On 8/29/2010 1:30 AM, Φραγκιαδάκης Παναγιώτης wrote:
> hello
>
> what do you mean soft start on filament ??
> linear slowly warm up ?
> step warm up ?
> and if so why is call it instant ?
> regards panos
Hsu and Bill covered it pretty well, but I'll add a couple points.
Although as has been pointed out they might better be called "quick
start", but the instant start comes from just throwing the switch and
going on the air. Normally the filament will be hot enough by the time
you throw the switch and pick up the mike, or grab the key. However as
was also pointed out this can be very hard on the tubes and results in
reduced tube life, often drastically reduced tube life. With the cold
resistance being so low and the current capacity of the filament
transformer being high running in the examples from 41.5 to 75 amps
(when at temperature) the cold inrush current is tremendous and results
in some very strong magnetic fields that can cause those
filaments/cathodes to really jump and possibly contact the grid.
The soft start can be *either* a step start, or a ramp up of the
filament voltage using a triac. The idea in either case is to eliminate
that inrush *shock* to the system along with a substantial increase in
tube life. Some tubes, I understand are particularly sensitive. I've
been told the GS35 is one such tube and it runs no where near the
current of those 4CX5000 tubes
I'm a fan of the "ramp up" but the step start is about as simple as you
can get while the ramp up requires a control circuit with a triac of
considerable heft to handle up to 75 amps with enough overhead so they
will last.
73
Roger (K8RI)
> SV4JYI
>
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