Phase inverter and negative feedback in Mk II-IV

[darkbluemurder]

I noticed that Mesa Boogie uses an unusual phase inverter and negative feedback circuit. The PI grid loads are 150k and 100k as opposed to the 1 Megs of the Blackface Fender. Is there any reason for making these so small and of non-equal values?

I also noted that the input coupling cap (0.1uF) is much higher than in Fenders. Is that to compensate for the low grid loads of 150k and 100k?

Finally the presence pot of 250k looks unusual. It looks like a variable feedback control. To me it is very effective, much more so than the usual presence controls. Are those parallel caps (the 0.001 in parallel with the 56k resistor) added to boost lows?

Any comments would be greatly appreciated.
Thanks.

 

[ursinus]

Calling Boogiebabies! Send the Bat signal.... 8)

 

[darkbluemurder]

I was just wondering why nobody else uses such phase inverter and negative feedback/presence arrangement. I doubt that these are patented or if they ever were that the patent is still valid. I just noticed that my Mark III has the best clean sound compared to all my Fenders and sounds more solid when putting pedals in front of the input. Since the preamps are so very similar I figured it must be in the phase inverter and/or the presence controls. I probably have to implement this in a Fender amp to find out. If I do I'll let you know.

Cheers and keep on rockin'