I ave a 6505 head and I replaced the tubes last year but I swear I am having issues with it. After about 3 min of playing, my amp suddenly get louder. Yesterday, it did the opposite. It suddenly got a little quieter. Also it used to hiss and pop a little.

What tubes should i use to replace them with? I thought I used some Ruby tube or something last time. Can anyone recommend a good brand? Even which ones so I get the right ones!

second question: I aways put the amp on standby after I am done and let the little green light go out before I power off the amp. Others have told me to just power it off at once. What is the proper way?