Problem is when the SS is switched to speaker position the tube amp has no resistance. Accidentally turning on the tube amp while not connected to a speaker load will damage the output transformer.
There is a workaround, although it requires a bit of design and construction (I know of no switch that can do this). You will need a three pole double throw switch (position A and position B or ON-ON) with a rating of at least 20-amps. You will also need a box with two pairs of binding posts (call them A and B) on one side for the amp hookup and one pair of binding posts on the other side for the speaker hookup (call this COMMON). Let’s say position A is the tube amp and position B is the SS amp. I suggest you look at a wiring diagram of a 3PDT switch....
Pole 1: Connect pole 1 common to the speaker negative binding post, position A to the Tube amp negative binding post and position B to the SS amp negative terminal binding post.
Pole 2: Connect pole 2 common to the speaker positive binding post, position A to the Tube amp positive binding post and position B to the SS amp positive terminal binding post.
Pole 3: Connect pole 3 common to the pole 1 common, which will ground pole 3 to the speaker negative terminal. Connect an 8-ohm resistor between position B of pole 3 and position A of pole 2. This will put the 8-ohm resistor in series with the Tube amp positive terminal and negative speaker terminal, protecting the amp. Position A of pole 3 is left hanging unconnected.
When you switch to position A, the tube amp is connected to the speakers and the SS amp and the protective resistor are connected to nothing, so the tube amp is the only hot circuit with the resistor having no effect.
When you switch to position B, the SS amp circuit is hot and the tube amp is protected by a circuit with an 8-ohm resistor, so if you accidentally turn on the amp nothing bad will happen.
(This is for one channel. For both speakers you will either need two of these or a 6-pole double throw switch with everything above times 2, i.e., poles 1,2,3 = poles 4,5,6).
There is a workaround, although it requires a bit of design and construction (I know of no switch that can do this). You will need a three pole double throw switch (position A and position B or ON-ON) with a rating of at least 20-amps. You will also need a box with two pairs of binding posts (call them A and B) on one side for the amp hookup and one pair of binding posts on the other side for the speaker hookup (call this COMMON). Let’s say position A is the tube amp and position B is the SS amp. I suggest you look at a wiring diagram of a 3PDT switch....
Pole 1: Connect pole 1 common to the speaker negative binding post, position A to the Tube amp negative binding post and position B to the SS amp negative terminal binding post.
Pole 2: Connect pole 2 common to the speaker positive binding post, position A to the Tube amp positive binding post and position B to the SS amp positive terminal binding post.
Pole 3: Connect pole 3 common to the pole 1 common, which will ground pole 3 to the speaker negative terminal. Connect an 8-ohm resistor between position B of pole 3 and position A of pole 2. This will put the 8-ohm resistor in series with the Tube amp positive terminal and negative speaker terminal, protecting the amp. Position A of pole 3 is left hanging unconnected.
When you switch to position A, the tube amp is connected to the speakers and the SS amp and the protective resistor are connected to nothing, so the tube amp is the only hot circuit with the resistor having no effect.
When you switch to position B, the SS amp circuit is hot and the tube amp is protected by a circuit with an 8-ohm resistor, so if you accidentally turn on the amp nothing bad will happen.
(This is for one channel. For both speakers you will either need two of these or a 6-pole double throw switch with everything above times 2, i.e., poles 1,2,3 = poles 4,5,6).