[Amps] Failure of a pair of FU728F

[Ron Youvan]

   Guido Tedeschi wrote:

> 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 
> overdrive, but the power was every time much lower than 50W and, anyway, 
> the amp "is protected", so, why it did happens?

> Any idea?

   You did not mention the actual heater power.
-- 
   Ron  KA4INM - Understanding is much better than
                                       knowing how.