2020 Gets Quieter Over Time

[nickrobinson]

My 2020 starts off at normal volume, then progressively gets quieter over 20 mins so I have to go from the normal 4/10 to 10/10 and the sound breaks up a little - any clues? The valves were last changed about 6 years ago but it's only used once a week or so for a few hours. It runs really hot, but then it always has, it's in a 2U rack with an empty space above.....

 

[maxxi]

i'd start off with changing the valves first, and troubleshoot further if the problem stays the same after valve-change