Unless you have an onboard preamp, it's easy. Just connect the switch so it shorts the ouput hot to ground. That will kill the signal.

hey Scott, is that Alamo a Capri? That has the same tubes as A.K.s Harmony. You guys can start a club.

My guess is the H-303 originally was AC/DC, but never found it's way to market. Unlike a radio, even if you have the chassis unaccessable to the user...

She? Well, since there wasn't any indication of gender, I too, assumed you were male. Guitar chicks are way cool, but way rare, so whooda thunk.ther...

12AV6? not likely, unless someone did some strange modification on this amp. Harmony never used a 12AV6 in any of their amps. This amp sounds like ...