I have a ampeg SVT 3 With i believe 5 tubes. This last week have noticed a ugly growl on E string from 3rd fret to 7th. Point is its using a lot power to reproduce sound there than g string 12 ft. I had a single preamp tube on last amp . which exhibited same type of problem so I replaced tube and it worked. Not sure if tube problem but besides cables basses ect. Now think time to go tubing. Truth I know nothing!! So I am looking for some simple tube inspection technics. You tube got a guy tapping on his SVT 2 with chopsticks . I got chop sticks. I know supposed to look for discolorations on tubes any thing else I should look for? Thank you.