So I've owned this Marshall Haze 40 for the past few years (Disclaimer: I bought it used originally). I recently had brand new tubes put in and biased by a tech (changed the EL34s to KT77s if that helps) Now all of the sudden the amp will fizzle out after a bit of playing 9sometimes 10 minutes, sometimes an hour). Everything will be fine and then it just fizzles out until there is nothing but loud HISSSSS. Earlier today, in fact, when I turned it on and took it off standby for 30 seconds, there was no sound whatsoever! Usually I can just turn it off and turn it back on a minute later and it'll be fine, but I know that's not exactly the best way to solve the problem. Is it possible the tech messed something up? The amp was fully functional even with completely shot tubes and never gave me this problem before. I'm hoping this is a speaker problem since I have a new one coming in anyway. Wish I could get audio/video of it but it's too unpredictable to catch.