I had occasion to measure transconductance as a function of
voltage and current for 6922 (Amperex, New) and 417A (W.E., New).
For those interested, here's the data:
6922 transconductance (mS)
Ip(mA) Vp=40 Vp=80 Vp=120
2 4.8 3.9 3.5
4 7.7 6.3 5.5
6 9.8 8.2 7.1
8 10.9 9.9 8.6
10 10.6 11.3 10.0
12 12.3 11.1
14 13.1 12.1
16 13.8 12.9
18 14.3 13.6
20 14.7 14.3
417A transconductance (mS)
Ip(mA) Vp=40 Vp=80 Vp=120
2 7.6 6.5 5.3
4 12.5 10.3 9.0
6 16.0 13.6 12.1
8 17.3 16.8 15.0
10 17.1 19.2 17.2
12 20.8 19.3
14 21.8 21.0
16 22.6 22.2
18 23.3 23.1
20 23.8 23.8
22 24.2 24.4
24 24.4 25.0
26 24.3 25.6
28 26.0
30 26.5
The relevance of these data is noise performance
when these devices are used in preamp stages
(particularly phono). In particular, note that the
6922 (6DJ8) needs to run at least 10 mA in order to
produce at least 10 mS, which is "pushing" the tube
a little. The 417 can also produce good gm at 10 mA,
which is not pushing that tube quite as hard, relatively
speaking.
Regards,
Steve