losing volume intermittently for all 3 channels.... HELP

[cchsiang2008]

Which tube I need to replace for losing volume Intermittently for all three channels?
The volume disappears for a half second and then on for a few seconds, continuously.
This problem happened few weeks ago, disappeared, and now it happens again.
Could someone recommend website for purchasing replacement tube?
I like the tubes that came with Mark V, so which set of tubes to purchase?
When do I need to replace the whole set?
Thanks in advance...

 

[Chester]

I had this happen once, through my Matchless. Rolled V1 and power tubes as first thought, but turned out the rectifier tube was starting to fail. Matchless also suggested I change the PI

Did both, cured problem

If you are based in Europe, Watford Valves or Hot Rox are excellent for Mesa valve replacements. Others will guide you if you are in the US ..