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I'll post some pics of this soon when I get some more time.
I modded my Mesa Boogie SOB to have a real presence control, and the improvement in tone was out of this world. Especially for distorted tone but also (though less dramatic) for cleans. IMO the old boogies really need a presence control.
The presence is different from the other tone controls, since it doesn't just shape the tone in the middle of the preamp section, rather it adds negative feedback from the output of the output transformer (i.e. the speaker terminal) to the front/input of the phase inverter, with a capacitor, having the effect of controlling the really high trebles.
The SOB has been regarded as a less desireable (probably the least desireable) of the vintage boogies. It comes with a "limit" control, which is meant to allow power amp-like distortion at bedroom volume levels, but according to basically everyone sounds totally crap and is best left in the 0 position (having no effect). Turning it up starves the PI causing it to clip at lower volumes. One forum post I saw said it should be labelled a "suck" control. The few people who said the control was useful said it should only ever be used in small amounts, at settings from 0 to 1 or maybe 2 out of 10.
More recent posts about the SOBs in general are more favourable, especially with regard to the clean tone.
The long head versions of the SOB's come with both a presence and a limit.
The other SOBs have either one or the other, and, don't be fooled, the label on the front panel is no indication of what you have since many of them (possibly most of them apart from the long heads) that say "presence" on the knob are actually wired as a limit control.
The happy news is to mod these back to having a real presence control is ridiculously easy (assuming you have some experience working with tube amps).
***WARNING*** tube amps contain lethal voltages, and can do so even after the power has been disconnected, unless the filter capacitors in the PSU are discharged. (Is is necessary to say this warning on a forum specifically about music electronics? Or is it assumed people will know this)?
My SOB says presence on the front, but it was really a limit, but someone had modded it (presumably to make it sound less crap) by adding a resistor and capacitor across the pot. It didn't seem to do that much at all other than change the volume slightly.
I looked at the schematics for the SOB and found two versions, and I could see how the presence was meant to be wired. The nice thing was that the capacitor needed for it was already there on the PCB, with one end attached to nothing.
All that was needed for the mod was to remove the limit pot, and wire the other end of the TCR current limiting diode (that looks like a 2 legged transistor) that was connected to the pot to ground, to remove the limit completely (so its like its set on 0 permanently). If you wanted, an additional pot could be added to have both but I didnt think the limit was worth bothering with.
Then, add a new pot of the right value (I'll post later with the actual values and a pic, I think its 250K) , and wire the wiper (i.e. middle lug) of the pot to the free end of the capacitor on the PCB (right next to where the pot is), and the side of the pot closest to the power switches (with the pot lugs facing up like all the other pots) is wired to the 8 ohm speaker output with a long wire that goes all the way from the pots at the front to the speaker jack at the back. That's it.
The improvement in tone was amazing. With the distortion, there was previously a harsh fizz that couldn't be removed by any of the other tone controls. The original configuration was equivalent to having the presence turned to 10 all the time (actually perhaps to 11 if you consider the 250K of the pot is still in the circuit when its on 10, and originally it would have been infinite resistance). With clean sounds, higher values of the presence can be used and give the amp some "vintage sparkle", but with distortion I find lower values of at most 6, or 5 or 4 are much much nicer sounding and smoother. This applies to both the internal distortion and to using pedals for distortion.
I've got a studio .22 and I find the presence control on that (which is a similar style of operation) to be extremely useful. I think its more useful than the graphic, unless you want a really scooped sound for metal, the graphic isn't absolutely necessary IMO but the presence control is what makes the amp as it has such a powerful effect on the tone. I haven't heard a Mark I but I'd go as far as to suggest that having a presence control would probably be the majority of the difference in sound between the Mark I and the SOB.
I modded my Mesa Boogie SOB to have a real presence control, and the improvement in tone was out of this world. Especially for distorted tone but also (though less dramatic) for cleans. IMO the old boogies really need a presence control.
The presence is different from the other tone controls, since it doesn't just shape the tone in the middle of the preamp section, rather it adds negative feedback from the output of the output transformer (i.e. the speaker terminal) to the front/input of the phase inverter, with a capacitor, having the effect of controlling the really high trebles.
The SOB has been regarded as a less desireable (probably the least desireable) of the vintage boogies. It comes with a "limit" control, which is meant to allow power amp-like distortion at bedroom volume levels, but according to basically everyone sounds totally crap and is best left in the 0 position (having no effect). Turning it up starves the PI causing it to clip at lower volumes. One forum post I saw said it should be labelled a "suck" control. The few people who said the control was useful said it should only ever be used in small amounts, at settings from 0 to 1 or maybe 2 out of 10.
More recent posts about the SOBs in general are more favourable, especially with regard to the clean tone.
The long head versions of the SOB's come with both a presence and a limit.
The other SOBs have either one or the other, and, don't be fooled, the label on the front panel is no indication of what you have since many of them (possibly most of them apart from the long heads) that say "presence" on the knob are actually wired as a limit control.
The happy news is to mod these back to having a real presence control is ridiculously easy (assuming you have some experience working with tube amps).
***WARNING*** tube amps contain lethal voltages, and can do so even after the power has been disconnected, unless the filter capacitors in the PSU are discharged. (Is is necessary to say this warning on a forum specifically about music electronics? Or is it assumed people will know this)?
My SOB says presence on the front, but it was really a limit, but someone had modded it (presumably to make it sound less crap) by adding a resistor and capacitor across the pot. It didn't seem to do that much at all other than change the volume slightly.
I looked at the schematics for the SOB and found two versions, and I could see how the presence was meant to be wired. The nice thing was that the capacitor needed for it was already there on the PCB, with one end attached to nothing.
All that was needed for the mod was to remove the limit pot, and wire the other end of the TCR current limiting diode (that looks like a 2 legged transistor) that was connected to the pot to ground, to remove the limit completely (so its like its set on 0 permanently). If you wanted, an additional pot could be added to have both but I didnt think the limit was worth bothering with.
Then, add a new pot of the right value (I'll post later with the actual values and a pic, I think its 250K) , and wire the wiper (i.e. middle lug) of the pot to the free end of the capacitor on the PCB (right next to where the pot is), and the side of the pot closest to the power switches (with the pot lugs facing up like all the other pots) is wired to the 8 ohm speaker output with a long wire that goes all the way from the pots at the front to the speaker jack at the back. That's it.
The improvement in tone was amazing. With the distortion, there was previously a harsh fizz that couldn't be removed by any of the other tone controls. The original configuration was equivalent to having the presence turned to 10 all the time (actually perhaps to 11 if you consider the 250K of the pot is still in the circuit when its on 10, and originally it would have been infinite resistance). With clean sounds, higher values of the presence can be used and give the amp some "vintage sparkle", but with distortion I find lower values of at most 6, or 5 or 4 are much much nicer sounding and smoother. This applies to both the internal distortion and to using pedals for distortion.
I've got a studio .22 and I find the presence control on that (which is a similar style of operation) to be extremely useful. I think its more useful than the graphic, unless you want a really scooped sound for metal, the graphic isn't absolutely necessary IMO but the presence control is what makes the amp as it has such a powerful effect on the tone. I haven't heard a Mark I but I'd go as far as to suggest that having a presence control would probably be the majority of the difference in sound between the Mark I and the SOB.
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