After spending the afternoon thrashing the living daylights out of it, checking out YouTubes and pricing attenuators I think I'll try the PPIMV.
Dmoney in what way does it screw with the presence?
Also, if it doesn't give power valve saturation at lower volumes does it still have all that gain when whacked up to 10?
Sorry sgmypod, computer says no
Basically, the presence works by taking the output of the amp (tapping it from one of the output transformer secondaries) and feeding it back into the phase inverter 180 degrees out of phase. so when you turn the presence, you mix more or less out of phase high frequencies with your output. more out of phase signal cancels out the high frequencies more and vice versa.
less output = less negative feedback... which is why presence and depth controls can be less responsive at low master volume settings.
The relationship between the phase inverter output, power valve input & output, and feedback is usually fixed. If you add a post phase inverter master volume to lower the volume of the amp, you alter that relationship.
It means you can be driving the PI quite hard by increasing the preamp gain/volume, but the output of the power section can be small (quieter amp), which means you get less negative feedback into the PI, which in turn effects the presence control. Any master volume will effect the feedback for those controls, but a normal Pre-PIMV that you'd find in a JCM800 for example, doesn't sit between the PI and the power valves and doesn't effect the power section the same way.
It's a while since I've used a PPIMV (not the Lar/Mar suggested) but basically, if you turn it right the way down and the preamp all the way up (which is dumb), you can get this sound which is the PI is distorting with no power valve breakup and very little feedback to tame the high frequencies as your presence won't really do anything. As I recall the result can be a bit harsh... BUT... using a PPIMV that way is a bit silly.
Different PPIMV's sound and react differently. The Lar/Mar is detailed in Ken Fishers trainwreck pages. I think It's meant to be the one to go for to really retain the sound of the amp, and I think a lot of Trainwreck enthusiasts opt for this PPIMV in Trainwreck Express clones, since those amps don't have master volumes either. when on '10' the amp should be the same as it is 'stock'.
I've never used the Lar/Mar though, I've used other methods (methods 3 & 1 in the trainwreck pages). Others here may have actual experience of the Lar/Mar. I don't think it will give you power valve breakup and lower volumes like an attenuator would.
what videos did you see?
since this requires modding the amp you should be sure its the way you want to go.
http://home.comcast.net/~jbjdav26/1992/1992%20Dual%20B+%20PPIMV.JPGhttp://blueguitar.org/new/schem/trainwreck/the_trainwreck_pages.pdf (page 14 & 24 of the pdf)
