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.

That's about right for the power amp tubes to be showing their age. I would go to and select a set. Read the descriptions, they seem quite accurate, but bear in mind there is just not much difference even from the very best to the very worst. There is however a pretty big difference from old worn out to brand new!

Don't try and do one or two, replace them as a set. Do the power tubes first as that is almost certainly what's going on and even in the unlikely event its not after 8 years you'll want to be ready with a set anyway. While you're at it, or later, you could do the pre amp tubes. They're cheaper and last a lot longer, but again better to have a set and not need them than to need them and not have em.
Oh, how do you know? No two are quite the same. One can suddenly burn out in a blaze of glory. Or die when you're not looking. If they don't do that, old worn tubes tend to lose dynamics and slam and sparkle. But it can be so slow and gradual its hard to notice. Or fairly sudden. Sometimes they will start making a kind of static noise like fabric pulled across a microphone. fffffttt.  fffffft.  Then nothing. By that I don't mean nothing it dies I mean nothing the noise stops. Its intermittent. Which can happen with any tube at any time even brand new. If it starts happening more with tubes that weren't like that before that would get me thinking maybe they're getting a little long in the tooth.

That's why I like to keep a spare set on hand. Any time I get to wondering in goes the new set. If they sound about the same that means the old ones are fine, and the new ones go back in the box for later.
I agree that the best approach is to keep a new, or tested NOS, set of tubes to put into the amp for comparison purposes.  At some point, the tubes have to be changed, so it makes sense to buy the set now.  You could get "lucky" if the set you buy now goes way up in price by the time a new set is needed.

By the way, tubes are never "done."  They can always be sold on ebay "as is;" if they you have a set of totally dead tubes, you can sell them as a "matched pair."
Tubes are finished when the silver getter flash turns brown. I ran a set of four 7591's in a Heathkit AA100 integrated amp for years until this happened. Background noise increased under the music. I ended up discarding that set of output tubes and bought a brand new set of EH 7591's. Pristine clarity restored!
Having a good tube tester on hand is recommended! That way cathode emissions can be checked before the getter flash goes increasingly brown. When emissions fall below 50% that is time to replace tubes!
Speakers were the Quad 57's. The AA100 was purchased from the original owner/builder (who built it in 1963!). So the 7591 output tubes already had plenty of use when I got it ($25!). He even gave me the construction manual! After I put some more use on it, the silver getter flashes turned increasingly browner - and noise increased, too!
tubes are towards or end of life when dynamics are gone. Preamp tubes may produce funny noises or extra hiss sound, power tubes will lack power to drive efficiently. If in doubt get or borrow a good tube tester and check before rather than sorry later.
roberjerman you are showing your age! Heathkit? Us Dynaco guys use to sneer at the Heathkit guys but you had to be braver to do a Heathkit because you had to build the boards, Dynacos you just had to do the point to point wiring. Bravo! I wish manufacturers would offer kit versions of their stuff again. Anyway, I always keep a spare set of tubes on hand. If you think something is off you can always swap to make sure. Actually, I always keep a spare set of everything around which I think is a neurosis. I would have a spare wife but I'd get shot. Think of what would happen if I got Atma-sphere MA-3s. I would be buried in tubes then get shot.
Steamboy with less gain in both channels I would suspect a power supply problem. Check your voltages. Great that you built your own stuff!
Change all tubes asap!!

Thanks everybody for the help.

Now I have to find a tube store!

And check the power supply!

I didn't make these myself. My landlord, David Yee, built these from scratch. He gave me several amps and pre amps to try and I picked the ones I liked best.

I can find no trace of him now and I wonder if he's back in Hong Kong.

He found Vancouver "too slow"! He called himself "DavidYeeAudio"

all posters gave good advice. IMO, the best was to keep a replacement set on hand at all times. If your tubes sound "tired" the new set should easily allow you to hear a difference.
A tube amp owner is often a tube geek to some degree, and "replacement" sets become laughable as one likely has a drawer full of the things. I could go another 10 years on my current stash. I recently broke a preamp tube (and had the power amp driver tube start showing its age) which launched me into a 6SN7GTB maelstrom of tube option messing about (mostly NOS various things moved around until the mojo returned)...hard to pinpoint when I got it all dialed in other than to say it all got right somehow eventually...changing out a rectifier (!) tube was surprisingly helpful, as I hadn’t noticed it needed changing, but there ya know when tubes need changing when while listening your brow becomes furrowed instead of not so much...
I have replacement sets of Western Electric 348a tubes (my stereo amp runs four of these at a time).  But, after many years of operation, two tubes started to go weak.  Although I have replacements, these tubes are so expensive, that I have, for now, replaced them with much cheaper "equivalent" tubes.  The disparity in price is crazy--the last time I could even find a 348a offered for sale, it was being sold at $1,500 each for old, but testing "good" tubes.  The 6J7 equivalent sells for around $7.50.  Now THAT is a big difference.
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Usually a browning of the getter flash indicates a weak tube, but there are a few types in which this is not the case and the tubes can still be viable. A black thick getter flash is fine. I think it was GE 6L6GCs that had a browning tendency that turned out to not indicate problems.
You sneak. Is the Yee a one-off custom or regular product? Either way kinda cool.
I have a Croft pre and power amp. I had the ffffff noise from one speaker, dropped the amps off for Glenn to service, the tube had reached the end of their lives, had them replaced. Sounds amazing now. However there was no loss of volume Glenn described it as a noisy valve! 
Thanx Veridian! My first ST-70 cost $79 in kit form. Can't remember what the built price was. Those M-125 amps look very interesting maybe for a small system up stairs. Let's see...Rogers LS3-5As. TD 124 with an SME on it. A PAS 3X. Just like old times:)
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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
When the timer goes off your tubes are done.
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.  ;> )
Thanks Pixelreffic,
I will trundle off to my local repair shop.
And get the rest of the amp tested as well.
I am not a tube geek. I know nothing.