Help on cab/amp switcher

[JimSalabim]

Hi!
I have two 50W tube amps and I want to connect them to a cabinet – only one amp at a time. The other amp should be connected to a 16 ohm dummy load at the same time (four 4 ohm power resistors in series, each 50 W).

Can I use a 4PDT toggle switch to do this and would it work as shown in my image?
Of course I would switch both amps to standby while flipping the 4PDT switch.

[peterg]

I think you should be able to use a dpdt switch. All the grounds are connected and there would be only one resistor.
pole 1 top to cab                        pole 2 top to resistor
pole 1 middle to amp 1               pole 2 middle to amp 2
pole 1 bottom to resistor            pole 2 bottom to cab

[JimSalabim]

Ah, yes, I see. So it's no problem, if the grounds the two amps are connected then?

[R.G.]

Ah, yes, I see. So it's no problem, if the grounds the two amps are connected then?

Only if you get hum. Well, OK, you're likely to get hum. Sorry. Sometimes not. The connection of a guitar to both input jacks is more likely to give hum than connecting the output grounds though. How are you solving the connected input grounds problem?

A bigger issue may well be that you need to use make-before-break switches to ensure that neither amplifier is ever open circuited. Some amps won't die because of this, some will, and that's where the prevailing wisdom on the internet not to run a tube amp without a load comes from. It is *possible* that even the few milliseconds between one set of contacts opening and the other closing on the dummy load, one or both amps will be open circuited long enough to cause a voltage spike that damages the OT. Or worse yet, it doesn't damage the OT with any ONE spike, but the accumulation of many of these spikes does eventually damage the OT.

Switches come in break-before-make and make-before-break versions; these last are also called "shorting switches".

[JimSalabim]

The input grounds problem is solved by this ABY box:
http://www.sabrotone.com/?p=3864

Thank you for the info about the make-before-break switches!
If I use a relay instead of the switch, would this be better?
What kind of relay would I need? Could I use a 9V relay (or maybe two relays) and switch it with a simple SPST toggle switch?

[JimSalabim]

So maybe like this but with 4 relays instead of 2?