Using a common Bias resistor for two SE tubes.
Some others have discovered this glorious idea as well, I was told.
We had an issue with EML Tubes, in an amplifier of which no schematic was handed out by Alan Eaton. So I had to create the schematics from looking at pictures, one of his customers send us. When I had the output stage on paper, I almost fell of my chair. Both 45 tubes are on ONE Cathode bias resistor. Sorry, but this is so dead wrong. This works only, if both tubes bias completely identical. Which is never the case. Moreover, when tubes age, their bias points changes as well. This is normal, and no reason to replace them. Either auto bias will correct this, or your hand-done bias is going to correct this. But with the Alan Eaton, neither of these things can be done. So if one tube changes it's bias point over time, the other must make up for the difference, and OVER-Bias itself. This will damage the tube, and it will loose emission. It hard to predict how this will damage the tubes, but it's pretty sure it will.
Another very difficult issue with one common cathode bias resistor, is the warming up phase. There is no "must be" behavior of a warming up tube. It's perfectly legal, if the one 45 tube needs 3 seconds to become 80% operational, and the other 4 seconds. Then after that, they both meet each other at 100%, sharing the common bias resistor.
But...specialists will know, the wear out of one switch-on cycle, equals the normal use of 2...4 hours. So switch a tube on and off 10 times, and you just added wear out, same as 24 hours continuous. Until some degree it is normal, and some micro-fine bits of emissive layer gets lost at a switch on cycle. That's what it does. However, this effect, this problem, stays only low if the plate current is not exceeded.
With a single cathode bias resistor, the tube which begins to work before the other, will get hit by theoretically twice the intended current. In real life not twice as much, but tubes have zero tolerance against this. So small differences can have bad effects. The effect is not, a good tube suddenly breaks down, but rather strange noises, or hum, and bad sound.
In any case, it voids guarantee on EML tubes.