[ARC5] Cold filament protection

[Bob kb8tq]

Hi

One of the lab experiments we did in High school physics was to vary the filament voltage
on a tube and observe it’s impact on the plate current. Yes, this was a *long* time
ago in a lot of ways. The tubes they gave us were all 6SK7’s or something similar. 
I suspect that was so they would bounce rather than break if we dropped them. 

Turns out, a receiving tube has a fairly broad range of voltages where it works pretty
well. The filament acts as a sort of constant power load over a reasonable voltage
range. I never worried a lot about 6.3 vs 6.4V on the filaments after that lab. We did
more “hands on” with tubes in that lab than any other lab work I any college course. 

You can likely guess which physics teacher also was the one who supervised the 
radio club :)

Bob

> On Mar 15, 2018, at 11:47 PM, Tim <timsamm at gmail.com> wrote:
> 
> I used a 2 watt, 1 Meg resistor as a "coil form" and wound about 5 feet of 30 Ga. magnet wire around it for a resistor (electrically in parallel).  Shrink-wrapped.  Cheap insurance for tube filament protection in my PRC-25.  Works great, no tube problems with my periodic use after 30 years or so.  I don't notice any performance loss in use, although I didn't measure the RF output with/without.
> 
> I use 2 alkaline D Cells in series plus 2 rechargeable 6 volt, 6.5 A-H SLA batteries in series for 12 volts and it all works fine.  Fits in the battery box nicely.  I recharge the SLA batteries via the front panel connector; the D Cells seem to last forever.
> 
> (Required ARC-5 content:  Did a PRC-25 ever fly in an aircraft equipped with an ARC-5 set?) 
> 
> Thanks for your great website Brooke - very valuable! 
> 
> Tim
> N6CC
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
> 
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html