1987x Acquired - Would love your comments on the mods.

[splitslim]

So I passed on the JCM800 2203 which was heavily modd'ed and grabbed this 1987x instead.



Master Volume installed - pre-phase inverter at the effects loop (before or after?). Green lead connects to board at BN4 and the blue lead goes back to the effects loop card ;



Cap installed on across R11, 330uf if I'm not mistaken;



Hum balance VR removed and replaced with 2 resistors and what looks like 2 caps beneath (at R31, R32 and VR2)? Would love more information on this one. Why the caps under the resistors and what benefit does this mod bring?



I'm considering having the master volume re-done to a post-phase inverter Trainwreck style (professionally). What are your thoughts on the pros/cons of making that change from what is currently installed here? Any comments on the proximity of the master volume control to the power and standby switches? Am I asking for trouble leaving it there, introducting AC noise into the signal path?

Thanks!

[neikeel]

There has been some input to this on Marshall amps forum.
The heater anti hum pot has been replaced by the 100R resistors, a sensible mod. No idea why the caps are there, I can think of no reason why you might have capacitors bypassing the resistors in that circuit.
I quite like the pre-pi Mv in a typical cascaded v1 (2203/2204 style). I have no experience with the crosswire mv. Although the Fischer mv that Mark and Larry made popular works very well particularly when you use a high quality pot (like a PEC). I personally use an attenuator to manage overall volume even on an Mv amp.
Regarding the 330uF on v2a cathode resistor. Guys chasing the brown sound like it but I find you lose a lot of definition in the preamp. I like the oem 0.68uF or even 0.47uF both give a nice upper mid gain boost without all the crap the 330uF brings with it.

[jarrodthebobo]

I've found that a 10uf cap over v2a is pretty much a full gain boost without a big deal of mush the 330uf cap brings. I'm not too sure on the science behind it and what frequencies are technically being boosted here, but that's just from experimentation.

I do however have a switch on my current project that switches between the 10uf and a .47uf cap over the v2a stage. Offers either full boost or a mid/treble boost. They tend to 'sound' like they have the same amount of gain, but the feeling is definatly different and so is the bass response.