Before answering, an earlier post asked you if the filter caps had been replaced...and that is a critical point. Additionally - are you inside the chassis or just replacing tubes? Primary sources of parasitic oscillation has already been answered by Rob, but you should not open the chassis unless you know exactly how to work safely and competently. Even unplugged the amp can potentially kill you. The question regarding filter caps is a critical one. If they are original you are probably chasing your tail - your voltages could be varying when they shouldn't, which renders all technical answers (as far as "fixing" it) irrelevant, and tube changes will be a waste of time. If you don't know - the amp need to go to a tech. Don't even turn it on again - there's a chance it could be damaged (probably not - but do you want to risk it?). If those caps are original they HAVE to be replaced. If over 10 years old they should be replaced because you have no idea of their condition. Many have date codes and a tech can tell how old they are (not necessarily how long it's been since they were installed, but if they're old they're old - don't take a chance). Once that's done if needed, a tech will have a much easier time tracking down the drop-out problem. Also - the first channel uses a different preamp with a lower gain structure. So the problem could be in the vibrato channel preamp, or it could be related to the amount of gain hitting the driver our output tubes. Any problem like that means an amp should go to a tech, and should not be played. Even if you never play that loud, the problem could be something that will get worse and start to appear at lower volume levels, and/or damage components instantaneously. Get it fixed now rather than take chances.