How are tubes to be purchased in general?

I have never had a tube power amp. Although I have a tube preamp (AI3) it is relatively unfussy with tubes. If a power amp has a matched quad set of tubes, how do you replace them if only one goes bad? Most are sold in matched pairs. How many do I have to keep around for emergencies? Do all four ever go bad at once, or can I just pop in one or two. I talked to the manufacturer (Rogue) but the rosy picture of tubes lasting years finds me a little bit skeptical. Of course if this question is like an owner of a gas guzzler car wondering if he can afford the gas (tube replacements) please tell me so or to go get a SS amp. It's just that a cheap pair of matched KT88's/6550 go for about $70.00 new, two monoblocks mean 8 tubes so that is $280.00 investment.
You need a auto ranging multimeter to check the bias on the power amp tubes. If your lucky whan a tube starts to go you will hear a little distortion . With the multimeter you can see which tube has a bad bias reading, which cannot be corrected with the bias adjustment. Then you replace that tube and your set.

If the tube blows it may take out a fuse or a safety resister depending on the amps design.If the resister goes you need a trip to the shop to replace it.

I prefer tube sound so I thinks its worth the extra cash, and hands on attention... for the next step closer in Musicality.
I agree with Weiserb. I recently blew a tube(while biasing) and thought a new matched quad was in order. But as stated above one tube, some biasing, and your back in business. I did manage to find another quad of the same tubes(JJ's) so now I have extras for backup. The attention Weiserb mentions to me is very important. Best of luck. (BTW I did have to replace a resistor).
I'm not familiar with the biasing process for Rogue amps but some amps have a bias trimpot PER TUBE (CJ) which makes tube matching less critical. My ARC VT100 MKII amp had just one bias trimpot PER FOUR tubes. In this case tube matching is more important.

You can buy matched pairs or matched quads from most tube dealers or just a single tube. If just one tube blows, I would probably buy a matched pair anyway and just keep the spare handy. Over time, you'll probably own several sets of tubes anyway! I have KT-88's and 6550C's that I like experimenting with. It's part of the fun.

As for expense, in all honesty I have never had a tube blow or wear out on me yet but I've only been playing with my tube gear for about 1 year. Even if you had to replace ALL your tubes in 1 year (not likely) thats a small investment compared to the quality sound you'll enjoy with tubes.

I've tried ARC VT100 MKII, Conrad-Johnson Premier 11a, Classe CA-200, Classe CA-300 (bought and sold used). So far I like the CJ the best. And so far I still like the original 6550C tubes. Have fun!
Agree with the posts above. Nice to measure the voltage (bias) running in your tubes - get a meter. The only time you are going to run into trouble is if your tubes are running hot and some amps change over time. Not like Sand amps where you plug them in and never think again for 10 years. Get to a local club or post on the tube section. Someone will have your amp and will be able to tell you just how to take readings and what they should be. I think you will find the audioasylum site invaluable if you are new to tubes.
Good Listening!
Over the last 7 years I've owned two different all tube setups and I've had it pretty easy when it comes to tubes. I've never had a failure and the tubes were not expensive. I have Rogue equipment now and the amps are auto biased (you don't do anything). My only qualm with tube amps is that they produce a lot of heat in the summer. A small price to pay. Btw, I just purchased two quads of Electro Harmonics 6550 tubes from for $99.90/quad as an upgrade. The original tubes still work fine. I'll test the new tubes in a year and if they're ok then keep on truckin with them.
If you are careful and don't do anything stupid (hooking up speakers to a powered up amp...) then they will last you years. I have a bunch of amps with very old tubes. My EAR amps got 10 years out of the tubes without ever blowing them. After some time, the sound lost some dynamics and it was easier to distort at loud volumes. I tested the tubes, ended up replacing them for very little and expect many more years of use. Try or to get an idea on tube pricing.
By the way, it is best to replace worn output tubes in pairs/quads. You also want to make sure that both channels have the same quality tubes or you will notice a difference. Tube biasing helps but, if you push them hard the older tubes will stand out.
i'll try and help--i dont own tube hifi but ive owned lots of tube guitar amps over 20yrs..
if a output tube goes early on deffo check the bias and change the tubes in a QUAD i wouldnt just change one---if ya cant find a quad buy 2 matched pairs and put one tube from one pack in first then next to that put one in from the other pack soforth---i change my power tubes once a year frequent use if i use the amp more than usuall in a year i change them in 9 months time.......
pre amp tubes last a lot lot lot longer so you probally wont want to change them unless you feel you must(anyways they aint exspensive unlike pwr tubes)
check out a guitar magazine/paper called ''vintage
guitar'' and there are so many ads from places that sell all sorts of tubes in quads pairs ect JJ,SOVTECK,SVETLANA, ECT ---in my own experience i thought the russian tubes sounded a bit better than the chinese tubes like RUBY...
some help maybe..cheers
It is usually best to replace tubes, in the units, they are being used. ie: pairs, or quads. It is a push-pull circuit, so replacing the pair, would be best. Tube matching, in power tubes, is important, because of the nature of push-pull designs. If you replace one, which doesn't match, it will unbalance the circuit.
As mentioned above, if your amp is self biasing or has individual bias pots, you don't have to get matched tubes, though it does not hurt as it may help to assure you that there is nothing whacky in the way the tube measures.

If the amp is cathode biased like the Rogue I prefer to use matched sets. If you have an amp with one bias adjustment for each pair or quad you really should use matched sets.

The issue here is how the tubes's like having four carbs on a car...they should be synchronized. If one tube is a current hog, it will run too hot in relation to the other tubes and fail early.

If you need matched sets, when buying a re-tube...ask the tube seller to give you the measurements to refer to if you lose a tube. That may help. Though most tube sellers matching rigs do not have regulated power supplies, so the batch they test six months later when you call may have a bit of a variance from what they tested for you.

That's why we had to get our test equipment custom-built. Our single tube tester is regulated.

Our new tester was built by Eric Barbour, the former Chief U.S. Engineer fro Svetlana Electron Devices.

The Hewlett Packard Power Supply we use cost more than entire test apparatus that are sold. Together between the two racks it will test/burn 40 tubes and weighs over 200 lbs.

I don't know why the commercially available test rigs don't have regulation. Even the Sophia curve tracer does not last time I read. It would raise the price, but makes for a really usefull tool.