Slightly OT: MM 65 preamp

[Ben N]

I didn't get much of a response to this at Ampage--tube snobs!   --so I figured I'd try this where "opamp" is not a dirty word.

I just brought one of these puppies home: An early Music Man Sixty-Five head, no reverb or trem, just Normal and Bass channels, which are essentially identical BF-Fenderish channels rendered in opamp (LM307H), feeding a 12ax7 driver/MV/concertina into a pair of EL34s.  Sounds pretty good, if a bit nasal, with single coils; you can crank it for a moderately overdriven sound, and clean it up at the guitar.  Hates HBs.

The input stages have these wierd paralleled pairs of diodes (like clipping pairs) across the + & - inputs of the opamps.  Jaysg at Ampage says these are to prevent input latchup, whatever that is.  In general the configuration of the opamp stages is strange to me (not saying much, I know).  Can anyone give me the lowdown on this preamp?  Would an opamp upgrade be in order, and what else would I have to change?  Can I improve the input headroom so that it handles HBs better withoiut dramatically dropping overall gain (which isn't that high to start with)?

Also, it looks pretty simple to add an fx loop between the preamp board and the PI.  Is that right, or should I be looking elsewhere?

(Long term, I think it would be cool to substitute a JFET ROG/DougH style preamp (Meteor? T-chief?) for the normal channel, tweaked for more headroom and higher voltage +/- power supply, but I guess I'll have to do some breadboarding for that.)

Ben

[Ben N]

Whoops, forgot to add this:

http://www.ernieball.com/mmonline/techinfo/old_amps/2100-65.pdf

[Elektrojänis]

I don't know about everything else but I think this article on geofex explains the diodes on the opamp inputs pretty well: http://www.geofex.com/circuits/what_are_all_those_parts_for.htm

...and then some.

[Ben N]

Wow, thanks.  That was perfect.  RG's drawing is exactly the MM circuit.

Any word on replacing the opamps with, say, JFET input opamps?

Thanks,
Ben