Mark IIc+ plates

[Fischerman]

It's probably pretty well known around here that the Mesa/Boogie Mark IIc+ is similar to the ODS. I was looking at a Mark IIc+ schematic (I think it was IIc+) and the first thing that pops out is that OD1 has an 82k plate resistor and OD2 has a 270k plate resistor (IIRC Rks are 1k5 and 3k3 respectively). These values are not only different but they are 'reversed' relative to an ODS; i.e. in the Boogie OD1's plate resistor is smaller than OD2's.

Is the intended goal to reduce distortion in OD1 and increase it in OD2? Is the intention to get OD2 to cut-off more? Does the low OD1 plate influence the grid current on OD2?

I don't want to imply that a Mark IIc+ sounds as good as or better than an ODS...not even addressing that. I just think that when designing/tweaking the Mark IIc+, someone thought that this 82K/270K arrangement sounded better and perhaps investigating 'why'...I might learn something. I know that when I was tweaking my 50W HRM (diff aminal I know) that it seemed that 'keeping OD1 clean-ish and getting almost all the distortion from OD2' always seemed to sound better (edit: although it does seem important to have OD1 'right on the edge').

[massygt6]

It could make sense...
It is more logical to build up the gain stage by stage crescendo, than shoot all your cartridges in the first gain stage and after, to cut-off bass and gain...

[llemtt]

The gain stage that gets overdriven first in a Mark IIc is the fifth not the fourth like the ODS.

The Mark II Boogie concept originates from a Fender AB763 reverb circuit removing the reverb springs and connecting the reverb driver directly to the reverb recover. This is one thing that even Fender did in his master volume silverface later circuits.

Teo

[Fischerman]

Hi Teo, Thanks for the response.

The gain stage that gets overdriven first in a Mark IIc is the fifth not the fourth like the ODS.

The fifth stage? It looks very similar to an ODS in that there appear to be only 4 gain stages (not counting the fx loop). What 'fifth gain stage' are you referring to?

EDIT: In my small amount of testing/'scoping I did on my 50W HRM...it appeared that when the Input Gain was set to ~1:00...that was right where the third stage just started to pull grid current. So in my amp the third stage wasn't perfectly clean per se. Don't know if that's normal or whatever.

[llemtt]

The "fifth" gain stage in the original schematic is labeled V2A, it acts also as the fx loop "send" stage.

Anyway five stages are too much, the ODS sounds a lot better but this is something we already know

For a smooth sound V2A in an ODS should be overdriven only a little, the "low gain" concept is at the basis of the HRM circuit.

Teo