The CK1916 is equivalent to an NL841 and is indeed a nice tube - it has a
left hand DP - the NL840 differers only in having no DP.
This tubes should (probably) work fine - there is (probably) something else
wrong... Have you checked you've still got HT at about 180V?
Nick
On Monday, 9 April
It's a well known problem with the 1916's.
I have about 500 NIB, and most of them show that problem.
Most need a high firing voltage (200 V and up) and some are completely dead.
The only thing I can imagine is gas leakage.
I have experienced that problem on some other American made tubes, but
Thank you all for your replies! Dieter, thank you both for providing the
original datasheet, and for your suggestion - increasing the voltage seems
to have done the trick. Jonas's kit suggests using 160v which has worked
fine for all the Russian tubes (rated at 170v), but at 190v I seem to get
