a master volume?

It has a 0.1uFd cap in series. You could call it a gain control that only works on high frequencies. Guitarists call it Presence.

Two 1M A channel volumes. 250K A Treble, 1M A Bass, 50K (no taper noted) Mid. 5K Lin Presence, and an internal 25K Bias.

It is a Marshall tone stack with a Fender output. Nothing exotic. Simulating the tone-stack, I'd think the Mid pot should be 50K Audio. If you use a Linear instead, the "basic" setting for the Mid knob would be about 2 or 3 instead of 5 (out of 10 or 11). Like all these tone stacks, there is a lot of interaction, and no flat setting: you diddle the knobs to taste and the numbers are just for convenient re-setting.

> 1/2 hour luch breaks are a bit limiting

Do it at home. You need a $15 soldering iron, a wrench, maybe small-nose pliers.

> the dangers of valve gear

Play a LOUD sustain power-chord and immediately flip the Standby switch OFF. Listen to the chord fade, fade, fade. Shut off the Power and pull the plug out of the wall. Make a nice cuppa tea and some scones, eat and drink. Be SURE no helpers plugged-in the amp while you were boiling water. Plug in your solder iron, clear some space on the bench, pull the amp out of the box. By the time you are ready to stick your hand where it shouldn't be, the stored charge will be nearly gone.

There is a missing resistor on the plan: where "HTR V1 V2" is biased. I'm sure the amp is correct and the transcription to paper is wrong: if it were wired that way, you'd have big problems keeping V1 and V2 alive.

Wow, that was a fantasic detailed response!! :green: :thumb: :thumb: :green:

Thanks PRR

now i'm off to order some pots an start a soldering :grin:

Do as the tech said and replace all of the pots and do not even bother trying to get the same kind. These are some of the worst pots I have come accross on an amp. If they are not dead yet they will be dead in awhile. Other then the pots these are really pretty nice amps. Need the occasional work but sound pretty great. Build quality is not the best, but once you replace those pots you should be good to go.

adam

These are some of the worst pots I have come accross on an amp.

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You're not wrong there. I've just opened her up and taken out the pots, and man they are C.R.A.P!!!!! :shock:
2 pots are def dead, and the others feel really rough and dirty. Well I've had her for 10years so i guess i can't complain too much :wink: Everything else looks ok.

I can't wait to shoot down to jaycar tomorrow and get some replacement pots. :thumb: :thumb:
This, along with wippin out my strats lance sensors & replacing them with SD alnicoII's is gonna take my rig back to simple, pure and PHAT!!! :green:

What size are the pots? IIRC they have some tiny shafts. Are you going to drill out for some big boy pots or find replacements?

Luckily the sovtek has holes big enough to fit the standard pots sizes that i can get from jaycar (checked with some last night).

I wont be able to use the chicken head knobs, but I'm not too concerned and i've got a few knobs that'll fit the new pots.

I would be suprised if Bruce drew this wrong.

I would not be suprised if the normal RC decoupling between V1 and V2 was not used in this amp.

I seem to remember a friend telling me a mig 50 they worked on had higher voltages than one might expect at the B+ and Fill supply(7.2VAC?) You might want to check the cathode to heater voltage at the CF triode of V2

At Duncans site there is a tone stack modeling program it even lets you use the the out Z of the stage before and the in Z of the next stage you can use A or B pots etc.

Right!! it all done. and it sounds soooooooo good.:green: :thumb: :thumb: :green:

I can't remember it sounding this good. even when it was working.
Clean and big and with so much more headroom than before. And I didn't even have to drill new holes for the bigger pots. woooohoooo!!!!!

so again big thanks to PRR and everyone else who chimed in on this one.
A :sam: :sam: :sam: for you all

regards

Steve

Those are really under rated amps, and you can get them cheap since there are always ones rendered unuseable by those useless pots. It seems staggering your pots made it 10 years. You can also expect to replace some tube sockets down the line, they tend not to keep tension after awhile. I think the power supply filter caps were also really cheap and very leaky, but its been quite a long time since I had my head in one of those amps.

adam

I think the pots have long been on their way out, it's just that i used to use the sovtek just as a power amp for my jmp-1 pre (with the mod i had done a long time ago.)

but i got sick of midi controlled this, and organising that patch etc and when i tried it with just the guitar, amp and overdrive pedal the tone just seemed so much bigger and more organic sounding. I guess thats the beauty of simplicity :roll:

I did the same thing to a first run Mig 50 a year ago. Removed all the pots, marked them and then used a multimeter to astimate the resistance.

There is a *but*! The pots are russian and there are no known fysicle (?is that the correct spelling?) subs for them. The original pots are mounted on a rails, where the new pots won't fit. You'll have to remove the rails and mount the pots on the chassis.

I highly recommend to replace the input jacks as well and for a better sound to output transformer could be replaced, as it's the weakest part of the signal chain. I haven't don this, but have played on ones where they were changed.

To finish the amp with some nice options I did add a master volume, dual input switch and pentode/triode switch. The master volume is really worth trying and not that hard to install. I chose a Post Phase Inverter Master Volume. If you do a search on PPIMV you should find some interesting links and topics on the web regarding this mod. The volume is placed after the phase inverter and has "little" affect on the tone. I used one input jack place for the master volume.

I wired the input jack to a switch, so I could chose between channel A, channel B or A+B. This last option is VERY interesting, cause you can really dail in some amazing sounds. It's the best tip I could give anyone with a similar amp. On older 4 input amps people simply did it with a wire between 2 inputs, which gave similar results. If you do this, changing the pull down resistor is an option, otherwise you'll go from a 1M input to a 500k input.

