Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mtyes on August 24, 2013, 01:04:22 pm
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I noticed that a couple of capacitors have been sniped off from the C14 and C15 positions on my Sovtek Mig 50. The picture below is not my amp. I've seen a number of pictures of the same model amp some have those caps and some don't. Mine did have them at one point as the pins are still there. I've provided a link to the schematic. http://www.blueguitar.org/new/schem/sovtek/mig-50.pdf (http://www.blueguitar.org/new/schem/sovtek/mig-50.pdf)
(http://franck.davot.free.fr/mig50/optimisation/Mig50-Mod0010.jpeg)
*Does anyone know what values these capacitors are and what effect removing/replacing them would have?
Also, most of the Mig 50s I've seen have a resistor going from what looks like pin 7 to pin 3 or 4 (can't tell from the pic) on both power tube sockets. My amp as well as the amp pictured above does not have this configuration. Instead of having those resisters,it has a single wire with three resistors inline coming from pin 7 of the first socket and going to pin 7 of the second socket. Is this just two different ways of accomplishing the same thing?
Thanks for the help.
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I don't see a link.
Brad :dontknow:
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I don't see a link.
Brad :dontknow:
Are you blind? it's right here http://www.blueguitar.org/new/schem/sovtek/mig-50.pdf (http://www.blueguitar.org/new/schem/sovtek/mig-50.pdf) :icon_biggrin:
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Are you blind? it's right here http://www.blueguitar.org/new/schem/sovtek/mig-50.pdf (http://www.blueguitar.org/new/schem/sovtek/mig-50.pdf) :icon_biggrin:
Ok, thanks, I see it now.
Brad :laugh:
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*Does anyone know what values these capacitors are and what effect removing/replacing them would have?
Your schematic has no reference designators so it's impossible to say based on this info. If you could look inside your amp and tell us exactly what the caps connect to then we could make a better guess.
Also, most of the Mig 50s I've seen have a resistor going from what looks like pin 7 to pin 3 or 4 (can't tell from the pic) on both power tube sockets.
It's highly unlikely that there is a resistor between pin 7 and 3 (or 4). A better guess would be a resistor connected between pin 6 and pin 4. That's a popular place to install a screen resistor.
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Post a pic of your amp showing where C14 and C15 were snipped. According to the attached schematic, both of those caps are Biggg filter caps and are necessary for proper operation.
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FWIW both drawings have to be wrong for the dc stand-off voltage going to the heater CT for V1 and V2.
The way their drawn tapping from the screen B+ dcv with no dropping R in series with the B+ to the CT you'ld have well over 400dcv at that point. Just guessing, they list no B+ dcv but it's gotta be high with them stacking 2x350v filter caps.
That would instantly short V1 and V2 heaters to the tubes cathodes and fry the 25v (C21) filter cap.
Brad :icon_biggrin:
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Post a pic of your amp showing where C14 and C15 were snipped. According to the attached schematic, both of those caps are Biggg filter caps and are necessary for proper operation.
I did'nt take a picture because the picture above shows the caps I'm talking about. Mine would show just the pins sticking up and no caps. They seem to be connected to each volume pot. One to each volume. The picture above shows the volume pots missing.
For what it is worth, I've had three of these amps over the years and this one sounds different, not much change after 3-4 on the volumes. My other amps would need to be pushed to 6-8 on the volumes to get real gainy. This amp is very gainy at 3 and the sound does not change a whole lot after that. I'm wondering if these caps would effect the way the volume works.
As far as the second issue. I did take a picture of my amp to show the wire going from pin #3 of the first power tube socket to ground and then to pin 3 of the second tube socket. There are three resisters inline after it's grounded. This is unlike other Migs I've seen. The others I see usually have a resister connecting pins 3 and 6(?) on each tube. Click on the image to make it larger.
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I have one of these boards in my shop......what are the odds? :undecided:
On Mon. I can measure the values and trace the location in the circuit (if they're there), if you need me too..
VR1 and VR2 (pictured near those caps) are the two input volume controls,,,,so maybe they are "bright caps"? :dontknow:....although, I don't see those on the schematic
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They seem to be connected to each volume pot. One to each volume.
They are likely bright caps. Values are probably somewhere between 100pF and 500pF. They increase the treble at lower volumes.
As far as the second issue. I did take a picture of my amp to show the wire going from pin #3 of the first power tube socket to ground and then to pin 3 of the second tube socket. There are three resisters inline after it's grounded. This is unlike other Migs I've seen. The others I see usually have a resister connecting pins 3 and 6(?) on each tube. Click on the image to make it larger.
