How do you know when tubes are done?


I have a homemade pre-amp and amp - both tubed.Recently, it became necessary to turn the volume  up quite a bit to get the same sound level.It still sounds good but I started to wonder if a tube or two was the culprit. The tubes are about 8 or 9 years old and  get light to medium use.
Suggestions?

steamboy
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to Millercarbon,David doesn't have regular products. Each one is different to some degree. He's always trying to get the best sound. The best sound I heard was from a SET design but bass was weak. He's really more interested in circuit tweaking and very minimalist designs. On/off toggle and volume knob only. He has to sell them to recoup the money spent on his 'hobby'.He might still be on Facebook but I don't use it. Can someone check, please?
I just got a pair of Klipsch RP-600M horn speakers. Very sensitive.Good sound and a good bargain, I would say
I’ve worked on tube gear for years, and judging by physical signs is not reliable. Especially with power tubes. Running a power or integrated amps on weak output or driver tubes risks damage to the amp. Amps with bias adjustments should be checked periodically. A significant change in voltage can be an indication of a tube in decline. For small signal tubes, the only truly reliable way is to use a tube tester. Ideally a Hickok or B&K. These can be expensive, but you can still find old Sencore "Mighty Mite" testers on ebay now and then for around $100 or so.
1947...courtesy of Bell Labs.  1967 courtesy McIntosh Labs when Frank determined his solid state amplifiers sounded better.  ;> )