[Amps] soft start on filament

[Bill, W6WRT]

ORIGINAL MESSAGE:

On Sun, 29 Aug 2010 08:30:04 +0300, ???????????? ??????????
<panos2 at otenet.gr> wrote:

>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

REPLY:

All tube filaments or heaters have much lower resistance when they are
cold compared to when they are hot. If you apply the full voltage when
cold, the current will be very large at first and may possibly cause
damage, even filament bunout. 

The "soft start" circuit applies a reduced voltage at first for a few
seconds, and when the filament has partly warmed up, applies the full
voltage. The stress on the tube is much less.

Some tubes such as the 3-500Z are called "instant warmup" but that is
not strictly true. "Fast warmup" would be more accurate. They warm up
much faster than cathode-type tubes but they are not truly "instant".
They take about one or two seconds, compared to 90-180 seconds or more
for most cathode-type tubes. 

73, Bill W6WRT