I did not due this, because the owner liked to have a darker sound in general and with all the options you have between the two channels, he could get his sound. But in general it's a good idea to change them.

As for the pentode/triode switch. This is easy to install, but you have to do a nice/clean job, since you're workiing on the output tubes. I installed a switch so he could set it on pentode (normal operation) or triode. Triode means that only 3 of 5 parts of the valve are working. The two parts which are disabled would normally add to a higher rendament. Disabling them drops the overall volume, drops the highs and lows a bit and makes the amp a bit more spongy.

As for the tubes I used JJ's for the poweramp and Sovtek LPS and JJ ECC81 for the rest.

Good luck and enjoy the amp.

Wow!! more excellent information. :shock: I wasn't expecting anyone to even give a rats arse about this amp. Sweet!!

You'll have to remove the rails and mount the pots on the chassis.

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Yeah i did this, and just used short hookup wire to the PCB. It seemed to work out sweet.

I wired the input jack to a switch, so I could chose between channel A, channel B or A+B. This last option is VERY interesting, cause you can really dail in some amazing sounds. It's the best tip I could give anyone with a similar amp. On older 4 input amps people simply did it with a wire between 2 inputs, which gave similar results. If you do this, changing the pull down resistor is an option, otherwise you'll go from a 1M input to a 500k input.

Click to expand...

I was just thinking of usin a good a/b or splitter box, but this could be worth looking into.

I highly recommend to replace the input jacks as well and for a better sound to output transformer could be replaced, as it's the weakest part of the signal chain. I haven't don this, but have played on ones where they were changed.

Click to expand...

I didn't even think about the output transformer. How significant was the change? I'd like to better my amp if possible but it sounds pretty nice already......

As for the tubes I used JJ's for the poweramp and Sovtek LPS and JJ ECC81 for the rest.

Click to expand...

I went with the EHX 12ax7's for the pre and some Winged C 6l6GC's for the power section. Did a litle bit of research b4 goin with this combo, but there were soo many options and opinions out there that you could go crazy b4 you even chucked them in your amp. I'm very happy with the tubes i went with :thumb:

I think i'm gonna build a stombox tomorrow for a bit of fun with the amp :green: :green:
Thanks again :thumb:

Steve

There are indeed so many tubes to choose from. I've tried several NOS and current production tubes and went with the above combination for my friend.

With the transformer (OT) upgrade you essentially have a old Marshall on high voltage. Have to check the schematics, but I remember that it was a direct copy of a Marshall with only few small changes. I believe ax84 site has some topics on OT's. Worth checking.

What type of stompbox are you gonna build? A "treble boost" would be nice, imo. Fortunatly there's loads of info on diy pedals.

BTW I always use a buffer between my guitar and amp. The essential design is *very* simple but the results are great. Check my site for the schematic.

Maarten
www.tubefreak.com

I do not really see why anyone would replace the transformer on these amps, they a great sounding amps and they do not sound like a Fender or a Marshall like most amps out there try.

The Mig50 fallows the same topology as the old 50 watt Marshalls but is a little different, Mainly the input stages, the two channels do not share cathode resistors/bypass caps, they both are biased and voiced differently in the Mig50. Other then that they are pretty much identical, but I think the Mig50 works out with more gain in the preamp. So putting in a JTM45 output transformer will get you pretty close to the Marshall sound if that is what you want.

adam

I thought about buildin just a simple overdrive pedal. Found a couple of things on building a tubescreamer type pedal, but with IC's i can get from down the road. Or i could just save up some $$ and get me a nice G2D pedal (built in NZ too) as they seem to be the best pedal ive tried so far. :thumb:

I was keen to make a tremelo pedal too, but some of the parts seem a little harder to get hold of. Looked at this one. http://www.tonepad.com/project.asp?id=42 Only part i have trouble getting here in NZ is the VTL5C2 or LED/LDR. Anyways that prob for another forum :roll:

I found this site has PCB layouts done already, which is easiest for me.
http://www.tonepad.com/
i also had a look at the www.diystompboxes.com/sboxforum forum too. Some good stuff there.

A treble booster looks like it's pretty easy tho. I could make one of those too. I'll probably end up making a whole bunch of stuff.

But it seems the sun has come out today, and i'm not a fan of being inside in summertime, so the ol stompbox may have to wait till later in the week. Maybee when it's raining.

> no known fysicle (?is that the correct spelling?) subs

Stupid english language. Sometimes when we say "f" we spell "ph". Also, at least in middle-US english, all 6 short vowels sound the same. The spelling you want is "physical".

> pentode (normal operation)... means ... .... a higher rendament.

Not sure what "rendament" is. The second grid in the Pentode isolates the input signal from the output signal. As you say, that makes more power, and lets the speaker "ring" in bass and treble. (The third extra electrode reduces problems that the second grid causes.)

> I would be suprised if Bruce drew this wrong.

It seems a fine plan. But it shows the V1 V2 heaters biased to the supply for the output tubes' G2, which would be over 400V, or 399V heater-cathode voltage on the poor 12AX7. Also the 56K there does no good but eats 3 watts. A more likely plan is a 150K-330K from G2 supply to V1V2 heater, 56K to ground, nice +60V to +100V on V1V2 heaters to reduce heater leakage hum and the strain on V2b heater-cathode insulation.

http://www.groupdiy.com/index.php?topic=11705

:wink: