Spare tubes? How many?

Being the anal sort of worrier that I can be, I'm thinking to buy some back up tubes. 4 KT88s, 2 12AX7s and 2 12AT7s in place now. I believe only the power tubes are really prone to fail. Welcome correction if I am wrong in that assumption. Should I buy 1, a pair or a complete Quad set of KT88s. If only one tube fails (assuming no sort of catastrophic fireworks) can you simply replace the single and re-bias? Will that make everything OK? (tubes were purchased from the Tube Store so are matched and have the matching number). Thanks for the input.
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always have a full set ready and a used pair ( that still work ) ..Always keep your last working set ( thats just tired ) ...Accidents can happen!! I keep 1 fresh complete set and 1 tired set...I use EL34 ( quad ) 2 pairs 12at7, 1 pair 12bh7a and 2 12ax7...
Any tube can fail, especially if you don't have a spare.

If you have individual biasing pots for each tube replacing the blown one and rebiasing it works fine, so long as you're not too anal. You don't need to replace the other three. Now if the tubes have been used a lot you can replace the entire quad and have 3 'spares' for future failures. :-)

Seriously, the small tubes die unexpectedly, even though they typically have a long life. At least keep a few new production tubes available to keep you in music until you can get some replacements.
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Of course it's easy to get carried away in this process.

I have one amp which uses a quad of KT88's. So I bought a spare quad. They were a different brand. Tried the spares and liked them MUCH better. Then I bought a new spare set (different brand). Tried them and liked them MUCH better... I think I'm up to 5 quads now hanging around. And of course I've got a set that I truly love in the amp, and lots of sets that will work, but no same-brand replacement as my favorites.

It's a slippery slope if you start getting into tube rolling while buying your spares! And that's without considering all the perils of NOS tubes. Saw a nice set of Genelex Gold Lions going for more than I paid for my amp!

Fair warning!
Thanks all...looks like I need to start budgeting for some backups. (Dfhaleycko - Thanks for the warning. Ain't going down that path! I'm running upgrades over the stock tubes now. Should be able to order same same for all 3 types).
I guess I am an tube hog... :-)
I have four extra pairs; but they are vintage 1940's tubes so worth keeping...
I have always found that 1940-1970 tubes are unequaled by newer tubes. Years ago I began buying good buys on a broad range of miniatures as well as output tubes. I have nearly 1000 tubes. They have been a great investment, even though now I use both a solid state amp and line stage. Tubes that one could buy for $8 in 1970 now sell for hundreds of dollars, some for more than $1000.

It is one of the costs of using tubes that you need to have a reserve stash.

It is great to see that I am not the only one with a sh*t load of extra tubes.

Luckily, I have a Lamm hybrid amp (M2.1), which only uses
one 6922 tube in each monoblock.
But to make up for the fact that I only need two tubes, I have the following pairs of tubes:

The stock Sovteks;
Two pairs of inexpensive new tubes;
Two pairs of medium priced NOS tubes (Orange Globes);
And, one pair of expensive NOS tubes (Pinched waist);

6 pairs of tubes which supposedly will last me a couple of thousand hours per pair.

6 pairs x 2,000 hours = 12,000 hours.

12,000 hours divided by my average of 4 hours per week (which includes an hour of warm up time) for my listening sessions = 3,000 weeks

3,000 weeks divided by 52 weeks a year = 58 years.

My conclusions from my calculations are:
One, I am not listening nearly enough!
Two, I have way too many pairs of tubes!
And Three, apparently my daughter, who will inherit my gear, is set for life too!

PS Anyone know of a cheap (and reliable!) place to buy more NOS tubes?

Kurt, I used to know of several. They are all gone now. Most of the Western Electrics are in Asia now, as they appreciated them before we did.
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Elizabeth, I had the Opera Consonance 211 which give about 16 watts. I was quite impressed with them relative to my Reimyo PAT777 which only has 8 watt output. Unless the 300B amps are PP, I would be concerned about have enough headroom for your music.

I have 93db efficient speakers and found the 8 watts were just not enough. I have no experience with the Dared products. Whichever you buy, 300Bs or 211s, NOS American output tubes will blow away what you will get in the amps, but they are pricy. I bought some of my WE 300Bs when they were $15@, some at $300@, and one pair at $750@. I use neither a 300B or 211 amp now.