[Amps] Failure of a pair of FU728F

[w9oj at indy.rr.com]

A friend of mine ruined the finals in his OM4000 by running the amp at
low power.
Array solutions told him the amp is designed to be run full power or
very close to it.
73 de Mike W9OJ

	-----------------------------------------From: "Ron Youvan" 
To: "amps"
Cc: 
Sent: Sunday September 15 2019 1:36:28PM
Subject: Re: [Amps] Failure of a pair of FU728F

 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.
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