Funny sound from tube setup

Looking for some sage advice on how to find out what tube needs replaced in my tube amp/pre setup. What tubes do you start with?

I checked my bias on the KT 88's and they were spot on, I imagine this means these are ok. One of my other amp tubes has what appears to be a black top instead of a silver top. Never looked that close at the tubes until now so I'm not sure if it just like that. What should I try next?

Far as I know, the best way to find the offending member is to switch tubes between channels in pairs -- and when the sounds moves, that's your culprit. This, of course, is assuming that there's some audible sounds coming from a single tube in a given channel. If you're just wondering, generally, when to replace tubes -- there's no real answer or bright line to that one that I know of. Over time, the sound can degrade, and eventually go sour all together. Generally, it's a gradual process and one you might not even notice (like boiling a frog slowly, won't even notice until it's too late). If that's what you're wondering about, then the best bet is to suck it up and get a full replacement. Replace them, and if it sounds a lot better, it was time. If it doesn't, the put the old ones back in an rest assured that you'll need the replacements eventually....

The sliver v. black on the top of the tube is perhaps an indication of where to start, but not necessarily the answer. The silver is the "getter." (Layman alert, by the way). It's an oxygen-absorbing substance that they put in there to absorb any oxygen to enters the vaccume -- and when that happens it turns black. That doesn't mean that, if it's black, that your vaccume has been compromised on that tube -- in fact, I understand that's pretty unlikely -- as that can happen when applying the vaccume in the first place. But it's possible, so if you've got a noisy one, perhaps a place to start.

All that said, you sure it's a tube?
Depending on how long its been since you've changed tubes or purchased as a used tube unit...I would do it right and buy a whole new set and keep what you have for back up tubes..Create a log on when you installed the tubes and roughly the hours per week you usually use your system.This will give you a better handle on when the next time comes around you need to figure out if and what tubes you might need to replace..if you get 1500-2000 hours on them you know its time reach in the pockets.................Its just something I do to get a rough idea on when to change tubes...
BUY A TUBE TESTER! They are cheaper than a set of kt88's. Usually your power tubes will last 1000 hours or so. Pre/gain tubes will last 5000 to 10000 hours. So, if you got your tubes all new, the power tubes would wear out first.