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.

To buy an argument. Why even consider tubes in a Pre-Amp, as Solid State devices work so much better in low level applications, lower Noise and Distortion, no Hum and no problems with Microphony. I firmly believe that tubes are superior to Solid State in the actual Amplifier Power Output stage, and also by necessity to get sufficient drive to the output tubes. I have just created a superb Commercial 60 Watt Monitoring Amplifier (KT88) for use in Recording Studios etc. It uses solid state Phase Splitter, and common garden variety 12AU7 tubes for voltage gain in the driver section, and for cathode followers. The voltage gain stage has local "Nested" feedback restricting the 12AU7 to a gain of 8. Tested 12 random 12AU7 tubes of various ages and makes in the voltage gain stage, and found a gain difference of less than 0.1 dB, and the plate voltage to come up within + and - 3 Volts of the designed 160 Volts DC. The Amplifier does have Bias Control for adjustment of Output Tube Cathode Current for each output tube, and AC Drive Balance, mainly to compensate for gain differences in the output tubes which despite matching and identical cathode current settings, still have slight gain differences. The Amplifier trims out to give 0.01% distortion at 1 Watt at 1 KHz and at 10 KHz. Changing driver tubes required a slight, but relatively insignificant "Tweak" to get identical performance from every one of the 12 tested 12AU7 tubes. I firmly believe that any healthy tube, of any make, should give identical performance in a correctly designed amplifier. If it does not, the design of the amplifier is suspect. Honestly, valve Pre-Amps, "Forget it", go for solid state any time.

You can purchase matched some NOS pre-amp tubes from Kevin Deal at www.upscaleaudio.com, or try the new Tesla tubes (best value) from the folks at www.tubestore.com. Leave the theoretical electrical analysis to the amateur physicists. Matched tubes that have been pre-tested for your specific pre-amp will image and balance significantly better. Let your ears be the judge.

If you don't have a ton of cash, using a tube preamp an SS amps is a decent way to get the signature sound of tubes in your system without having to sell appendages or organs to keep up with the cost of replacing power tubes in a tube power amp. Most small preamp tubes are quite cheap. I agree with most of the above posters that matching isn't really as critial in a pre.

poulkirk, i paid less than $2k for a nice used tubed preamp. i can't think of anything solid-state anywhere near the price, even used, that would come close to it. the linn kairn solid-state pre i was using before - a wery nice pre for the price, imho - can't touch my new toobed pre...