I jammed this weekend with my old friend guitarist Tom Gage. I brought about half of my rig: a Digitech RP500, iStomp running Swing Shift, and JamMan Stereo looper. I didn’t bring my Peavey KB2 keyboard amp because Tom had a Fender Deluxe Reverb available for my use, and I wanted to hear my rig coming through that amp. I’m glad I did, not because it sounded great, but because I know now that it doesn’t.
It doesn’t sound horrible most of the time either. In fact, when I play basic amped-blues sounds through it, it sounds very good. What makes it sound really horrible is a pitch shifter set to an octave down, which of course is one of my bread and butter sounds. When I use that effect, the speaker just craps out; it puts out noise, not a sound with clear definition in a wide range of frequencies. It gets worse if I use the iStomp to introduce additional parallel frequencies.
A keyboard amp takes an incoming signal and makes it louder without fundamentally altering the nature of the sound. A guitar amp does a lot of things to an incoming signal: it adds distortion, it compresses the signal, it adds some noise, and it makes it louder too. You want the keyboard amp when you’re working with an amp modeler. You set your sound up on the amp modeler exactly the way you like it, and the keyboard amp delivers it to the audience just like that, only louder.
Anyway, let there be no further room for doubt: don’t plug your amp modeler into a rock guitar amp unless there is no alternative. Stick with keyboard amps and PA systems. You’ll be glad you did.