Mark IV problem

[fatmatt3568]

When playing on my Mark IV sometimes there will be this crackling sound when I put it on standby and switch it back it will either start the crackling again or will be gone its only when im not playing anything. And at practice today when I was playing it made a high pitch noise like when you have feedback but i was playing and had the noise gate on , then it just stopped and worked fine for the rest of the day. Do I need a new set of tubes or do I have to have checked out by a tech?
Thanks

 

[t0aj15]

Sounds like output tubes to me. Time for new ones.