Bizaare experience today!!! There is no question here, I just want to share a really bazaar experience that I desperately working on for like 4 hours today: I am designing a guitar amp with high gain. I did run into sort of " motor boating" due to inadequate filtering of the B+ a day ago and I fixed it. So today I was going to finish up the amp and did final testing.When I did the final test, I can hear a tremolo sound when I hit a few notes. I went through the whole thing looking at the various HV point with a scope, play around with the ground, filter caps and all. I was stumped, I just cannot figure this out. It's kind of warm today and I have a big fan close by blowing towards me. All of a sudden, I remember the spinning blades can give reflection of the sound. Just like if you stand in front of the fan and talk to the fan, you'll sound funny like you have mucus in your throat and has a vibrato. I turn off the fan, everything back to normal!!!! This is a crazy day for me. I thought I post this for a laugh!! BTW, for someone that is into tube amp, here is a little discovery. Do not put a filter cap right after the choke!!! It makes a good resonance circuit. It was giving problem this morning, I give a tremolo sound also, I can see the junction of the choke and the filter cap give me a slow ring when I hit the guitar hard. I was puzzled for a little before I figure out it's the resonance. You need to have a resistor at least 2K to dampen the oscillation. After I did that, then it works. That's actually part of the reason I got fooled by the fan. The symptoms are very similar. I was flat out of idea before I started to think out of the stupid box!!!