I was having intermittent crackling on the right channel of my 500c. I have just replaced the left channel OPT and have the unit vertical on the bench so I am able to see the output tubes while running the unit. I finally noticed that the crackling was coming from the V11 tube as I could see slight flickering inside the tube. The tube appeared to be just slightly more orangish than the others. I swapped V11 and V10, and there was a little improvement. Checked the pin 5 ground resistors V11. 367mV V10 421 mV V9. 275 V8. 306 Thought I'd swap V10 and V8 to try and balance this mess out a little. Well after that swap there is not crackling from either channel for the last 90 minutes or so. Any idea what's going on here? I am afraid as soon as I put it back into service it will start crackling again.