Matching pre amp tubes?

It depends on what the circuit is and how the tube is being used. Miniature duo triodes ( 12AX7 etc.) have two triodes in one glass envelope. Each section can perform differently: transconductance, noise, and so forth. Power tubes are matched up to provide balanced drive to the primary of the output transformer in power amplifier. Measuring your small signal tubes will require you have a mutual transconductance tube tester, characteristic curve tracer, or some other device to do this. Such test equipment runs into the hundreds of dollars for a good used tube tester and much higher for the curve tracer. George Kaye makes a small signal tube tester that is currently manufactured and allows you to plug ear phones into it and listen for microphonics. Neat! Unless you want to invest in tube test gear you may want to obtain your tubes from someone you trust who can test them for you and provide you with tubes with balanced sections. Under most preamp conditions I'm not sure obsessing over whether your tubes are precisely matched is either necessary or critical.

You don't have to match the tubes in the preamp section because they're bias in class A operation not class AB or class B as in the ouput push pull stage. The matching of the ouput stage tubes help reduce the distortions create by the difference in cathode current from one tube to another.

It is more important to understand the noise characteristics of the tubes. There is noticable differences both in noise and sonic characteristics of various preamp tubes.

I have found that there is a need for matching gain of preamp tubes. I ended up with 6922's and 12au7's that were supposed to be matched and subsequently threw out the L&R balance so much that I was questioning my hearing for a while ... kept thinking my left ear was going deft. Sounds funny till you experience it. Gain is critical for stereo imaging.