[Amps] Failure of a pair of FU728F

[Roger (K8RI)]

That was an expensive two weeks:-D
A normal Tetrode tune up for a manual tune amp (know the approximate 
tune and load settings)before starting
start with just enough drive to be able to peak the output. Increase the 
drive in small, but increasing steps. This avoids excessive grid 
current. Screen current will soon go normal. Beyond this point peak the 
screen (G2) which is a more sensitive indicator than Anode current. IOW, 
tune it just like a triode until the screen current is in the proper 
direction. Tuned like that there should be no worry abt excessive grid 
current.

With the amp being auto tune the tune and load should be a non issue.

73, Roger (K8RI)

On 9/23/2019 9:49 AM, dj7ww at t-online.de wrote:
> 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
> owner.
> Since rewiring and swapping the tubes the amps runs flawless.
>
> 73
> Peter
>    
>
> -----Original Message-----
> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Roger (K8RI)
> Sent: Montag, 23. September 2019 10:55
> To: amps at contesting.com
> Subject: Re: [Amps] Failure of a pair of FU728F
>
> 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)