Wel this is ironic. I was just getting ready to ask a very similar question.
The other day I turned on my Herbert to let it warm up a bit. 5-10 minutes later I came back and turned it off standby, stepped back and started playing. No sound. I immediately realized what I had done and since I was closer to the cab than the head I plugged the speaker cable back in. It was still working but didn't sound quite right. So I shut it down and used one of my other heads for band practice. Later, I fired it back up and looked at the back to see if the fuse lights were lit up and they were not. And it sounded better. But it still seems it's not quite right. It sounds good sometimes and other times it doesn't. I bought new power tubes to try out but haven't put them in yet. Is there anything else I should check?
Also, I have a 2 probe Weber Bias Rite and 1 bias probe from Eurotubes that I hook into a multimeter. Can i hook all 3 up at the same time?
Thanks, Jusup