Adding a choke to Sovtek Mig 60

[evil79]

So I would like to add a choke to my MIG 60, and wondered what kind I need for this amp. Classic Tone transformers have some Marshall chokes, a 3Hy 250 ma jmp type and a 5Hy 120 ma jcm800 type. I'm assuming the 800 type might best suit this amp, but don't know shit about chokes. Also, where would I put it, physically and electronically? And what resistor/part is currently doing the job of a choke in this amp that I'll have to remove? I'm having a hard time posting attachments at the moment, but the schematic is easily found by googling. I'll post it and pics when I figure it out. Someone school me please....

[evil79]

Here's a screenshot of the easiest to read schematic I've found for the MIG 60. If you need amp pics let me know.

[pdf64]

R41 220ohms is doing the job sometimes done by a choke.
However, as the amp's filtering is pretty good, replacing R41 with a choke is unlikely to result in any tonal or measurable benefit.
I understand you may need to prove that to yourself though
If you want the amp to have a stiff response then use a choke with a lowish resistance, eg ~150 ohms.
Whereas for a bit of HT sag, use one with a higher resistance, eg 500ohms - 1k; also you may prefer a lower value cap at the screen grid node C18, eg 22uF.

[evil79]

Thanks man. Are you talking about DCR? Specs have the 2 i mentioned at 115 ohms. And that cap you mentioned is actually 150uF. What would lowering it do? I've read so much about how a choke gives life to amps, and I wanna hear it for myself. They don't cost much, so why not?

[evil79]

Here is a revised schematic with the mods I've done. Any thoughts?

[evil79]

Can anybody give me more info? Enzo, what's your 2 cents?

[Enzo]

I generally stay out of mod discussions, because I am not into that. In my view you won't hurt it by adding a choke, but I don't see it as doing anything useful either. As to changing cap values and stuff, you will probably have more fun trying things than I will telling you what I think about it. In general on tone, it is important to think about too many highs and not enough bottom are two solutions to the same problem sometimes.

[evil79]

I generally stay out of mod discussions, because I am not into that. In my view you won't hurt it by adding a choke, but I don't see it as doing anything useful either. As to changing cap values and stuff, you will probably have more fun trying things than I will telling you what I think about it. In general on tone, it is important to think about too many highs and not enough bottom are two solutions to the same problem sometimes.

Right on Enzo. I got this amp for $200 in 2003, and it never sounded as good as I hoped it would. After the recent mods I've done, it sounds remarkablly better. Less fizz, more mids, tighter bass. With plate voltages at ~640v, those fizzy highs are gonna linger. I posted another thread about replacing the PT to remedy this that I hope you'll chime in on. You've talked about it before in a similar thread. I feel like the OT and PT are where this amp really comes up short. Maybe I'll forget about the choke for now.

[Enzo]

I have a pair of Mig 60s new in the box, I bought them about 1994. And I have a Mig 30 serial 0045. I first bought that wholesale from New Sensor just to check out, it was made with Russian military parts mostly, the Russians had not yet learned to make consumer crap. I decided the amp was not so special, but at the time they were cheap, well made and sturdy, and the perfect vehicle to convert into my own design amps. I later decided I had other things to do before converting amps, so they remain in boxes in the warehouse. That is why I bought a couple Mig 60s not long after. Now they fetch big money on ebay.

[evil79]

I have a pair of Mig 60s new in the box, I bought them about 1994. And I have a Mig 30 serial 0045. I first bought that wholesale from New Sensor just to check out, it was made with Russian military parts mostly, the Russians had not yet learned to make consumer crap. I decided the amp was not so special, but at the time they were cheap, well made and sturdy, and the perfect vehicle to convert into my own design amps. I later decided I had other things to do before converting amps, so they remain in boxes in the warehouse. That is why I bought a couple Mig 60s not long after. Now they fetch big money on ebay.

I think the Mig 60 is the best Sovtek amp made. Almost entirely point to point. All the caps and resistors I've measured are almost dead on point. I'm just not a fan of the stock fizzy-ness. I dropped all my pre amp voltages to brown it out a bit. I got V1-a down to 127v, put a 150pF cap from v1 plate to cathode, a 100pF fizz cap across the PI plates, a 50k mid pot, a Trainwreck ppimv type 2, 68k grid resistor on v1, 5.6k instead of 1k at power tube grids, sozo mustard caps for tone stack, silver micas, replaced all electrolytics, changed cathode sections to match jcm800's, .33uF presence cap (which does little now with ppimv), moved gain pot to right after 1st gain stage. Been slowly doing these things over years when I have time and it's been interesting. I've traced the entire circuit to understand what's going what, especially in the PT department. Most of the wires are white coming from the PT, which is annoying. Can you help me with a PT replacement? Current one has 4 windings: 50v bias (mine measures -78vdc after D10), 6.3v heater (mine's at 7.25 vac for v5-v3), and 2 more windings for B+, (one for el34 plates, one split for pre amp plates and screens). How would I wire a jcm800 PT to this circuit?

[pdf64]

Thanks man. Are you talking about DCR? Specs have the 2 i mentioned at 115 ohms. And that cap you mentioned is actually 150uF. What would lowering it do? I've read so much about how a choke gives life to amps, and I wanna hear it for myself. They don't cost much, so why not?

Yes, I was thinking of DCR; some Marshall types have a higher resistance eg http://www.hammondmfg.com/pdf/EDB194C.pdf
If the resistance in the HT supply to the screen grids is reduced, eg from 220 to 115 ohms, I suggest that the value of the individual screen grid current limiting resistors is increased, eg from 700 to 1k ohms.
And yes, you really need to hear for yourself the difference between a stiff HT and one that has some bounce / sag.
But be aware the effect only occurs when it's being pushed hard.

[soundmasterg]

My brother had a Mig 60 that he bought new and the fizziness was annoying. Then the Sovtek EL34G's that were in there stock self destructed and took one of the transformers with it, so he sent it back under warranty. When he got it back repaired he traded it for a vintage Silvertone 1484 and never looked back. I had a couple Mig 50H Midgets which in my opinion were the best amps Sovtek made. Very close to a JCM800 circuit-wise. That said, I sold it years ago once I started learning to build my own amps and didn't look back. Enjoy your MIG 60.

Greg

[cyclone]

Those Sovteks are hard to dink with. I bought a mig 50 when they came out, convinced that a conversion to cathode bias was a good idea by an article in VG. Like a lot of those articles "rip out your vintage coupling caps and replace wholesale with modern junk" it was not a good idea. I suggest keeping it stock and building from scratch, more fun anyway.