Preamp Tubes

I make a point of testing all of the tubes I install and sell, and as a consequence there are some that don’t quite meet my standards. By design, a 12AX7 should produce a voltage gain of 100, and a transconductance of 3200 (for both triodes) when operated at certain conditions (anode at 250 VDC, grid at -2.0 VDC, and anode current of 1.2 mA).

New commercial tube testers report gain and transconductance referenced to the above standard conditions, which means anyone can plug a tube into a tester and get a number…which may or not be accurate! All tube testers have certain assumptions cooked into the design, and knowing how to compensate (and calibrate) for different test conditions is a critical aspect of generating good data.

With or without good data, I suspect everyone who tests tubes quickly realizes that many new 12AX7s do not perform to design! This raises two questions:

1) What is acceptable performance for current production tubes?
2) What happens to the tubes which do not meet these standards?

Some tube designs are almost as likely to test high as low, but high voltage gain shouldn’t be the only goal. Also keep in mind the transconductance will decrease with time as the tube ages. In practice this means a worn out tube will deliver less current than a new one.

One of the reasons I default to electro-harmonix as my tube supplier is they publish specs, which can’t be said of most companies. For example, several of their 12AX7 tubes list a minimum gain of 82 or 83, and no maximum. For a tube with a design gain of 100, that is a wide range of values which fall into spec, but at least it is something.

I stock several varieties of 12AX7s in order to try and use the best preamp tube for a given application. In terms of the standard offerings from electro-harmonix, I broadly categorize them as follows:

EHX 12AX7EH – Good all purpose preamp tube. Output more often slightly low than slightly high, but still a great performer.
Mullard CV4004 – More likely to overperform compared to the EHX, not uncommon to run up to 110% of spec, and non-microphonic.
Sovtek 12AX7WA – Reliable with elevated cathode voltages (cathode followers) but slight lower performance – assume 90% nominal.
Sovtek 12AX7LPS – Most consistent based on my testing, and one of the best 12AX7s. Long plates may be microphonic in some amps, but this tube is good for more than just the PI.

With these things in mind, this is how I grade them:

For preamp tubes I use +/-10% of design transconductance as my standard limit as I am skeptical that more accuracy than that is needed, or practical. That’s my answer to #1 above.

When a tube fall below this value but still meets published specs, I categorize them as B-team. That’s my answer to #2.

They work, but they are not as strong as the best. I sell them at a 20% discount because they are too expensive to throw out, and they are still useful. There may or may not be a perceptible difference when installed in an amp, but at least it establishes a standard.