[ARC5] [Milsurplus] Cold Filament Inrush Current

[Kenneth G. Gordon]

On 15 Mar 2018 at 15:22, DSP3 wrote:

> One of the factors mentioned in the Eimac power tube book is to 
> address in-rush by ensuring that the filament transformer (especially 
> in thoriated tungsten types) is rated for the actual current of the tubes, 
> and no more.  That way the lower, in-rush resistance it initially 
> mitigated by sag in the transformer.  The opposite scenario is that an 
> overrated transformer will sink whatever current necessary during the 
> cold, heat-up period.  I have seen tubes "bloom" when power is 
> applied where the extra current is available.  Not a good situation...
> Jeep - K3HVG

True.

FYI, according to Rich Measures (SK - Jan 4), the transformers in the Heathkit SB-200 and 
the SB-220 are especially designed to limit inrush current to both the filaments and to the 
HV.

I have watched the tubes in both amps light up from a dead-cold start, and in both, the tubes 
appear, visually, to "ramp up" to full brightness.

Even so, some additional "help" certainly wouldn't hurt.

Ken W7EKB

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