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post-pi mv AND pre-pi mv together...

1/3/2006 4:06 PM
Ian Anderson
post-pi mv AND pre-pi mv together...
I've just wired this up in my high-gain homebrew and this really offers a wide range of tones by mixing varying volumes of each master volume.  
Anyone else tried this?  
... Ian :)
1/3/2006 6:52 PM

Yes. It gives you the oportunity to vary how hard you hit the power tubes no matter what you do with the preamp... Sort of. I like the PI MV.
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1/4/2006 9:06 AM

Sundown amps used both in their amps in the 80s.  
Rebel schematic:  
Pre PI MV is labled "Master" and the post PI MV is called the "Govenor."  
1/4/2006 3:37 PM

I've been meaning to do some taste tests along this line.  
What would you say the general differences in tone are for a pre-PI master vs a post-PI master?
1/5/2006 10:41 AM
"What would you say the general differences in tone are for a pre-PI master vs a post-PI master?"
The only amps I've ever done it in are Marshall types and one oddball homebrew experiment. One thing a post-PI MV will do is to help debunk the myth that Marshalls are all 'power tube distortion'. You can get full-on Marshall distortion at low volume levels using the post-PI MV which illustrates that almost all the distortion (i.e. crunch) in a non-MV Marshall comes from the preamp and mainly the phase inverter.  
Typically in a non-MV Marshall the PI is the first thing to distort and having a pre-PI MV can allow you to stay clean at higher volumes and/or adjust how hard you slam the PI at different Volume settings (i.e. preamp volume settings).  
There's much more but I'll stop there. :)  
PS - just curious Ian, which post-PI MV did you use? I usually use the Ken Fischer type; i.e. the dual-pot replaces bias splitters and 'grounds' to the bias supply (so no extra caps are required). There's also the 'Rich mod' type where the dual-pot is in parallel with the bias splitters but still 'grounds' to the bias supply and is connected slightly differently, and then there are types that use caps to preserve the bias and the dual-pot is connected to true ground. And then there is the 'crossline' (aka Matchless) post-PI MV (basically a Cut control with no cap) which I don't like at all.
1/5/2006 12:03 PM
Bruce /Mission Amps

Please describe, if possible, what you don't like about the Matchless X-cut MV?  
My preference is the KF dual gang but I recently retried the X-cut and it sounded pretty good too.  
1/5/2006 3:32 PM
Ian Anderson

The post-PI mv I used was from the Fender Tonemaster... 47nF off the plates of the PI >>> dual-gang 500k pot >>> 47k off the wipers >>> 47nF into the output tube grids (via 4k7 grid stoppers)  
The pre-PI mv is good at adjusting how much the PI gets slammed and gives you more/less bass.  
The post-PI mv sounds pretty much the same at any volume and is thick & chunky even at whisper volumes.  
By mixing the two MV's to varying degrees I've been able to tune in some very tasty tones and balance out the flaws of each type of MV (i.e. pre-PI can be too trebly/thin and post-PI can be too thick/bassy)  
At home I use the post-PI mv to control the overall volume and mix in the 'pre' to taste. At gig volumes I'd imagine it will be the other way around though as the amp gets a thicker tone when cranked anyway.  
... Ian :)
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