Help me mod my MIG 50

[powerlutin]

Hi all.

I own the first version of the MIG 50 (midget) bassman/jtm45 clone. I am looking to learn how to work in amps and I feel like this is the perfect place to start since it has componants that need replacing and that I like the sound and think it would make a nice modding platform. I will be assisted by an amp tech while doing the first few mods so that I learn the basics of working in an amp without killing myself.

I already modded guitars, built cable and about 5 guitar pedals (some being a bit complicated) so I know how to solder.

Here is what I'm looking to do: replace the pots, switches, tube sockets and old caps.
Jumper the two 1/4 imputs and use the leftover hole to install a master volume. Install an effect loop. While recapping, I'd also like to switch some componants to tighten up the low end, and to add some balls. I'd like the preamp volumes to react nicely to control the overall gain.

Thanks in advance!

One more thing that needs to be solved. The overall voltage fed to the power tubes is way to high. How can I lower that? Changing the output tranny?

Here are its schematics http://www.schematicheaven.com/newamps/sovtek_mig50.pdf

I'd like to know your suggestions regarding the mods I'd like to do, and what pots could fit in. (I was looking for CTS 1 meg linear at the moment)

[Gibsonman63]

Not to sound negative, but why exactly do you need to change the pots, switches and tube sockets?

FWIW, that's a fairly extensive modification turning an apple into an orange. It would be cheaper, faster and have more predictible results if you bought and amplifier that has these other features or built a kit. Tube amps are very sensitive to layout problems due to the high voltages and that's a lot of extra stuff to cram in there.

Don't get discouraged by my comments. Hang out for a while, use the search function, and read some posts from the guys with 10,000+ posts. There is a lot of good knowledge here.

[powerlutin]

The pots, tube sockets and switches are pretty crappy and I'd like it to be as reliable as it can be. One pot actually broke last week

No money for a kit and I'd rather learn the impact of what I do rather than just build something from a plan

The master volume knob mod has been done before: http://www.ceriatone.com/images/layo...0Ceriatone.jpg

[NorCalTuna]

That amp is a clone of a later marshall. You could part swap it up to a JCM800 clone really easily which still has some earlier amp mojo plugged into the 2 channel. Go ahead and do the caps, switches and pots but the sockets? brother, you have more time than I.

I'd spend that time building myself a nice, cheap champ- less work, literally. You can get a box, trannies and all the parts for like 100 bucks if you shop around or buy half an amp of ebay. If you've modded pedals you know about rooting parts and skip the choke for economy. Then I'd spend at least a week- a month- f'in with it- if you wanna learn... Build it so you can change the blocking caps and the cathode resistors and caps over and over again without nuking the build. Add a tube. Build a SE trainwreck. Maybe you used a hammond 125, its 43 bucks from newark, and build a push pull amp. Eat a sando. Also, haunt the forum for a long time, learning to read and compare schematics. Then, when you're done, you will have something of your own and a real journey taken. Multi stage amps kinda swamp small changes and can easily go badly awry as mentioned above, making for a frustrating experience. Champs- which don't have to be wimps, incidentally (that JTM doesn't really have much more gain than a brawny champ- oh and you can crank it and granny won't mind! try that with your MIG...) are very versatile and can be quite loud with the right speaker. I built another two weeks ago just to get back to a piece of myself, and now have a bangin version in my pocket. I digress.


The power transformer is the Achilles heel of your build- the thing you want to build has a rec tube, and your transformer has no tap for one. Even with a transformer and socket for the rec tube, it won't work because you don't have a center tap on your transformer. SO- you're pretty well stuck with High voltage, bridge rectified rock and roll. Incidentally, when I think of JTMs, early bassmen and rectifier tubes, things like tight, "ballsy" etc aren't the first thing on my mind. I think of, say, eric clapton, holding a note for like an hour. Eddie van halen once described his tone as brown and, uh, ballsy.

[powerlutin]

Thanks a lot! I'll take your suggestion and study for a bit.

I was actually pretty much going for the sound you described (the two channels, jcm800 ish on 6L6's). A loop is also something I'd really like.

