[Amps] Failure of a pair of FU728F

[Guido Tedeschi]

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