A good friend has an RD50 112 I worked on, recapped and modded the Distortion circuit. He never uses the Bright switch and would use it instead to get more bass, similar to what the Deep Switch does on the 65 series amps. Researched this site, didn't find anything really applicable. I have experimented by shorting the cap the Bright switch goes to, can't tell if it actually allows more low frequencies through or just makes everything louder.
Alternatively, if I could increase the low frequency range of the Bass control, that would work too.
Any suggestions welcome.
Shorting out that cap
On an RD50, shorting out that cap in the Bright circuit will make it louder across the full frequency range, but you already know that. Take a look at the RD65 schematic for ideas. The RD65 messes with the feedback loop of the 2nd op-amp of IC-1 in the signal path for bass boost. You could probably do a similar thing to the input (1st) Op-amp on IC-1 on an RD50. This would boost bass for both clean and limiter channels. If bass boost is only desired on the clean channel, then do the mod on the 1st Op-amp in the clean path. This may be your best bet as many folks turn off the bass boost switch when using the dirty channel on an RD65. It gets too dirty. -mgriffin
On 2nd thought
Hello Inertian, After taking a closer look at my previous comment, I realized the RD50 is already hardwired like an RD65 with the Deep switch closed. Long story short is I don't think there's a lot you can do to enhance the circuitry to get more bass. Like when Cap'n Kirk said "Scotty, I need more power!" and Scotty said "Cap'n, I'm givin' you all she's got." Maybe your friend just needs to set the bass control to a higher setting. Sorry for throwing a curve ball at you. Good luck with the repair. -mgriffin