Hi Carl,
I have read/heard that about filament transformers before. I am not sure
where that idea started but have some doubts about the validity in all cases.
The cold resistance of a filament is close to a short circuit at turn on.
How much current can a filament transformer deliver when the secondary is
shorted? 2X rating? 4X? 10X? Voltage is not important; current is the
culprit here.
You are right about the 3-500Z; it is not known to have open filament
issues. Perhaps the current requirements are low enough to limit destructive
magnetic fields inside the filament during turn on?
Larger tubes definitely have open filament issues due to surge currents;
learned that first hand. This leaves a bit of a dilemma: do you or don't you
pay attention to filament turn on surge current for a given tube?
Those big tubes are expected to be turned on/off for a limited number of
times in their lifetime. We amateurs expect unlimited on/off cycles, a very
different scenario.
There is a lot more to be learned about this.
73,
Gerald K5GW
In a message dated 6/3/2011 6:15:46 A.M. Central Daylight Time,
km1h@jeremy.mv.com writes:
If the filament transformer is sized properly it should provide sufficient
inrush protection at ham tube power levels. The 3-500Z certainly isnt
known
for open filaments nor are any of its tetrode cousins.
I do bring my 250TH's and 304TL's up with a Variac.
Carl
KM1H
----- Original Message -----
From: "Jim Garland" <4cx250b@muohio.edu>
To: <TexasRF@aol.com>; <amps@contesting.com>
Sent: Thursday, June 02, 2011 8:24 PM
Subject: Re: [Amps] Should a blower run after amp shutdown?
>
>
> Some tubes, such as TH327/347, caution in the data sheet to not run the
> filament without air flow. In their case, the filament power is about 200
> watts. If 200 watts is bad for a tube then it seems latent anode heat
> might
> also be ba
>
>
>
> You're right about tube data sheets cautioning the need for airflow with
> only the filaments. But that's because the filaments disspate enough
heat
> to
> raise the temperature of the tube seals too high without cooling.
However,
> that's a different situation from the latent heat of the anode, since
that
> can't increase once the power is killed to the amp, assuming the
filament
> power is killed simultaneously. Of course, if the filaments remain on
with
> no cooling, then that's asking for trouble!
>
>
>
> On the issue of thermal shock with blower left on, remember every time
you
> switch to standby the same scenario exists. No problem there, so how can
> there be a problem at shut down?
>
>
>
> Good point, Gerald. I think you're right.
>
>
>
> In my view, the largest gain in tube life comes with careful management
of
> filament/heater current at turn on. I keep thinking about how light bulbs
> typically fail at turn on and how that applies to transmitting tubes. One
> tube manufacturers' web site suggests a loss of 60 hours of life during
> every on/off cycle. Yikes!
>
>
>
> I've wondered about the light bulb analogy. Obviously, reducing the
> inrush
> current at turn on is desirable, but I've hardly ever known a
transmitting
> tube to have a burned out filament (excepting really old tubes, like the
> 250TH, or tubes that were gassy). Actually, I did lose the filament on
an
> 8877 years ago, but that was because of a bad weld to the filament
support
> structure. Eimac replaced the tube, even though it was long out of
> warranty.
> More typically, filaments (or cathodes) lose emission, or the grid
becomes
> contaminated, or the tube becomes gassy or shorts internally. That said,
I
> still think filament inrush protection is a good idea, particularly with
> thoriated tungsten filaments.
>
>
>
> 73,
>
> Jim W8ZR
>
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