(You set the bias of this amp by turning a trim pot until both LED's are of equal brightness, they also pulse in time with the music when it's loud. These LED's are sensitive to AC fluctuations and will go up and down in brightness. I say all this about these LED's so you'll understand why I mention them flickering with my issue)
I instantly turned off the amp and did a quick look and sniff. No burning smell nor anything that looked wrong.
I decided to pull the six tubes, I inspected the socket seats and saw what looked like a tiny bit of carbon deposit on a couple of them. I grabbed a thin wood dowel and some isopropyl alcohol and gave them all a good reaming. Then I retensioned the seats and slapped the tubes back in. I turned on the amp and it is running smooth so far, cross my fingers.
This is the first time I was able to fix (hopefully) this type of crackling and popping with socket cleaning and retensioning. I had a very similar type of crackling and popping happen a different time and it turned out to be a bad driver tube. Anytime I remove and install a tube I re-tension, but never did much in the way of corrosion removal. These sockets are about 1.5 years old and this amp runs several hours a day.
Is what I experienced typical of what happens when tube/tube socket contact is compromised?
Also, what do you use to clean tube sockets?