OK, a Princeton only has a Vibrato channel, it's only one channel, not like the bigger amps that have a Vibe channel and a non Vibe channel.
Couple of things you can do to get more distortion out of the amp. Increase the value of the nfb resistor, try 5K. Increase the value of the mid resistor, try 15k to 25k. Experiment, see what works for you.