A bit off topic.
I wanted to check my working/understanding against some known tubes and their operating points, which as a byproduct, deepened my understanding of these interesting devices.
I like and use the 2A3. The 2A3, at classic operating points, generates about 3.5W of power at 5% distortion and roughly 5W before its grid is driven positive, albeit approaching 6% distortion. A decent, if not outstandingly linear small DHT. Its rp and load requirements mean it runs about 2.5Ohm output impedance, about as high as I'd like for full-range duty, even for speakers designed for high source impedance drive. The 2A3 can get better with different operating points and loads, but not near 300B territory, I think. So why do I like it? Maybe it just sounds good despite (because of) the distortion... or the distortion is low enough in use (say <1W). The possibility of using AC filaments is a bonus. I do enjoy my amps that run it - at least with JJ or EML - at admittedly a more linear part of the curves, so I just dunno. Maybe time for something different.
Although I have not done anything extensive, I'm rather impressed with what has been achieved with the venerable 300B. At similar operating points to the 2A3, the 300B produces more power at about 60% of the distortion, with similar gain and lower practical output impedance. Linear. And it scales to comfortably achieve more than twice the output power of a 2A3 while still maintaining linearity. It also is easy to implement, though almost mandates from DC filaments. Sound? Well, that is a personal choice, but I reckon it can be had in the desired flavour - just need to find the right supplier and operating points. I have a newfound respect for this tube: I think it is cool in its competence. Which probably makes me decidedly uncool.
The PX4 was also interesting. Some folks love the sound of this tube. It appears to trounce the 2A3 in linearity... and could be better even than the 300B looking at some of the figures, but seems to me similarly linear as the 300B when working the curves. It has more gain than both, though if you want really low distortion you lose the benefits of the gain in use due to the greater step-down when using the higher (but not prohibitively-high) load. The PX4 also so has an output impedance in the ballpark of the 300B. The EML curves and figures are similar but different to the Osram curves; maybe just the natural variance that can be seen in manufacture. I raise the EML option because I don't entertain using unobtanium NOS tubes. If you have the UX4 socket and filament current covered, and you like their take on sonics, the EMLs could be nice if looking for 2A3-ish power... at around half the distortion and a lower practical output impedance.
I'm curious now about the AD1 - I will take a look at some point. I have not looked at the 50 - a quick eyeballing of the data suggests the output impedance when loaded as recommended (to get decent power) to be too high for me - maybe it would ideal if relieved of bass duties? Nor did I look at the 45 - power is too low for me, I think.
This is pretty obvious stuff to those who have been around, and yeah, I guess I've "known" it through osmosis... but it was real nice to work through the curves and charts to "see" it (I hesitate to say understand it - I really don't!).
And finally, back to the subject of this thread, the EML 12B, which I thought could be used as an output tube. It can, if you are looking for 45-like output power at about 70% greater plate dissipation. The high rp of the tube means that you simply can't have low (enough) output impedance and sufficient power at the same time, assuming you can find a good enough output transformer to provide the load and bandwidth. At about 17W plate dissipation, a little over 2W can be managed at an output impedance somewhere around 2.5Ohms in use, if my calcs are right. On balance, it is not what I’m looking for in an output, even with distortion far less than the others mentioned here. The 12B would be an awesome driver, and could be used in a 2-stage amp with modest 1:2 step-up somewhere in the amp... just as Jac intended.
Okay, enough on this topic. I need to get my posterior into action and finish a design and start a build.