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Re: [Amps] Failure of a pair of FU728F

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Subject: Re: [Amps] Failure of a pair of FU728F
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Date: Mon, 23 Sep 2019 15:49:17 +0200
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Yes, anode tuning must be for maximum screen current with the anode
capacitor and loading capacitance for maximum output power and screen
current limit.

OM Power is mixing it up in their "how to tune" video with grid current
which is not indicated anyway.

Last year I repaired at two weeks old OM4000A amplifier.
Both tubes had failed.
The only problem I could find was too high heater voltage.
The transformer was wired for the wrong voltage, not for the voltage of the
Since rewiring and swapping the tubes the amps runs flawless.


-----Original Message-----
From: Amps [] On Behalf Of Roger (K8RI)
Sent: Montag, 23. September 2019 10:55
Subject: Re: [Amps] Failure of a pair of FU728F

As I have an Emtron manual tune amp I can only comment on the FU728F 
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, 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)

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