The subject says it all; I want to retrofit a double valve to my Olds S-20 bass trombone. I realize it probably isn't worth the trouble, but it's an old friend I'm not ready to part with. Also worthy of consideration would be a way to add a D valve circuit to the F loop without doing extreme violence to the flat wrap. The existing small F rotor doesn't disturb me so much.

The denizens of the For Sale area suggested asking here too. What would be involved in retrofitting a dependent D loop to a flat-wrap Olds S-20 bass trombone? While small-ish, the existing F rotor seems adequate for my needs.

Any shared wisdom appreciated.