[Milsurplus] [ARC5] Cold Filament Inrush Current

[Ian Wilson]

I did some experiments a few years back with a PC power supply (of
sufficient power rating)
to power the filaments of a 304TH (5V @25A). The current limiting of the
PSU refuses to
deliver enough power to increase the filaments' resistance enough to light
the filaments even
after several repeats.

In the end I used a relay and RC time delay to switch in/out a series
resistance.

73, ian K3IMW

On Thu, Mar 15, 2018 at 1:07 PM, Kenneth G. Gordon <kgordon2006 at frontier.com
> wrote:

> 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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