I'm kind of embarassed to ask this. I've been playing for 20+ years, but I'm just not a gear-head. How do you know when the tube in your amp needs to be replaced? I have a SWR SM900 that was working fine, now when I power it up it's really noisy and distorted for about 15 minutes, then seems to work ok when it's warm. Is the tube the issue, or something else? I don't like my local service center, I don't want to bring it in unless I have to. Thanks.