Mk IIC+ "squish mod" to preamp??

[dodger916]

Per this auction:http://cgi.ebay.com/VINTAGE-Mesa-Bo...39:1|66:2|65:10|240:1318&_trksid=p3286.c0.m14
What's that mod do?

 

[Elpelotero]

great question...It's legit. He's got the receipt with MB initials...BUut what the heck is it?

 

[Boogiebabies]

$ 50, it's just the bright reduction mod. there are no parts billed out, but two .06 cent caps are all that's required.

 

[igfraso]

I heard that the mod is only available to Simulclass IIC+ amps, not for 60/100 watters.

 

[Platypus]

I heard that the mod is only available to Simulclass IIC+ amps, not for 60/100 watters.

They have the same preamp so I don't see that being true.

 

[dodger916]

$ 50, it's just the bright reduction mod. there are no parts billed out, but two .06 cent caps are all that's required.

Ed: Could you explain what the bright mod does? Also, my C+ sounds a bit "grainier" than I'd like. Could that be due to caps? Thanks again! Frank

 

[Boogiebabies]

The 60/100 and Simul Class have slightly different preamps due to the power section. The bright mod is basically using two capacitors normally stock in a 60/100 and using them in a Simul Class preamp. The point where the lead channel is inserted in parallel with the clean channel there is a 3.3M resistor with a 10pf cap on 60/100's and 3.3M and 20pf on Simul's. This cap regulates some of the treble that passes on through the preamp. The difference of 10pf and 20pf is how much treble each lets pass. The 10 is a tad less bright which helps tame the brightness of the preamp, specifically for 6L6's. The 20pf was used on Simul's, to some degree, to pass a bit more high end to counter act the effect of the outer two sockets being wired in triode for the effect of a rounder treble response. The same principal is used on the second lead gain stage. The 60/100 uses a 1000pf cap in parallel with the 270K plate load resistor to trim the high end response of the gain stage, again due to the use of a 60/100 power section. The Simul class preamp does not have this cap, so the treble response of the second lead gain stage sounds as designed. So basically the bright reduction mod is changing the Simul preamp to that of a 60/100 by reducing the 20pf to 10pf and adding a 1000pf cap to the second lead gain stage plate load resistor. The effect takes some of the high end sizzle and allows you to run higher treble settings with maintaining an even smoother preamp.

 

[Boogiebabies]

I heard that the mod is only available to Simulclass IIC+ amps, not for 60/100 watters.

You are correct. The parts used are already in the 60/100.