The Stovebolt Site Forums The Shop Area The Electrical Bay The Radio Bench A D Radio 12V to 6V Reducing Resistor

I couldn't stand the suspense any longer, so I went and set up a test with a reducing resistor. I used a 1.35 ohm resistance in series with a 12V supply to a 6V AD radio. I monitored the radio voltage from cold to fully warmed up and it worked perfectly. At cold startup the voltage dips to about 5 V for about 5 sec and then it levels out at about 6.5 V. I can see no problem at all with this approach.

I have read so many negative comments about using a resistor and I really see no reason to fear this method. I would suggest a resistor of about 1.2 ohms and at least a 40 watt rating.

Did you use the typical ceramic resistors you see at auto parts stores or something special? Do you have a part number and a source?
Thanks

I actually used a combination of high wattage resistors to get that combination. I will look up a part number from a mail order place if that would be helpful. I am thinking of purchasing one of the ones from a truck supplier, to try out.

there's a resistance unit available from some of the repro places [Filling Station?] called "VOL-TA-DROP" that does the job - about 3" sq ceramic slab w/ a heavy wire wound resistor unit and 2 different current taps, from 5&6amps to 10&20amps - I've got one [10-20] from long ago that I used to run a nice 6V fan in a 12V truck

Bill

Big Red,
what is that in your photo? Is it an aviation related part?

It is the " Volt-a- Drop " Red 58 refered to.

Box is marked
" Airplane ,Boat, Trailer, Automobile"

looks like a vintage model BigRed! mine doesn't mention Columbia, just the VOL-TA-DROP Corp, VanNuys CA, and came in a more 'modern' lookin box

same thing tho - not sure if it was the filling station I got mine, they don't have a picture now of the one they offer [#FS-129 $30] - Chevs of the 40's has a high tech one [part# 1119005 $76.25]

any other place I've seen has something like an ordinary ceramic resistor or larger coated one that looks wire-wound, and w/ prices around 4-8$, they're not good enuff - wonder if a marine supply might have'em?

Bill

We use the Volta- drops not only on our old truck bike haulers but we use them on our bikes also.

http://shutter01.pictures.aol.com/d...CoNOJu8B3cEpSEaR21OJvD7StAiFa8U40300.jpg

[img]http://shutter01.pictures.aol.com/d...BYkZxJyxPHTJW9R65NQaOwX-cQuHecJS0235.jpg[/img]

The problem with using a resistor is that the radio draws a different amount of current (less I think)until the tube filaments warm up. So to set up a resistor to run the radio at 6 volts normaly results in a higher voltage as it is warming up. This shortens tube life. I may have this back asswards but do know it can be a problem. I had my radio converted to 12 volts by replacing the vibrator, transformer and tubes with 12 volt ones. Not as bad as it sounds as the 12 volt tube radios are plentiful for parts.

Grant,
actually that is backwards and the filaments draw more current before they are warm. This results in less voltage across the radio for maybe ten seconds or so. There is actually less strain on the filaments at startup because of this.

Thanks for the correction. I know I was told it was hard on the tubes filaments. Do you know if that is fact or fiction?

Thanks

Grant,
under normal conditions, 6V filaments with a 6v supply, when the power is applied, there is an immediate surge of current that exceeds the warm operating current. With some tubes you can actually see the light from the heater flare up for a second and then settle down. This is the most stressful moment in the life of a filament. An analogous situation is the incandescent lamp. Light bulbs often "blow" when they are first turned on, because of this affect.

With the 12v supply and suitable resistor, what happens is the initial current surge is much less because of the resistor. The filaments will take slightly longer to warm up, but not by a noticeable amount. This affect is also known as current limiting.

I don't have any scientific data or studies to point to for this application but the analysis is correct and my observations with crude instruments support this explanation. One could hook up a time trace on a scope and record the peak waveform in both cases and I believe the data would tell the story.

As far as the rest of the operating conditions, the resistor solution seems to function quite well.

Fred

Grant,
I have been thinking about my answer for a while and I am going to add to it!

If a 6v radio is run from a 12v supply using a correctly chosen resistor value, the radio will perform normally. If then one of the filaments burns out, the radio will stop working, which is normal. What will happen though is that the voltage to the radio will go up some because less current is passing through the resistor. If the radio was continually left on, it is conceivable that another filament could blow and of course the voltage would go up again. Ultimately, leaving a nonfunctioning radio turned on while powered through a dropping resistor could eventually result in a radio being fed with a voltage close to 12v.

This whole scenario could be avoided by using a solid state regulated supply which delivers the same voltage constantly.