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Re: [Amps] Failure of a pair of FU728F

To: amps <>
Subject: Re: [Amps] Failure of a pair of FU728F
From: Ron Youvan <>
Date: Sun, 15 Sep 2019 13:36:28 -0400
List-post: <>
  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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