[Amps] Failure of a pair of FU728F

[Roger (K8RI)]

As I have an Emtron manual tune amp I can only comment on the FU728F 
Tetrode.
I would not run a single one of these tubes at 200-330W output, let 
alone a pair.
There are at least 3 versions of the FU728F.  The earlier versions tend 
to have some problems and the later ones say they have a 1200W plate 
dissipation, they really are 1500 watts.  Other tubes have had this 
change on paper as well.
The FU728F is a relatively  inexpensive tube for its power capabilities 
as "were" the Chinese 8877s
Like many Tetrodes, when run much below 75% output they may exhibit a 
negative screen current.  The tubes should be operated in the normal 
screen current and tuned for a peak in screen (G2) current, for best 
efficiency and linearity keeping at less than the maximum ratings .
IOW, the tube should be tuned for a peak in screen current, NOT for max 
output, or max plate current

73., Roger (K8RI)

On 9/15/2019 6:40 PM, Guido Tedeschi wrote:
> Thanks Joe,
>
>    but the manual says that the amp protection is activated from Grid 
> current too high...
>
> 73 de Guido, ik2bcp
>
>
> Il 16/09/2019 00:20, Joe Subich, W4TV ha scritto:
>> On 2019-09-15 4:40 PM, w9bs at w9bs.com wrote:
>>> I was running the OM4000a at 200-300w out (with around 3-4w of
>>> drive), and tuning it as normal to the "carrot" on the tuning
>>> indicator.

-- 
Roger (K8RI)