There is nothing to calculate. Once you determine desired voltage drop, which you do precisely the same way for any biasing method (by reading the number off the chart, given a pre-selected loadline) you pick a suitable LED (Vf of LED = -Vg of tube) and make sure you don't exceed its rated current for operation (which with preamplifier tube such as 6922 you won't).
You can also approach it from the opposite direction. For example, since I know that cheap red LEDs (Vf = 1.7V) tend to have among the lowest dynamic impedances, I might look for bias points that are multiples of 1.7.
I suppose that I have to measure the voltage drop across cathode resistor to know the voltage and current needed & after change for LEDs, that's OK?
That's a pretty low bias voltage- you're edging close to the grid current region. An IR LED will get you close- 1.1-1.2V, but you might want to go higher for the same of linearity and headroom.
Looking at the datasheet, the dynamic impedance seems to be about 25 ohms- that's higher than you'd want. A cheap red LED will give you 5 ohms or less.
In theory, yes, that can help a LED with high impedance. In reality, it's much easier to use a good LED in the first place. With 5R or less, a bypass has no significant effect.
In practicality there is no need, as a diode or LED will bypass all frequencies similar to a very big capacitor.
Any diode or LED can be used but the major factor between choosing them is a forward voltage drop. For example, a typical power diode like the 1N4007 has a drop of about 0.6v, a red LED is 1.5v, Green is 1.9v (estimated) while a zener may be 24v or so.
Choose the bias point in the typical way and match the diode need to the voltage across the cathode you want.
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What happends if a LED connected between cathode and ground breaks down/ fails. Will it then not lead any current and our precious tubes are safe, or will it conduct so that the tube leads current heavily and breaks down?
Never had one fail in a small signal situation. I did fry some during experiments with output stages and they failed open.
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