[Phlogiston]

Yes, that is why I was considering using the 3S4 as a replacement for the UX120 output tube. But, the 3S4 really isn't any different spec.-wise than the 3V4 except for the 67V screen. I don't think that there would be a noticeable improvement in audio output. In the actual application of the sub. 3V4 there is plenty of output to cover a UV120. At this point my interest would be in adding a resistor between the plate and screen for the 3S4 and evaluating it as an additional option.