[Amps] soft start on filament

[Roger]

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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