[ARC5] Filament Question - in RT-66

[Kenneth G. Gordon]

On 2 Oct 2015 at 23:07, Dennis Monticelli wrote:

> Ordinary incandescent light bulbs fail with repeated on/off due to the high
> current surge at turn-on.  The positive tempco of the cold filament causes the
> surge and the rapid change in dimensions that results shocks the filament into
> failure, usually at a support post.
> 
> I would think that vacuum tube filament failure would be similar.  If so, a
> controlled inrush is the remedy.

In the case of battery tubes, such as those Wayne is involved with, the 
answer is, generally, inrush surge does not occur.

Those are specifically designed for "instant-on" service, their filament voltage 
is very low, either 1.5 or 3.0 V, their current draw is also very low, and current 
limiting is designed in.

IMHO, you folks are talking about high-power, or even "reasonable" power 
output tubes. Battery tubes are a "different animal".

In my experience, the only possible caveat with battery-filamented tube 
circuits would be drift in oscillators from cold to "hot", but since battery tubes 
never really GET hot, that is of very little actual concern.

My advice to Wayne is to not to bother with running them constantly on. 
Doing so would limit their life.

Ken W7EKB