Do small preamp tubes 12ax7 need to be matched?

I think "matched pair" for small signal tubes is a misnomer. What, exactly is being matched? Transconductance and current figures are a poor indicator at best as to how any given small signal tube will perform in situ. Anyone with extensive tube testing and utilization experience would agree that sometimes even tubes that read NOS on a calibrated tester can be absolute duds sonically be it noise, microphony or other sonic issues. My strategy is usually to find a half-dozen tubes that all test good, and then listen to all of them to find a good sonic match for a useable pair. Generally, I find that of the half-dozen, at least two will be unacceptable for the given application (which doesn't preclude their use elsewhere in a circuit). Hence, the old saying of 'if it sounds good it is good', applies quite well to small signal tubes. Power tubes, of course, are an entirely different story.

-Richard

I have been told that the small signal tubes do not need to be matched but that the power tubes do unless you can adjust the bias on your amp.

Chuck

Palasr is correct on all points.

I would only add that perhaps the bigger issue is whether to bother with (i.e., pay for) tubes that are "low-noise" in the context of preamps. While it obviously depends upon the preamp, my experience is that low noise is not a necessity for line stages, but that for phono stages, it is very important.

The only small signal tubes I closely match are ones used as long-tailed phase invertors. Most amps don't have any provision to adjust the AC balance of the driver stage. For that I use a VTV small signal tube tester. It also lets you listen to the tube so you can check for noise and microphonics. Very helpful if you need to screen alot of tubes.

I always use matched tubes. Many amps allow output tubes to be biased individually but I have never seen that for smaller tubes.

Note: Tubes like the 12AU7 are stereo tubes, i.e. they each have a pair of plates, grids and cathodes. I am unfamiliar with the circuitry of the Aesthetix but many of the preamps and phono amps I have built and modified I found some tubes where half of the tube is used for one channel, the other half of the tube for the other channel. So if you are really concerned about matching tubes (like I am), make sure that the tube is matched with itself when a single tube is used as amplification stage for both channels.

I buy from a reputable dealer who always sends me matched tubes at no extra cost.

Regards
Paul