The three "resistors" are DIODES. They provide some arc protection to the socket and OT in the event you may try to wail away on the amp with no speaker connected. The schematic shows one 3KV diode. Your's have been replaced with three each 1KV diodes. The brown and blue wires on pins 3 of each tube connect to the OT. I highly suspect there are three diodes in the white heat shrink.
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I am sorry this print is so poor but it's all I have. I think this schematic may be the amp in question.
I wouldn't trust these old Sovtek drawings verbatim without tracing it out in the amp to confirm. I recall finding a few glitches on the schematic for the Mig60.
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I have one of these boards in my shop......what are the odds? :undecided:
On Mon. I can measure the values and trace the location in the circuit (if they're there), if you need me too..
VR1 and VR2 (pictured near those caps) are the two input volume controls,,,,so maybe they are "bright caps"? :dontknow:....although, I don't see those on the schematic
Thanks. No need, I think the riddle is solved. Bright caps make sense as the amp is darker on both inputs than what I remember from my others.
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They seem to be connected to each volume pot. One to each volume.
They are likely bright caps. Values are probably somewhere between 100pF and 500pF. They increase the treble at lower volumes.
As far as the second issue. I did take a picture of my amp to show the wire going from pin #3 of the first power tube socket to ground and then to pin 3 of the second tube socket. There are three resisters inline after it's grounded. This is unlike other Migs I've seen. The others I see usually have a resister connecting pins 3 and 6(?) on each tube. Click on the image to make it larger.
The three "resistors" are DIODES. They provide some arc protection to the socket and OT in the event you may try to wail away on the amp with no speaker connected. The schematic shows one 3KV diode. Your's have been replaced with three each 1KV diodes. The brown and blue wires on pins 3 of each tube connect to the OT. I highly suspect there are three diodes in the white heat shrink.
Thanks for all the info Slucky!
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I was still curious, so I took a look only to find both of these were snipped off of this board too....
So this was either a very popular mod, or maybe a production line change :dontknow:
This board came out of modified unit,,,,that appears to have been worked on by someone who would be better suited as a bomb builder,,,,,who thought it was ok to make connections by just twisting wires together and suring them up with a little black electrical tape
The posistion of those caps is very obvious, as the traces come directly off of the wiper and one terminal of each vol. pot.......bright caps for sure :thumbsup:
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I was still curious, so I took a look only to find both of these were snipped off of this board too....
So this was either a very popular mod, or maybe a production line change :dontknow:
This board came out of modified unit,,,,that appears to have been worked on by someone who would be better suited as a bomb builder,,,,,who thought it was ok to make connections by just twisting wires together and suring them up with a little black electrical tape
The posistion of those caps is very obvious, as the traces come directly off of the wiper and one terminal of each vol. pot.......bright caps for sure :thumbsup:
Silvergun,
A goggle search turns up some of them with and without the caps. I've owned three of these amps and it's hard to imagine someone wanting to tame the brightness as they are kind of dark sounding to begin with.
Would those caps only effect low volumes as Brad said? This amp is always seemed darker than the others I've owned. Really great sounding amp but since I bought it a few months ago I've always wanted more top end. I use it with an attenuator with both volumes around 4. Would putting some caps there help brighten the amp? If so what values would you suggest?
Also as I stated earlier, This amps seems to break up much quicker than the others I had. Are there any obvious mods I could look for while I have it apart that might be obvious which would cause it to be a gain monster at 4 instead of 8 like my previously owned?
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Would those caps only effect low volumes as Brad said? This amp is always seemed darker than the others I've owned. Really great sounding amp but since I bought it a few months ago I've always wanted more top end. I use it with an attenuator with both volumes around 4. Would putting some caps there help brighten the amp? If so what values would you suggest?
A bright cap is only effective at less than full volume. At full volume the cap is shunted by the pot. As you turn down toward zero, the cap become more effective. It will definitely brighten your sound if the pot is set at 4. I suggested anything between 100pF and 500pF, but according to the schematic alerich posted, the value should be .001µF (1000pF). Just get a variety and brighten to taste. There's really not a right or wrong value.
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Also as I stated earlier, This amps seems to break up much quicker than the others I had. Are there any obvious mods I could look for while I have it apart that might be obvious which would cause it to be a gain monster at 4 instead of 8 like my previously owned
Well yes,,,but we'd be guessing....
Can we get a nice hi-res pic of your board?
We need to confirm your model.....do you have a Master Volume?....or Vol. 1 and Vol. 2?
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Would those caps only effect low volumes as Brad said?
Sluckey said it and yes to your question.
It's a volume pot bright by-pass cap. It's wired to the top/x end of the volume pot to it's wiper. As you turn the volume pot up it has less affect as a by-pass cap because there's less resistance left on the pot to by-pass.
