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Hey you electrical wizzards! I finally tacked down an original 6 volt radio for the 'ol 54. The question is, since I'm running 12 volt on the truck I know that I need a voltage reducer. I got one of them and I went to bench test the radio before putting it in and, shoot! the dang voltage reducer sure got hot!! Is this normal? and if so, how hot does it, or is it supposed to, get? And if this is normal is there some type of insulation that needs to go around the reducer so it doesn't melt anything, (such as a stray wire or something), that comes in contcat with it? Since electricity is one of the world's mystries to me anyone with more knowledge on this then I have would be helpful. Thanks! :confused:

Did you get the ceramic type with the recessed resistor wire?

Yeah it's normal for them to get hot and you'll want to be sure Nothing that can melt or catch on fire comes in contact with it.

Last year I rewired my Uncles 41 Coupe to 12V and he came out with a box containing a full restored radio 6V so I installed it but had to wait to wire it up until I ordered a voltage reducer from Ron Francis. It also dropped the volage to the fuel gauge.

Be sure to check the voltage before connecting the radio. We had a problem of blowing the radio fuse when the headlights were on. It turned out to a surge in current from the alternator. I ended up finding it with my amp meter connected in-line with the wire going to the radio. I cured it by installed a slow blow fuse of the same amperage.

Really nice Original car with the Fulton visor, skirts, over-ride bumpers...almost every option except for the fog lights. Now that he's went for a few rides in my truck he decided to Street Rod it. I already built him a 350, 350 tranny, 66 Chevelle rearend, and have all the Mustang II suspension now so next month the fun begins!

Chevy1,

It is the ceramic type. Thanks!

My ceramic resistors are mounted on a piece of phenolic printed circuit board. Provides something to bolt to and helps keep it away from the other wiring.

By far the best solution is to have your radio converted to 12 volts. It will be much more reliable and will have a longer service life. The ceramic voltage drops have typically a 40 percent error rate which is quite a lot to a radio. Your radio requires about 12 amps when you first turn it on and then draws 8-10 contunious after that. So according to the electrical rules that are in charge here....when you reduce the voltage output by half, the amperage output is also reduced by half. So when you reduce the voltage from 12 volts down to 6 volts the amperage is also being reduced at the same time. So...in this case you will need about 20 amps on the incoming side of your reducer in order to get 10 amps output to power your radio. That explains why your reducer is getting so hot.

Think of this as driving down the street with the emergency brake on...the faster you drive the hotter the brake shoe gets, so in this case you are driving about 70 miles an hour!! In addition the flucation in voltage will eventually ruin your radio, and when that day comes you will be very sad.

By contrast if you had your radio converted to 12 volts it would require just 5 amps from your charging system making it much more efficent and reliable. Best of all you would have the option to add the FM band and nobody would know, it would look the same as it always did.

Just some food for thought....Randy :rolleyes: :rolleyes: :p :p

were chevy trucks ever positive ground?

I hope someone else answers with a complete answer. I am fairly certain that GMC trucks were positive ground prior to 1955. Chevrolet were always negative ground [I changed this later - I mistakenly first said positive, even though I new better - sorry ].

Tim

[ 12-14-2001: Message edited by: tclederman ]

good eye tclederman . I made the same mistake before, assuming that he was talking Chevy! Am I to assume the radio that you have 54GMC is for a GMC? If it is I also believe that it is positive ground. That will make a diiference and may be the reason for the extra heat. Even correctly hooked up there will be significant heat at the resistor. It is my understanding that all GMC's prior to 1955 were positive grounds and Chevrolet was always negative ground. BTW, As far as the amps goes, when you increase the voltage the amps drop equally ie: a motor rated at 6/12 volts and 3/1.5 amps, when you apply 6 volts you have 3 amps. When you convert it to 12 volts you have 1.5 amps. In your case you would also have to calculate the reducer load because of the heat loss in converting. It will be minimal but you will be able to read it with an amp meter. When converting to 12 volts you can also use smaller size wire/harness. If you use the original wire from the 6 volt you will be safe because it will now become oversized which is good as long as the insulation and connections are good.

..........g

Ok, here's the details...Radio is actually a '53 chevy car radio, which is nagative ground. It will fit the '54 GMC, the only difference being that the dial is oval shaped instead of rectangle, but it is basically the same radio. I cheated a little and got one of those repro radio dial face plates, (the little glass piece with the stations on it), for a '54 GMC and cut it to fit the Chevy bezel. The truck was, long ago before I got it, (and half-a**ed) converted to 12 volt negative ground. I will soon be rewiring the whole thing as I don't really trust the tangle of wires I see and it also has a bad ignation switch that shuts the engine off at odd intervals. I even had one of the dash lights, that was missing a bulb short out and there was no fuse on the line so I almost had a fire! Anyway, I'm still playing with the radio ideas. I have a wiper motor to install before the radio can go in so it'll be a while before I get it installed. Thanks to all who replied for the info. Anyone else who might want to add any comments are more then welcome.

The GMC was positive ground, Chevy negative.

Sounds like you're on the right track, and you're not cutting up the dash to put in a radio. If you werent so far away I'd buy you a beer. Randy has a nifty little book on converting these machines, call him and get one. Might help on undoing the p.o.'s mess.

Fifth, some of that is right but you need to go back to school and learn about amps and volts and watts. Amps are a measure of current and what goes in has to come out. Watts are power and volts are pressure. Ohms is resistance. If your 6 volt radio needs 10 amps at 6 volts that is 60 watts (volts * amps=watts, volts=amps*ohms, amps*amps*ohms=watts). If you want to run it off 12 volts you need to cut the voltage in half and keep the amps the same so you will end up with 12 volts * 10 amps across the whole system so you get 120 watts. The voltage reducer has to dissipate 60 watts, so that is why it gets so dang hot. A better idea is to either convert the radio or build a voltage regulator. A voltage regulator can drop the voltage without wasting so much power using the magic of transistors and such.

