The Gain pot is A50K, that's an audio taper.
This isn't a readily available part from places that normally stock Ampeg parts. Try contacting Invalid Link Removed
Ampeg customer service or an authorized service center may be able to help. With the recent sale of the company, ordering parts that aren't in stock from Loud may be difficult until the transition to Yamaha is complete.
You should remove the pot, grab some images. Look for a manufacturer's part number, then look for an equivalent.
Unless the pot is damaged, there are things that you can do that would avoid having to replace it. Sometimes the solder joints on the circuit board break and need to be reflowed. If the pot is scratchy sounding when you rotate the knob, the first thing to try is some Deoxit D5.
