I have been working to match my sovtek mig 100u with drawings for the sovtek mig 100h. there appears to be no drawings around for the mig 100u.
my tone controls are very lacking in adjustment so upon checking the wiring with the diagram this is what i found.
the 100 ohm resistor at the top of the mid pot is not there. there is an 82 ohm resistor from the bass wiper to the treble wiper . additionally the mid pot is a higher value.
I believe this has been modded or was changed at the factory.
if i am correct, i should clip out the 82 ohm resistor and connect the wiper of the bass pot to the top of the pot where it connects to the treb control?
i find that in all marshall 800 schematics the 100 ohm does not exist. also it does not exist in the sovtek mig 100 (not h or u).
advice?
here's the physical view
my tone controls are very lacking in adjustment so upon checking the wiring with the diagram this is what i found.
the 100 ohm resistor at the top of the mid pot is not there. there is an 82 ohm resistor from the bass wiper to the treble wiper . additionally the mid pot is a higher value.
I believe this has been modded or was changed at the factory.
if i am correct, i should clip out the 82 ohm resistor and connect the wiper of the bass pot to the top of the pot where it connects to the treb control?
i find that in all marshall 800 schematics the 100 ohm does not exist. also it does not exist in the sovtek mig 100 (not h or u).
advice?
here's the physical view
Attachments
Thank you both for your replies.
i disconnected the 82 ohm from the bass pot wiper and put a jumper across the lugs to match what should be the correct wiring.
after looking at this I am wondering if the 82 ohm was misplaced and meant for the mid control where it shows the 100 ohm.
it is already much better. i can actually hear the frequencies sweep when i move the controls. something it didn't do prior.
i may still try the 82 ohm in the mid control.
the program was interesting. i played with it a bit and found some interesting interactions.
thanks again.
i disconnected the 82 ohm from the bass pot wiper and put a jumper across the lugs to match what should be the correct wiring.
after looking at this I am wondering if the 82 ohm was misplaced and meant for the mid control where it shows the 100 ohm.
it is already much better. i can actually hear the frequencies sweep when i move the controls. something it didn't do prior.
i may still try the 82 ohm in the mid control.
the program was interesting. i played with it a bit and found some interesting interactions.
thanks again.
Attachments
82 and 100 Ohms make "no" difference in a network of 25K-100K resistors. They may as well be shorts. However sometimes in production, a resistor is "just right length" to be a jumper, and the labor to strip wire is more than the cost of a resistor. Or maybe they over-ordered 82r resistors and have to use them up before the boss gets mad.
I really think the MIDrange pot ought to be nearer 10K than 25K. Maybe this was done to make a less dramatic tonal change (why? to make a higher-price model sound better?). Or maybe they got a real good deal on 25K pots.
_I_ would consider Fender Blackface as the "reference" here. Millions of other amps plagiarize Fender's values exactly, because they work. True, there are enough Blackface-clones around already, and you may want something different. But IMHO it is very hard to find different and *better* (even for a specific style/genre).
I really think the MIDrange pot ought to be nearer 10K than 25K. Maybe this was done to make a less dramatic tonal change (why? to make a higher-price model sound better?). Or maybe they got a real good deal on 25K pots.
_I_ would consider Fender Blackface as the "reference" here. Millions of other amps plagiarize Fender's values exactly, because they work. True, there are enough Blackface-clones around already, and you may want something different. But IMHO it is very hard to find different and *better* (even for a specific style/genre).
I would agree with you that a low ohm resistor probably doesn't play a big role here.
however my guess would be its part of the "trim" of the mid pot.
looking at both the marshall and fender tone stacks they are nearly identical.
this amp is a clone of the jcm 800 so i would expect it to follow that fairly close.
While the tone pot is a 25k the presence of the 30k resistor would lower it by nearly half.
it may well also be an attempt to circumvent patent disputes, although as you have stated it wont make much difference.
What i noticed working with the simulator software above is that the mid control only creates the curves mid point shelf, moving the entire tone range up or down, it in itself does not boost or cut the mids above or below the treble and bass. As a matter of fact if cutting the treble and bass to their minimum it creates a nearly flat response. at which point the mid control boosts or cuts the entire curve.
I have positive grids bias software. i have been using this to change tone stacks and work with various manufacturers designs to find the best for my tastes. from there i will be able to find schematics to allow me to mod my amp for those characteristics.
however what i found was that tone controls on the sovtek were not giving me the range i should have to start. thats what brought me to this point.
I had even gone as far as running a white and pink noise generator thru the amp and viewed it with an analyzer. what i had found was the controls had made almost no difference.
now that i made the above stated changes it already is a huge improvement.
so this will be a work in progress.
however my guess would be its part of the "trim" of the mid pot.
looking at both the marshall and fender tone stacks they are nearly identical.
this amp is a clone of the jcm 800 so i would expect it to follow that fairly close.
While the tone pot is a 25k the presence of the 30k resistor would lower it by nearly half.
it may well also be an attempt to circumvent patent disputes, although as you have stated it wont make much difference.
What i noticed working with the simulator software above is that the mid control only creates the curves mid point shelf, moving the entire tone range up or down, it in itself does not boost or cut the mids above or below the treble and bass. As a matter of fact if cutting the treble and bass to their minimum it creates a nearly flat response. at which point the mid control boosts or cuts the entire curve.
I have positive grids bias software. i have been using this to change tone stacks and work with various manufacturers designs to find the best for my tastes. from there i will be able to find schematics to allow me to mod my amp for those characteristics.
however what i found was that tone controls on the sovtek were not giving me the range i should have to start. thats what brought me to this point.
I had even gone as far as running a white and pink noise generator thru the amp and viewed it with an analyzer. what i had found was the controls had made almost no difference.
now that i made the above stated changes it already is a huge improvement.
so this will be a work in progress.
Attachments
I believe both companies have used several different tone control circuits, but at least a few of the references I looked up, think that Marshall variants typically move the mid-scoop up to higher frequency, presumably because there is more treble content in an overdriven Marshall signal than a clean Fender one.
The other difference I've heard about is what happens when all three knobs are set to zero. For some Fender variants, no signal gets through at all if you do this...rather counter-intuitive for the guitarist!
These are some of the reasons why I am not a fan of the Fender/Vox/Marshall tone control circuit. Twisting the knobs feels like playing Russian Roulette sometimes - you never quite know what frequency response you'll get as a result! The treble control seems to be almost as much a volume control as a treble control, and the mid control pokes various peculiarly shaped holes into the frequency response, but cannot actually boost mids. Every control's frequency range overlaps its neighbour's, and everything interacts in unpleasant ways. It's all very weird.
-Gnobuddy
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