ECC99 has quite high dissipation for such a small tube, and this tresses the limits of what is possible. It is a good idea to test the grid current of this tube.

If grid current is too high, it can theoretically be gas, but with gas you will also see Plate current and Gm be far too low, after little use hours already.

Grid current caused by gas is not very dependant on the plate dissipation, wheras grid current caused by grid damage (over heated tube) tends to go up excessive when you are close to maximum dissipation. Once grid current has occurred, it can come back unexpected. Always distrust such a tube. To find this out, use both systems at maximum dissipation at the same time, to stress the tube to it's limit. This is not good for the tube, so don't do it by default without reason.

Particularly a sharp rise is NOT characteristic for gas, and you can diagnose such a tube with grid current, cauded by grid emission.

Grid emisssion will occur when the tube was accidentally over heated, or used a long time at maximum dissipation, or simply if factory processes were not good. Count with all three options for a JJ tube.

Since JJ datasheets provide only the elementary data, there is no factory specification for grid current.

For doing a grid current measurement properly, use instruction card #4 as we sell it, with order number 380-064-91, and combine this with our ECC99 test card, as this card sets the tester for grid current measurement properly, at 750nA full scale.

Test results we report here from a user from Holland:

Cold test: 10nA per system, for new and used tubes he has. Please note, cold test is NOT the right way, but so be it, this is what he measured.

With a hot test, by instruction card #4, he found 30nA for new and used tubes.

This data is provided here for what it is. These are not specifically values every tube must have. To my opinion the "good" level may be even higher, like perhaps 100nA or so, but in fact I can't say where the "?" or "bad" range really begins.