OK - realize I'm coming to this thread a bit late, and sorry if I'm being dense, but it seems to me that there are some commercial amps out there that advertise pentode-triode switches. Seems to me that even if they have disclaimers about turning the amp off before switching, there would be serious liability issues when someone inevitably ignores their admonitions. So . . . how do THEY do it? I've been searching for quite a while for an appropriate DB3T -- hell, even a DPDT -- with an adequate DC rating, and haven't found one. (Nothing under 600# anyway ;^)
For the record, I've implemented the function using DP 3-position rotary switches that were AC rated, with no ill effects, but it's not something I would trust, or give to someone else. I do like the ability, though, and would like to do it right.
I was just thinking about a circuit I encountered a while back -- I think it was in Craig Anderton's Electronic Project for Musicians book -- he used a quad bilateral switch, in combination with some strategically-placed diodes, to implement a time-delay for an electronic bypass. I'm wondering if it would be feasible to use a similar approach, in combination with appropriate relays, to break the B+, switch the HV lines to the output tube(s), and then reconnect the B+. . . . Yeah, kind of like driving thumbtacks with a sledge hammer, but the challenge is difficult to ignore . . . .