If it really is too much trouble. I might just to sell it and buy a kit of some sort, or a broken amp with the intention to fix it (hadn't tought about that). I really want to recycle a cheap or broken amp though

[NorCalTuna]

It's not hard at all, thats the point of my post- you just have to mod and rearrange the parts you have there.

BTW YOU HAVE A CAP DISCHARGER AND ARE AWARE OF THE HAZARDS PRESENT IN THIS WORK, I presume! Also, i'm going to kinda spell this out, but I may have accidentally (or intentionally?) left something out to make sure you're doing your homework, i'm Socratically stupid- if you can read schematics, this project just leaps off the two... SO.... lets get that under out belts, eh? : ^)

I personally hate working on anything with a PCB but having worked with pedals you're possibly is a good position to do that. The pots are, as long as they're not board mounted, easy and a good move. You only need to change a few parts. just look at this schematic:

http://www.drtube.com/schematics/marshall/jcm800pr.gif

In fact, print both your schematics and put them next to each other. Understand the signal flow.

the jack literally connects the circuit through- The two input jacks become your channel switching. Start, just like marshall- find the 2.7k .68 bypassed triode. Thats the first stage in the 800, see? Find that stage 1 blocking cap, and the 330k resistor. That could stay, but the stock 470k/470 bypass cap I suspect drops the needed bass and bypasses the highs better, so... your call. That goes to the switch on the #2 jack. Then confirm that the jack is appropriately connected to the input of triode 2.

OR, F that. Look at the schematic. What about a non bypassed, 10k cathode stage fed by a bunch of serial resistance says "plug your guitar into me, I'll sound great." Oh wait, the clean channel on that amp sucks. How about just skipping that?

So, yah, off your .022, into the 470/470 network, off to the gain pot, which you're changing, so no new work there.

You'll have to change the second triode cathode resistor to 10k, it'll be 1k/220uf now as long as you did your crafty mods right. Incidentally, the gnarliest JCM800s around were probably the caswell #36/#39s, and he did the same thing- just rearranged stuff. He used an extra triode, though, borrowed from the vibrato circuit.

Here's the tricky part. You may have to add a terminal strip or something but it's no big- get to that in a second. In the MIG, the grid resistor is the volume pot. In the JCM, the grid resistor is a 470k/470k with a 470pf bypassing the "signal" side. You could simply use a gain pot- you should have an extra one- here, with a cap over the signal side (if you put it on the wrong pins the amp will be very dark). Mr.Marshall's setup is analogous a pot permanently set on "5", with a bright cap. Then, drop the simplest of all master volumes, which sounds good here, after the tonestack, before the driver input. The lead comes off the wiper of the treble pot- and ends up going back to the same place on the board where it came from, the .022 input cap of the driver. You could put a 25k presence pot on if you want, since you're changing them.

Now, check your work, and walk away- go eat that sando. Make sure it matches the schematic like ten times, following the signal through, and then again. You're done. The 820/1k discrepancy on v2a won't make a real difference, but the 250 in the tone stack could be changed up to 470 if you want. Check for stray wire hairs, solder bridges etc, and if you're ballsy enough, fire it up. If you have a light bulb limiter or something like that you should use that... check your voltages (exact doesn't count, ballpark does. If something is way off, power down, get it right), and play some tunes. Once you have your 800 up and running, it becomes a bad, bad, badass mod platform, so take your time and get it right.

Your next project is to install bias monitor resistors and learn about that, perhaps? Then the loop. Read, read, read. The questions have been asked; read read read.

[powerlutin]

Now that's the sort of help I'm looking for! Thanks a lot man. I'll keep you guys updated in a few weeks. In the meantime i'm going to do my homework and get the necessary parts.

[powerlutin]

Uh, I am a bit worried about something. Are we talking about the same amp? The MIG-50H is the one that people refer to as being close to a JCM800. Mine is the regular MIG-50 and is compared to a bassman

[NorCalTuna]

Well, it's cool. Cuz, 1. You're gonna do your homework and make sure you have a full scope plan. Once you have that- a plan- it doesn't matter what was, it's all about what you doing NOW. Right? 2. There's nothing magic in here- just a careful selection of parts, frequently not even obeying good engineering practice. The most intrinsic part of an amp is the transformers (which is why people spend hundreds on original stancor 3801 trannies for their trainwrecks), which are kinda like the heart and brain of the thing. Luckily, your trannies are probably fine for what we're NOW talking about. So, arrangement and values, sans hype.

