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

[Dr. Gerald N. Johnson]

On Tue, 2009-11-17 at 14:05 -0800, Robert Jefferis wrote:
> Greetings all,
> 
> Adjustment of 30L-1 filament voltage to accommodate 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

Yes keeping the 811A filaments down to rated voltage is a very good idea
to promote tube longevity. Doing it with overloaded copper wire
resistors that are overheating the wire and its environment is not a
good idea. First there's the problem of overheat and then there's a
consequent failure of the resistor's insulation and damage to other
nearby wiring and the radio. Plus its not energy efficient. 0.7 volts
drop at 16 amps (four tubes) is 11.2 watts to be dissipated. 

I'd suggest the better way to reduce the voltage is with a bucking
transformer on the linear's supply line. The bucking transformer will be
a great deal more energy efficient and add quite a bit less heat to the
environment of the hamshack. If the line voltage is 12 volts high, a
small transformer with a 12 volt secondary rated at 15 amps (for 120
volt input) or 24 volts rated at 7.5 amps (for 240 volt input) is
appropriate. Since the bucking transformer only has to supply the change
in voltage, its power rating is lower than the linear's load. A standard
rating of 250 VA would do well. Something like a Square D model 250SV43F
might be available surplus or new. The standard industrial buck boost
transformers of commerce usually have a split primary and a split
secondary so they can be wired for 120 or 240 volts in and 12 or 24
volts change. They can even be wired to run from 240 volts in and
provide a lowered 120 volts for receivers and a lowered 240 for the
linear at the same time.

As an alternative to the 0.044 ohm resistor consider a 0.01 ohm resistor
for all four tubes such as a Mouser part number 684-MP916-0.01 which
requires a heat sink for its 16 watt rating. Or a 684-MP2060-0.01. Or a
71-RH25-0.01. Or for the individual tubes a 588-14AFR047E, 0.047 ohms at
4 watts. But lets stay from the smoking hot too thin copper wire.
-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer