Many thanks to everybody for all the answers.
The amp is the OM4000A, so the half power is not the 200-300W which
could damage the tubes.
About the TX spikes, I don't know if the FTDX-5000MP which is set to
15-20W and limited by menu to 50W could have power spikes problem, but
other amps will suffer power spikes problem going to error state without
any damage and this should be a behavior of an amp of that class.
He will measure the heater voltage and he will choose the correct
transformer tap, but, again, an amp of that class should protect itself
going to Fault state if the heater voltage is too low or too high.
Measuring the grid voltage, the voltage goes from 0 to -123V in 40-50"
after power up, then, 1' after power up the voltage rises suddenly to
0V, probably when the temperature is enough to have the short between G1
and cathode.
Without knowing why the failure happened, my friend rightly fears that,
after spending 800€ for a new pair of tubes, they will be broken again
after short time...
Thanks and 73
Guido, ik2bcp
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