Some members that use GM70 had informed around 2K hours
Hi Erik! 2000 Hrs should be within reach, if the tube was stored carefully.
The data sheet gives good starting information.
The first line « долговeчности / Dolgovechnost » means Lifetime, and the data is MINIMUM 1000 hrs, according to Technical [test regime] Specification CA3.310.003 TU1.
Second Line «критиeрии долговeчности / Kritieriy Dolgovechnosti» means end-of-life is judged to be when the cathode emission is down to 0.6A
Third Line «гарантии Cpок хранeния ... условиякх »
- says that the storage time guarantee is three years.
The fact the the GM-70 is a thoriated tungsten filament offers a good opportunity to extend the lifetime for audio usage:
The data sheet allows ±5% of nominal voltage for the filament, but if instead you can stabilise the voltage to -5%, you should be able increase the lifetime according to the guidance of the EIMAC graphic in post #7.
And further, given that audio should not need anything near to 0.6A of peak emission, it may be possible to reduce the voltage even further on the GM-70. If it sounds as good as at 19V, it should work fine.
As the tube wears out, or does not sound right, you can readjust the voltage upward, to draw the last emission capability out of the filament.
If you try this, please report back with any results!