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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