[Amps] Failure of a pair of FU728F

[Rick Robinson]

How about a key up spike in TX power ?

On 9/15/2019 1:36 PM, Ron Youvan wrote:
>   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.

-- 
Rick, W8ZT
WVDXA


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