OK, 3. You're way lucky that "People" aren't working on your amp, right? The same people who would look at a mig-50h and tell you it's 800 like? Do your homework, here's a mig-50h schematic. Open it, and the 800, and at least one HUGE difference should leap, f'in leap off the page to you, hint, near the end of the preamp.

http://www.blueguitar.org/new/schem/sovtek/mig-50h.gif

4. "people" don't know how to read schematics, often. People also suffer from confirmation bias, so when some guys like, blah blah blah, it becomes truth. Truth, what is that? Didn't you do your homework? New assignment. 5F6 scheme, Mig-50, Mig50h and JCM 800 2203/2204 schematics. Print them and hilight the changes. Not many huh, until you get to that 50h. It's actually kind of unique, compared to the other offerings. See why? For extra credit, figure out what's interesting about MIG's heater arrangement on that amp.

5. Bassmen and JTMs have similar values to pretty much ALL Marshalls, it's the branch of the family tree they sprung from. Thats why you can hype your values around and make a new amp, the way caswell did. HOWEVER the early amps had rectifier tubes, which is a HUGE component of the sound, especially those larger amps, and usually lower voltages (I have no idea about the MIG voltages). Where as some values may blur the lines between models and years (and parts drift, especially with heat, making some things like the 820/1k differentiation moot) the rec tube is a feature only of these very, very early marshalls. Folks who would claim that the (massively capped, FWBR'd) MIG-50 is a lot like a bassman are kinda right but also may not really understand the feel and effect of a tube recified amp. Luckily, You're doing your homework, right?

[powerlutin]

I like the way you think. Anyhow sorry for the confusion, It just hit me that you could have mistaken the two, as I wrote Midget in my first post and didn't know it refered to the 50h.

And yes, you can be sure that I will do my homework. I like to learn and I really want to know how to give this amp (and other amps) a second life. I thank you again for being so welcoming and helpful even though I'm new to this and that we're strangers.

[powerlutin]

Got around taking some pics and started reading/comparing schematics.

I need to get a multimeter to confirm that the componants in my amp are matching the schematic. One thing I noticed is that the diode bridge as been replaced.

[catstrat]

mig50 mods

Welcome. I use to own 4 mig 50s and they're great amps. The pots are really cheesy. They have really small shafts and i replaced the pots on all of mine. The Mig 50 is different than the Mig 50 H. The H is the one that's like the JCM 800. It was designed by Ed Pennypacker who also designed the Mig 100 h. Both of those amps scream. Back to your MIG. You can get a schematic from schematic heaven ,in case you don't have one. It's a pretty straight forward amp and most of the mods have already been mentioned. When i mod an Amp i like to play with the negative feedback circuit along with the other mods. Some amps have a switch to drop the feedback circuit out all together. As said before,,,, Please ,please be carefull around an energized amp, it can kill you. They're people on here that have a lot more knowledge on modding amps then i do. My mods are pretty straight forward. I wish i would of kept some of my Migs.. Hindsight is 20/20. Good luck on it.

[soundguruman]

The pots are, above all, the first issue to tackle.
The stock pots are pathetic. As you have noticed they have NO thread to attach them to the chassis, and break easily.

The best replacements I've found are PEC- precision electronics Canada.





It's worth it. These PEC pots are so much better, and have a perfectly smooth adjustment range from 0 to 10.

[soundguruman]

Sorry for the double posts, but I wanted to show you how I did it.
There is a limit to how many MB I can download in one post...





What you want is Cliff jacks, like a Marshall.
This will allow you to isolate the input jacks from the chassis ground, which is necessary to reduce the hum from ground loops.

Now, post some pictures of your amp, with plenty light, and I shall fearlessly guide you (somewhat).
(bless me James Marshall, for I have sinned)

More pictures of amp please.

[Tage]

2 years old? jst tell him he needs more power tube distortion and leave it sgm...