Then there is the 6GM5 which is essentially a 7868 in a "fat bottle" 9 pin miniature. These aren't very common, and as far as I know, there are no modern production equivalents.
I've been told, though I've never actually tried it, that the 6GT5 (and 12GT5 & 17GT5) is a rough equivalent to 7868, same pinout, just somewhat different ratings, especially the G2 voltage rating. Supposedly they will work as a drop-in substitute in a "pinch". The usual caveats, etc apply for trying this experiment YMMV, etc... However they do make decent triode strapped pseudo-triodes, as dpo most similar TV sweep tubes. As mentioned elsewhere, these horizontal sweep tubes are "insanely" linear.
I'm currently ginning up, at least on paper, an SE amp using 12AX7 or 12SL7 mu-stage (modified SRPP), driving a 42KN6 dual BPT (parallel plates) in triode-strapped mode. Should be good for at least 6W out in SE pT mode. An oddball tube with lots of interesting possibilities. One opf the NNETG guys swears by these tubes, in either SE or in PP modes.
I chose 42KN6s rather than 6KN6s because they're less cost, and I have a perfect filament trannie for these, two 40VAC CT 1A secondaries , and two 115VAC primaries. With 120 in, it should be good for 42VAC out. A flea market foundling for a buck,... I'll be running the 42KN6s at 75-80 mA, B+ 370VDC, and fixed bias (-80 to -110 V grid bias). Plate load 3 to 4K into 8 ohms. The filament demand is 18.9 watts, the 6KN6 gobbles up 3 amps, and the 42KN6 sips just 450 mA per tube.
A PA774, without a heavy load on the filament windings, should handle the PSU duties rather nicely. However, I'll probably use iron with more "heft" just to play it safe.
6KN6s still get a somewhat "premium" price because these have been used in various RF "linear" amplifiers for ham and CB use. The 42KN6s are moatly ignored...
The critical factor here is to use the dual unit (paralleled BPTs), and not the newer, more readily available single plate units. There is a major sonic difference...