RM 80 problem Please help me

[duffrouillard]

My rm 80 was performing great until lately. The sound goes from normal to hush quiet then back again. I power it off and it seems to help for like a minute and then back again. I changed the end on the speaker cable and checked the tubes any info will help thank you
duff

 

[bruce egnater]

Does the volume drop overall, both channels at the same time? When it happens, does it sound like the gain/distortion/tone change or just the volume drops? How did you "check the tubes"? Do you use the series effects loop?

Egnater