mikebeam
Yeah, I saw that. I might have to try that.
I have a Fender Hot Rod Deluxe. Could I unplug the 1/4" that connects to the loudspeaker and run a line into the interface? I was going to try that, but I was worried about frying something.
Never, ever run a valve amplifier without the correct impedance load (such as the speaker) attached to the speaker output)s). Valve amplifiers, because they have output transformers, require a particular impedance at the speaker output, which is why e.g. Marshalls have different outputs for eight or sixteen ohms.
If a valve amplifier's output transformer isn't correctly loaded down by a speaker or suitable dummy load, the transformer and/or the output stage valves can burn out, and generally take other components with them as well. Valve amps don't tolerate being run without a speaker connected for the same reason. On the other hand, a very brief short circuit on the speaker output might not kill them.
Transistor amps, on the other hand, will sometimes tolerate a lower load than they are designed for or even open circuit with nothing connected, but shorting out the speaker output generally kills them instantly.