My PRRI developed a hum in the reverb. Reverb works, but the higher I go on the pot the louder it gets. Sounds like an open ground in the circuit. I checked the tank, the cables, the tank position and V2. Everything seems fine externally. So, I opened it up this morning and took the meter to it. Seems the right footswitch jack (as pictured from the inside) might be the culprit. I get continuity all the way from the left (red) through the orange cap looking resistor to the left leg of the right resistor. No continuity through the resistor on the right. I guess that would the open ground. My question is, what kind of resistor is this? It's labeled Y5P, 221K 500V. By chance could I put a regular 220K resistor there? If not, where the hell could I get on like this?