I ordered the Tenma 72-505 for $34 which should be good enough for most basic stuff testing tube amps. Think of it this way: what wave form and frequencies does a guitar produce? You'd be looking for sine waves in mV, start at 150mV 1KHz.
I might buy a fancier generator for things like a Pulse function, which can be helpful for guitar amps. You can google the Hz for any note on a guitar if you needed to. Then you can pulse the frequency to simulate hitting the note if individual notes are causing issues.
I also setup a true isolated transformer and a GFCI outlet for safety.