[R-390] OT: HP-410B Tubes - The Saga Continues...

[Barry]

I did a little more testing on the 12AU7A's tonight.  In the tube tester, I ran the filament voltage down to approximately where the first amplifier is running them.  They're running about 3.8V in parallel so I set the filament switch on the tester to the nearest value - 7.5V.  In that position, the 12AU7A's really went off the deep end transconductance-wise.  Both tubes showed a significant reduction in transconductance on triode "1" and on triode "2", both tubes showed extreme reduction in transconductance.

The same experiment with the 5814's showed some reduction in performance but not much and the reduction in both triodes (at least on the one tube I tested) tracked together.

I then tried running one 5814 in V2 (first amp where the filament voltage is reduced) and a 12AU7A in V3 (second amp).  This actually worked okay.  It took just a little while longer for the meter to stabilize, but once the zero point stayed very close when switching between ranges and modes.

I tried running the 12AU7A's in both positions and jumpering out filament dropping resistor just to see what would happen and the results weren't pretty.  I know they reduced the filament voltage for reasons I don't quite understand and apparently it is quite necessary (although I did not try this with the 5814's).

Oh, and, neither 12AU7A showed any significant signs of gas on the tester (no surprise there).

Anyway, I thought some of you might find this interesting...

Thanks,
Barry - N4BUQ