Is it posible to convert a Sovtek Mig 100 tube amp into a bass amp?

[mr79redbird]

I recently acquired a Sovtek Mig 100 tube amp in a trade.And was wodering if it could be converted into a bass amp.Inside it is all point to point wired no pcbs.it is a very straight to the point amp hi&lo inputs,presence,bass,mid,treble,master,gain.I was told it is a straight up marshall clone except for the 6l6gc power tubes.It certainly rocks.If this is a stupid question i apologize in advance.Just need a head for my ampeg b25 2x15 cab.My ampeg svt 350h head recently crapped out & took my genz-benz 410 with it[amp squealed lights flashed and cab rolled smoke]Any info appreciated thanks.

[JimmyM]

Plug it into a bass cab and see what happens. The only thing I'd think is you might get too much distortion out of the transformers to play loudly, but it could work.

[Rockingbird]

If it's just a head, it'll function fine as-is. If it's a combo, trade the speakers for something more bass-friendly.

[Caca de Kick]

Yup it will work just fine. Simply plug in a good efficient bass cab and you have a bass rig.

I've used a 45w '63 Fender Concert amp (guitar amp) through my 8x10 cab in a pinch for rehersal, works fine.

[Yorick]

Lemmy's been playing through a Marshall 100w guitar head forever. If it works for him..........

[staindbass]

i dont think there are any differences besides guitar amps usually have less power and different eq frequencies on the tone knobs. it will work fine.

[Downunderwonder]

iirc it's a Soviet "plexi". So you should be able to swap some capacitors and tone pots and turn it into a "SuperBass", ala Lemmy.

[aledeville]

Sometimes the guitar amps have a special wiring that cleares up the bass when playing with a lot of gain (prevent muddy tone).

Tell us how it worked. These MIG amps are sometimes pretty cheap to get!

[BassmanPaul]

Originally Posted by Rockingbird

If it's a combo, trade the speakers for something more bass-friendly.

Unfortunately it's not as simple as that!

[mr79redbird]

Thanks for the info.And it is a stand alone head.Seems like i read somewhere that you can change a couple caps and be in superbass territory as Downunderwonder suggested.Just cant remember where or the cap values.Thanks guys for the replies & info.

[blackba]

As others have mentioned, just try it through a bass cab and see what happens. Just pay attention to the impedance. I tried a few guitar heads through my bass cabs with my Jazz bass. A few worked well, others, not so well.

[nashvillebill]

I'm looking at the Sovtek Mig100H schematic on schematicheaven.

1) The power supply uses a choke and it doesn't say what the plate voltages are. So I can't say for sure how much "sag" the power supply has but the PS has a reasonable amount of capacitance in it , so it's likely stiff enough for accurate bass use.

2) Part of the bass response will depend on the quality of the output transformer. I cannot tell that from the schematic.

3) The tone stack appears to be pretty much a standard Fender "FMV" tone stack, with a slight tweak or two. The EQ points are therefore probably a little bit more geared to guitar but IMO should work for bass as well.

4) V1A has a cathode bypass cap of 0.47uF. This will likely cause bass roll-off below 100 Hz or so. Good for guitar, it actually tightens up bass response, but that rolloff may be too high for bass. Perhaps a change to 2.2 uF would be better. However, it may induce too much bass boost and cause the bass to sound muddy. 1.0 might be a good compromise.

5) The smallest coupling caps I see are 0.01 uF. I'd stay with those.

I don't see any big obvious issues though.

[runmikeyrun]

I'm going to go out on a limb here and tell you to increase those coupling cap values. The amp might become unstable or get farty if you go too big, so buy a few different values and experiment. On my windsor I went as large as 1.0uF in the latter gain stages, but I found I couldn't increase the value on the first stage because it would become unstable. I think I added 0.47 caps in the 2nd and 3rd gain stages. Use smaller values in the first couple stages and get larger as you go on. Measure the voltages on your coupling caps to make sure you buy caps that can handle the voltage. Also, remember that caps in parallel ADD the values to get your total value. I soldered my caps onto the legs of the existing coupling caps. It's a bit of a ghetto rig way to do it, but if your board has thin traces (like mine) it's way better to do that than risk lifting and breaking traces. Plus they are easily removed.

I also put 1.0uF coupling caps in before the PI. All those mods DID make a difference, but just remember this thing isn't going to have the punch or headroom an SVT or 400+ will. I get a great Lemmy tone out of my Windsor with decent bottom end, much better than stock. You can expect that at the least with about $20 bucks in capacitors.

Have fun and post sound clips!

[nashvillebill]

Wow, 1.0 uF coupling caps? The exact roll-off depends on the grid-leak resistor and output impedance, but for a typical 1 Meg grid-leak even a 15 nF (.015 uFd) yields a bass roll-off at 10 Hz.

Merlin Blencowe's "Designing Tube Preamps for Guitar and Bass" has many pages on coupling cap design. Concerning the use of common .022 uFd coupling caps (yields 7 Hz rolloff), he writes on page 42, "In modern designs, even bass amps, there is no need for such a low frequency roll-off. It is much more desirable to use a smaller capacitor to attenuate very low frequencies--since these are almost entirely unwanted noise signals-and because it will reduce blocking distortion (discussed later) and promote a 'tighter', more controlled bass tone. Having too much bass response in an amplifier nearly always leads to 'woofiness' or 'muddiness', even in a bass amplifier." The italics are his not mine.

[mr79redbird]

Again thanks for the info/advice.The amp has both 4&8 ohm output jacks.So i plugged my 4ohm ampeg 2x15 into the 4ohm jack,and icouldnt believe how easy this 100 watt head could drive that cab.Plugged into the lo gain input with my p.It sounded big and round with plenty of attack.In the hi gain input with my epi explorer bass very ballsy &flat out mean very Lemmy-ish.Much better than i thought.Im excited.I think with some of the mentioned mods it could be great.Hard to believe a 100 watt amp can sound so big & strong.Tube amps DO rock.Once again thanks.

[nashvillebill]

If it's got plenty of balls now, I wouldn't mod it any further. Trying to mod it for more bass has a very real chance of making the bass too "farty". And in the mix with a band, you need those mids more than lows... IMHO of course.

[AwkwardLoudness]

Some 'guitar' amps have output transformers capable of low frequencies and some do not. I'm not sure about the Mig 100. The various Sovtek Mig amps sound good when they're properly working as they are not known for great reliability. I've played guitar through Mig 30, Mig 50, and Mig 100 amps. Simple circuit and good tone....sorta Marshall-like. As others have said, you definitely want proper bass speakers.

[bacehead]

I sometimes use a Marshall '59 SLP reissue and love it. No mods. Plug in to the top #2 input and its great. Gets a little dirty as the vol goes up but what would you expect.

[fudgemaster]

Don't know about sovtec. I bought a peavy Windsor for tube sound. By itself it toned more for guitar. So I plugged a small gkrugr straight into the amp return, set all the bass tone you. want going strait into the power tubes without the preamp. Sounds great and full thru my 69' peavy 2-15 cab. No mods necessary. Hope this helps. VF fan.

[throbgod13]

you might want to swap the 12AX7s for 12AU7s, to reduce the gain in the preamp..