Box to switch between two amps, one with an effects loop, one not

[walpow]

I've got a board that I use for both guitar and bass. My guitar amp has an effects loop; my bass amp doesn't. I've figured out how to wire up a switcher with a 4PDT that'll switch everything I need:

* For guitar: last pre-preamp pedal output goes to amp, effects send goes to first pedal in the loop, last pedal in loop goes to effects return.

* For bass: last pre-preamp pedal output goes to first pedal in loop (internal to the box), last pedal in loop goes to bass amp.

The 4PDT handles all the above plus a couple of status LEDs.

My question: this handles all the signal switching. But the seven jacks involved all have ground lugs too, and I'm concerned about that. Will a common ground give me hum? If not, which grounds should be wired together, given that certain jacks will be in the same circuit in one switch position but different circuits in the other?

[maarten]

Hard to imagine this one without some kind of block diagram. Seems to me that when you have everything lined up for your bass, there should be no problems (meaning not an excessive hum), but that when you switch over to your guitar a ground loop could be introduced if your last pre-preamp pedal is in the same box as your pedals in the effect loop (sharing the same ground connection? - this is not quite clear to me). If so, maybe you could use a separate cable to plug the box in the effects return which is grounded only at one end. So the cable shield is connected at one end (in the box) with ground, while at the other end (at the effect return) your plug is not connected to the cable shield. The box ground is thus connected to the amp ground only through the effects send cable. Hope this makes sense and that I did not misinterpret your problem....
Maarten

[maarten]

Sorry, maybe I did not consider this well enough: when you are using your guitar, it seems to me you have several ground connections between your amp and the effects which could cause trouble as this introduces a ground loop. Therefore you would have to be able to limit the number of ground connections with the amp to just one (as will be the case when you use your bass). Either cables which are ground connected at one end, or the possibility to switch off the superfluous ground connections could be a remedy.
Maarten