Please educate me on tubes

Since I know nothing about tubes, I have a few questions:

Vintage, re-issuem NOS, modern production (or if there are other types)? What do these types mean?

Is it safe to buy used? If so, what do you look for?

Should I buy as sets (pairs, quads, matched)?

Where do you get the most, least or doesn’t make a difference? Input, rectifier, output, etc.?

This would take a few pages to answer properly. You seem to be starting at the very beginning!

Q1: There are modern production tubes from different sources in Asia or Eastern Europe. There are also New Old Stock tubes, the best of these come from Western Europe and the States. It is somewhat gear-dependent, and somewhat dependent on your own taste, but to me there are NOS tubes which are much, much better than modern production ones, and others which are nothing special. Tube experts will talk about production dates and codes, year of manufacture, and factory as well as make.

Q2: Used premium NOS tubes can be a big bargain if bought from an experienced hobbyist or a reputable reseller.

Q3: Depends on whether your gear uses matched sets.

Q4: In my experience, small-signal tubes (phono stages and preamps) and driver tubes (amps) are the ones to upgrade first.

How do you find out what's what? Google, Audio Asylum and the archives here. Start your search with the tube types you need for your gear.
Here is some info to get you started.

From my experience and others early 60s NOS tubes are the best sounding tubes and the most expensive. I have bought tubes from members here on Agon and off of ebay.
The tubes I have bought from members here on Agon most were new NOS tubes. I did buy some used NOS tubes from Sam here on Agon about a year ago and they still test strong.

Most of the tubes I have bought off of ebay were used. Of about six pairs, half have gone bad, mostly due to grid emissions.

Buying tubes on ebay is about like shooting craps, throw the dice and take a chance.
Two pairs that I just bought recently only lasted about 50 hours. One pair of early 60s Siemens E88cc tubes, and a early 60s pair of Amperex PQ 6922 USA tubes.

About a month ago I won a bid on ebay for a pair of 60s Siemens CCa tubes from a gentleman in Germany. In his ad he said both tubes tested strong. He even gave the test results. When I received them the first thing I did was to test them. Both tubes failed on shorts and grid emissions. I filed a claim with paypal, now the waiting game.

If you are going to buy new or used NOS I would highly recommend you buy a mutual conductance tube tester.

As for new production tubes I have had good results with EH, Electro Harmonix, tubes. I run them in my Audio Research VT50 power amp.
Even new tubes you have to be careful who you buy them from.
The last set of EH 6922 tubes I bought from Jim McShane, great guy to deal with.

One of the biggest mistake I made early in the tube game was on a pair of new NOS Siemens CCa 6922 tubes. I bought them here on Agon. The ad said they were from the 60s, turns out they were from the 70s. Learn what to look for before you buy.

Here is a great picture of the early 60s and later 60s Siemens CCa tube.

Great points, Jim, and sorry (though not totally surprised) to hear about your Ebay tube buying woes. I recently was looking at some ECC803S telefunkens also from a guy in Germany, but decided the risks were just too high, esp at those prices!

Anyway, to reiterate, Audiogon (vs Ebay) this is a generally safer forum to buy tubes, IMHO.
There are some trustworthy private tube dealers on Audiogon, but eBay is more risky. Look at the seller's feedback. The crooks get weeded out of Audiogon, and regularly change user ID's to regain anonymity. Also, I've found that good retailers like Upscale Audio, Jim McShane, SND, Vintage Tube Services, etc., have good tubes often at lower prices than Audiogon or eBay. Good luck. If you make a few mistakes, it's just part of the learning process.