Hi, apologies if this is not the appropriate forum for this question...I am trying to troubleshoot an issue with a potentiometer between a tube preamp and source input. The issue is that the 20k Ohm pot measures correctly when not in circuit, and the wiper to ground/input measures correctly in its travel. When in circuit the resistance measured approaches just over 5k and then returns to 0 when turned clockwise, manifesting increasing volume to the halfway point, and then attenuation as the pot is turned from 12 o'clock to 5 o'clock. I am trying to resist the urge to just replace the pot until I understand the issue. The source (ipod dock) has a dc resistance of 250 Ohms and the input tube has a grid resistor of 300 Ohms, both seem pretty standard to me. The pot is isolated from chassis and signal ground connects to the preamp at one point only on the pcb. My guess is that this is a bad pot that misbehaves once there is current running through it. But most of my guesses in electronics are wrong. Any help is much appreciated.