That is a glaring error for someone who is moderating the electrical forum. Anyway, I have a GMC radio and it never consumed more than 6 amps once warmed up. The best solution, if you don't want to convert to 12 volt, is a DC to DC convertor. Using transistors, it very efficiently converts the 12 volts to 6 volts. I have been searching the surplus catalogs but haven't found one yet. This might be good product idea for someone with an entreprenurial spirit. Another option would be to use a series string of silicon diodes. The forward resistance drop is usually 0.7 volts so you could use 10 diodes in series. You may have to play around with the number of diodes since the voltage drop varies somewhat with temperature and the type of diode but it should be better than a resistor. It should also be mentioned that while the filament voltage of the tubes in auto radios has been reported to have a larger tolerance compared to typical tubes, operating above rated voltage (6.3 volts) will DRASTICALLY reduce tube life. Slightly decreasing voltage will usually increase tube life while only margially reducing transconductance. This is just something to be remembered when using resistors to adapt 6 volt radios to 12 volt systems. There have also been some people spending exhorbitant amounts of money on tubes and vibrators for these radios. Antique Electronic Supply (www.tubesandmore.com) has decent prices on tubes, vibrators, and high voltage capacitors. Fair Radio Sales also has good prices on tubes (you'll have to search for the web address.) Mouser Electronics (www.mouser.com) and Digikey (www.digikey.com) have more standard components (resistors, capacitors, etc.) I have no financial connection with these suppliers, I am just a satisfied customer. There are others, shop around. If you can rebuild engines, you can fix an old radio.

The other option is to make your own voltage reducer out of solid state parts, any electronic supply house should have a LM78H05 which is a 5V 5A three terminal positive regulator (-gnd) a couple of 1N1004 diodes and a couple of capactors and a heat sink. For + gnd use an LM79H05 instead, the 2 diodes connected in series to the ground pin of the regulator will increase the output voltage to 6.4V out @ 5A. the capactors are 1uf 20v one from the input of the regulator to gnd and the other fron the output to gnd.

54GMC - I believe that Carter sells the correct bezel for the 54 GMC radios??? I have a few of the oval '53 Chev radios too and they are an exact fit for GMC minus the corners. How did you manage the speaker?

This may be of interest if you haven't seen it before:
1954 GMC Radio Dealer Advertisement

[ 12-22-2001: Message edited by: OldCarKook ]

OldCarKook, Thanks for the ad, that's great! I looked through Carter's web site and couldn't find that bezel. I'll be searching for one at the Long Beach and Pomona Swap Meets soon.

Thanks!

4onthefloor/J.S. I looked for a DC-DC converter a while back in Digikey and a bunch of other places that I have forgotten about but no luck finding one that ran at 6V with any where near 5-6Amps. I think you are right though, you could design one easy enough and if done right could be made better than 95% efficient. Just use an adjustable voltage regulator that drove a 20KHz switcher with a cap on the output to get rid of the ripple / audio noise. Probably less than $10 in parts and you could make them with enough capacity to run the radio and guages and they would run cool as a cucumber.

Ken, Marlin P. Jones (www.mpja.com) has a 5V/5A DC-DC convertor for $15. Input range is 7-40VDC and the output is 5V, 5A. There is a trim adjust (voltage?), but it doesn't say how much. The part number is :12875-PS. They have a lot of 5 volt supplies at higher current outputs, but none that accecpt 12 volts. They also have some 5V to 12 volt DC-DC convertors. I don't know if they will work at 6-7 volts, but you could use a three terminal linear regulator to drop the 6 to 5 volts and feed that into the convertor. These convertors would be ideal for people wanting to use 12 volt accesories on a 6 volt system and are cheap (less than $10).

Those three terminal linear regulators if you float the ground pin with two diodes in series will increase the voltage by 1.4V giving a nice 6.4V output. LM7800 and LM7900 will put out 1A fine for running the temp guage and fuel and could be mounted right to the back of the cluster. They make the 3 terminal regulators in up to 10A.

And who says working on 50 year old radios ain't rocket science.One of you guys want to design a solid state 6 volt regulator for the generator on my 55 BSA cycle? LOL

Albert, thanks for the tip. I am going to remember that when I need an oddball voltage. You could also use an L7806CV (www.mouser.com) which is the 6 volt version and dispense with the diodes. Still, the problem with linear regulators is that they are no more efficient than a resistor.
Jason

Ok, with all this talk of space shuttle schematics just to get a 6-volt radio to work in a 12-volt truck I think I might have found the ultimate, (AKA simplest), solution. Has anybody ever used one of these 12/6 batteries? Seems like this would be the easiest way to go, just run a hot line to the radio off the 6 volt terminal and wire everything that isn't 6 volt the regular way. (Or is there a catch I'm missing here? Someone let me know if this is too simple or what?)

[ 12-24-2001: Message edited by: 54gmc ]
null 12/6 battery ]12/6 Battery[/URL]

[ 12-24-2001: Message edited by: 54gmc ]

If you don't mind forking over $180 - $220 for a battery . . .

I like the idea of a DC-DC converter . . . some of us deal with this stuff at work and see it applied in other applications. Not really rocket science. Guess it depends on what you spend your time on and where your interests are. I think it is time for an entrepenurial (sp? Mavis, can you look this up for me?) spirit among us to come up with one of these and make stovebolters proud to stay with 6Volt accessories. How bout it Albert? JS? (heck JS, being a newlywed and all without kids . . . you are the one with all the time on your hands!)