It is to be noted that in my previous article on the ECC99 I mentioned a peculiar effect where the tube pulled notable grid current at startup until placed in a circuit with almost zero grid resistance, whereupon the grid current stopped flowing. I have since purchased and used 3 more ECC99s, and did not have this problem whatsoever.
An examination of the circuitry I used would indicate that I possibly had too much grid circuit resistance, which would cause too much voltage to build up on the grid over time if the grid was contaminated with material from the cathode during manufacturing processes. You can read my speculations on this in my previous article. This contamination is not uncommon, due to the grid’s close proximity to the cathode. I can only speculate whether or not my grid circuit resistance was too high, as the JJ datasheet says nothing about how much grid resistance is too much. It would appear that 470K is about as high as you want to go, as I had no problems with that number, and it’s a common grid resistance value.
A final thing I would like to clarify is that when I showed how to “fix” the grid current problem, I suggested plugging the ECC99 into the first stage of the amplifier and turning the volume control down to minimum. This “fix” will, of course, work only for the stage of the ECC99 directly on the input.
Either way, do not be discouraged by this slight oddity I noticed when I began using it, for the ECC99 is an incredible-sounding tube and deserves to be widely recognized.