[Amps] Failure of a pair of FU728F

[Guido Tedeschi]

A friend asked to me why the pair of almost new FU728F tubes on its 
commercial PA goes south.

It seems that, when tubes are warm, there is a short between Grid1 and 
Cathode on both tubes and, when the PA PTT is activated,  it reboots 
again with the 3 minutes warmup delay.

So, he asked to me why both tubes were damaged but I was not able to 
answer, because:

- The PA is less than 3 years old and it has been used for less than 300 
hours.

- The amp was used 99% at about half of the maximum power

- The TX power is limited by menu to 50W (FTDX-5000MP)

- Mains voltage was always between 220 and 230V

- Antennas are fine

- The PA is claimed to be protected against almost everything...

Normally, I think that it would happen when the tubes grid is 
overdriven, but the power was everytime much lower than 50W and, anyway, 
the amp "is protected", so, why it did happens?

Any idea?

73 de Guido, ik2bcp