Buy a bunch of cheap 12-volt relays (at eBay, you'll get them for cents
each) and use them according to the principle of any two-valve stack
(in pianos and organs).  Aren't they called "relay valves" there also?

Regards
Harald Müller

 [ The French expression is "relais" but in English the signal from
 [ the tracker bar is amplified by a "primary valve" which sends the
 [ signal on (relays it) to the "secondary valve".  "Relay" and
 [ "relais" also denote the complicated switch system which is the
 [ brain of an electrically controlled pipe organ.  -- Robbie