Filament voltage for 20's radios: the light came on...

[Reece]

I recently had need to power a typical 1920's radio for testing. I eventually will build a power supply for these sets and have plans to do so, but needed power right away. I could steal B+ power from another AC set I built, and C power from a few flashlight cells. But I had trouble finding 5 or 6 volts DC for six 01A tubes. They require 5 volts at 0.25 amps, which would be 1.5 amps total. Back in the day of 6 volt car batteries I would have been set. What I needed to do was drop half the voltage of a 12 volt car battery. The resistor would have to be R = 6/1.5 = 4 ohms and would have to dissipate P = 6 x 1.5 = 9 watts. The closest power resistor I had was 8 ohms at 20 watts. I thought to put some other power resistors in parallel with the 8 ohm to lower the resistance, but none of the others that I had were suitable in either resistance or wattage.

Then I thought of the beleaguered incandescent light bulb. We use them for dim bulb testing on AC set where they act as great shock absorbers, so why not as a simple resistor? They do work as resistors, but aren't so simple. As they heat up, their resistance goes up.

I tried one 100-watt bulb in parallel with my 8-ohm resistor, but that wasn't enough, nor were two. Finally three bulbs worked fine. The resistor got hot, but not too hot, because I gently chucked it in my big bench vise as a heat sink. The bulbs glowed just enough to barely show the filament through the bulb frosting, and they got warm, too. I had several DVM's monitoring voltage everywhere, and was able to adjust the radio's filament rheostat to properly illuminate the tubes and get the radio to work.

[jr_tech]

Is that schematic drawn correctly? Seems like the bulbs a resistors (in parallel) should be in series with the battery and the load.

jr

[bob91343]

It's really a lot of trouble when in fact a simple variable power supply would be best. I have a few variable supplies with meters and adjustments that can be set all the way down to zero and will put out a few amperes. Check a swap meet and you can get one for a low price I'm sure.

[Reece]

Right you are, jr; don't know how I missed that, corrected it. Thanks!

Bob, sure a power supply would be better and I am going to build one, but I needed this immediately one day. Just listing it here as a stop-gap measure to get the job done.

[stusnyder]

6 volt car - tractor batteries are still available at tractor supply, and some auto stores, but are pricey.

[Reece]

This lash-up is the way I lit your Silvertone, Stu!

[stusnyder]

I figured that How's that unknown coming along?

[Reece]

Unknown is still on the bench. I was out of the country for a couple of weeks after I got it. I have several schematics printed out that use the same tubes that I am going to compare it to. That's a pretty crammed chassis.

[HiFiCanada]

We use to use a 12 volt car battery and find the tabs that connect the cells, the center tab we would use a wood screw and turn it in that will give you a nice smooth 6 volts. The older car batteries, had pitch tar like cover, and you can easily see were the tabs are. The new plastic car batteries there is no way to see that, but if you have a old car battery or lawn mower tractor type battery that you have no use for, you can still find the tabs, buried in there. The tabs stagger, and you can follow it by the battery post first.

Train set power supplies would work great, they can deliver a lot of current, at least more then what you will need and it is variable. Some of the good model railroad power supplies are very smooth in DC output.
Maybe something to keep in mind for the future, if you don't have one , you can find them at garage sales or Ebay for sure.

[old_coot88]

Another option might be one of these puppies (the first 3 items shown)..
http://search2.hobby-lobby.com/psear...hp?uid=2&q=bec
BEC= "Battery Eliminator Circuit", used in model aircraft to drop flight battery voltage down to 5V or 6V selectable, nice smooth regulated DC for powering avionics. These are switchmode, which means they don't heat up like the older 'linear' BECs which were resistive droppers.

[HiFiCanada]

Actually I forgot to mention that the power supply for model railroad is transistor pulse control it's been around for about 20 years.

[Gary T. Lane]

Why don't you use a battery charger that has both 6 and 12 volts? I do this all the time when I need 6 volts.

[maxhifi]

if you need five volts, consider that the adaptor used for USB devices will work, if it has enough current capacity. I have a number of 5V 'wall warts' hanging around from discarded modems, hubs, etc

[Reece]

The problem is that these 01A tubes use the filament as cathode, and the "A" supply has to be very smooth. I tried a heavy-duty DC wall wart, and a battery charger, but they had way too much ripple. I hung more electrolytics across the supply but to no avail. My car was right there so I hooked it up as an expedient. If I were to need 6 volts all the time I'd build a supply but I needed it "right then." I remember those old tar-top car batteries where you could hook to any or all cells.