[Collins] Thoughts on the "New Filament Saver Device for the 30L-1"

[Robert Jefferis]

Greetings all,

Adjustment of 30L-1 filament voltage to accomodate high AC line  
voltage has been on my back-burner list of things to do. So, Mr. Kim's  
article in Vol. 16, No.5 of the Collins Journal piqued my interest. I  
had been planning to use either a power resistor or a combination of  
modern current sense resistors to do the job. I keep a stock of PTFE  
insulated wire on hand, so my first question after reading the article  
was: he [Mr. Kim] says he uses "between 22 to 24 gauge wires cut to  
about 12 to 13 inches...". Thus, I was prompted to do some back of the  
napkin calculations.

If at some reference line voltage, the 811A filament voltage is 7  
VRMS, and we want to reduce it to 6.3 VRMS, we need to add 0.044 Ohms  
to the filament circuit if the tube filaments actually run at 4 Amps  
each. The added wire will dissipate 11.2 Watts. Allowing, oh, about  
one milli Ohm per solder joint ( a SWAG), the wire resistance needs to  
be about 0.042 Ohms.

With a reference temperature of 20 deg C, #24 AWG stranded copper wire  
resistance will range from approximately .0211 to .0261 Ohms per foot,  
depending on the stranding configuration.  Let's use one foot at 0.025  
Ohms/foot. We are short of the required resistance by .042-.025 =  
0.017 Ohms. So, the missing resistance must come from wire heating.  
Predicting temperature rise on paper is darned near impossible. But,  
since we have an answer from the article, we can work backwards to  
estimate what temperature rise must occur. The temperature coefficient  
of copper wire is fairly reliable and I have seen numbers that range  
from 0.00396/deg C to 0.00427/deg C. Using 0.004/deg C, we get a  
temperature coefficient of resistance for the one foot length  of  
0.025 x 0.004 = 1e-4 Ohms/deg C. So, the estimated temperature rise is  
0.017/1e-4 = 170 deg C. The nominal temperature rating of PTFE  
insulation is 260 deg C, so this estimate, although it is probably on  
the high side, is within the working range of the insulation.

If #22 AWG wire is used, the same length would require a temperature  
rise that is beyond the insulation rating and, there would probably  
not be sufficient temperature rise to make the desired voltage drop  
anyway. So, the answer must be #24 AWG (or, I have missed something ?).

So, what's the point of all this? Well, I wonder if it would not be  
better to start with a longer piece of wire of larger gauge so that  
the voltage correction can depend far less on temperature rise (and  
attendant warmup interval) as well  as ambient temperature?  I don't  
think you can design this on paper, it is an empirical, cut and try  
approach. Oh boy, another winter tinkering project?

73, Bob KF6BC