Brad :icon_biggrin:
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Would those caps only effect low volumes as Brad said? This amp is always seemed darker than the others I've owned. Really great sounding amp but since I bought it a few months ago I've always wanted more top end. I use it with an attenuator with both volumes around 4. Would putting some caps there help brighten the amp? If so what values would you suggest?
A bright cap is only effective at less than full volume. At full volume the cap is shunted by the pot. As you turn down toward zero, the cap become more effective. It will definitely brighten your sound if the pot is set at 4. I suggested anything between 100pF and 500pF, but according to the schematic alerich posted, the value should be .001µF (1000pF). Just get a variety and brighten to taste. There's really not a right or wrong value.
Thanks for all the information. You really helped me out.
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Also as I stated earlier, This amps seems to break up much quicker than the others I had. Are there any obvious mods I could look for while I have it apart that might be obvious which would cause it to be a gain monster at 4 instead of 8 like my previously owned
Well yes,,,but we'd be guessing....
Can we get a nice hi-res pic of your board?
We need to confirm your model.....do you have a Master Volume?....or Vol. 1 and Vol. 2?
I have the Mig 50 non master volume. I'll take some pictures and update in a few days.
I've owned all the Sovtek models including the "H" and the mig 50 is the only one worth anything. I've owned three. I bought my first one brand new for $250 many many years ago after reading an article in a magazine. I saw one in my local store and I was shocked at how good it sounded. My old 63 Bassman was one of the best sounding amps I ever owned and the Mig isn't that far behind. Pretty incredible amps if you like classic tones.
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I have a Mig60 but it's been hot rodded and has Mercury iron so it's really barely a Mig60 anymore. It's the best high gain amp I have ever played. I bought a Sovtek slant 1x12 cabinet a few years ago. The seller demoed it for me with a Mig50H midget head. Can't say I was too impressed. It got really flatulent when you turned the gain and volume up. The little cabinet is awesome. There is a MIg50H midget on Craigslist in Nashville right now and I will be up there on vacation next week. I may give the guy a shout and go check it out to see if I just heard a dud before. That little 1x12 is awesome.
Sovteks were cool little amps. I remember when they first hit the stores. If Mike Matthews would reissue them with better components and without all the reliability issues they once had I bet he would sell a boat load of them. I could not find suitable replacements for the Russian cap cans in mine. I had to retrofit axial replacements. Luckily there is plenty of room inside the Mig60. I left the old cap cans retired in place for posterity.
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I have the Mig 50 non master volume. I'll take some pictures and update in a few days.
Great, that's the same board I have here, so at least I'll be able to cofirm/deny part values......it's pretty easy to tell the difference between stock and modded components, especially the resistors, because they are pretty unique looking
From what I could tell, the schematic that sluckey posted is pretty accurate, except for the component labeling (C14,C15,,,etc....all wrong)
Minimally I would try some new high quality power tubes....one of the first things to go in worn tubes is high end.....this thing runs B+ well over 500vdc on 2 - 5881.....I would not expect those to last, especially since most "modders" tend to bias everything hot.
BE CAREFUL-----these amps could be time bombs, left over from the cold war :icon_biggrin:
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BE CAREFUL-----these amps could be time bombs, left over from the cold war :icon_biggrin:
I'm the proud owner of two of these beautiful MIG50's and indeed, both of them were timebombs ...
MIG50 #1: Marshall 1986 clone (shared cathode / 250pF/56k tonestack)
MIG50 #2: Marshall 1987 clone
I've drawn schematics of both amps which use completely different component numbering. :dontknow:
Ask me anything, that's why I joined this forum ;)
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I've drawn schematics of both amps which use completely different component numbering. :dontknow:
Ask me anything, that's why I joined this forum ;)
I'd like to see your schematic drawings,,,,if you can get them posted
When you say Marshall 1986 and 1987 clones,,,do you mean that your Sovtek amps were modded to be like those amps?
Thanks
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I've drawn schematics of both amps which use completely different component numbering. :dontknow:
Ask me anything, that's why I joined this forum ;)
I'd like to see your schematic drawings,,,,if you can get them posted
When you say Marshall 1986 and 1987 clones,,,do you mean that your Sovtek amps were modded to be like those amps?
Thanks
There are a few errors on my digital schematics wich I'be scribbled on the printouts ;)
The 1986 clone has:
* Shared 820R/220uF cathode on V1
* 250pF/56k tonestack
* 100nF as PI couplers
* 39k NFB
The 1987 clone has:
* 820R/220uF and 2k7/680nF cathodes on V1
* 500pF/33k tonestack
* 22nF as PI couplers
* 82k NFB
... amongst some other changes, and they were 100% stock!
I'll try to fix the schematics before I leave for 3 weeks vacation ;)