Tube life for emission labs tubes. Eml20b example

[MaciekP]

it is a custom AM core. No, I didn't measure the damping factor. I have Audio Note E speakers now, rather easy to drive but in fact I don't need more than 10W of power so 10k transformers would be fine also.

[J. Leven]

you have a PM

[Jac]

Hello Maciek,

20B is a fine tube. Lifetime is a LOT higher than AVVT. That is for sure.

gm is difficult to measure. I have little trust in measurements done with any other equipment then ours. For the box data, we use the AT1000 by Amplitrex. This is not a very good tester, but we use it because many people have it, and it can be bought new. So any other equipment may as well be good, but all gm testing methods have their own specific problems, and differences will result quickly otherwise. L3-3 is good also, but can not do 420V.

Anyway if you repeat the factory measurement, you need to use the same settings as on the box, and that measurement is specified with DC heating.

So adjust the grid voltage until you have 420V 42mA, and then: What is the grid voltage needed for that with DC HEATING. Not AC heating. Compare that with the box data. Then, measure Gm at 420V 42mA. Not any other voltage or current, or any other tester.

When people use electronic heater circuits, I am no friend of such devices. These are a dangerous source for problems.

Also any slow heating up of tubes, under full anode current is extremely BAD for the tubes. The condition with under-heated cathode and full anode current damages the active layer of the cathode quickly. This changes new tubes in old tube very fast. I have seen crazy things with Variacs for the complete amplifier, or slow electronic heat up circuitry, that takes many times longer than normal, trying to do "good" to the tube. In fact that is very bad. Best is put immediately 5Volt DC on the tubes, nothing else.



Jac