The Stovebolt Site Forums The Shop Area The Electrical Bay radio polarity

I scored a rebuilt radio for my 54 GMC at the Petit Jean swap meet. The seller, who claimed to be the rebuilder, told me it would work on positive or negative ground. It makes no difference since it uses a vibrator.
One of you radio guys tell me if he knows what he's talking about. He apparently used negative ground to test it (if he did) because he talked about the wire from positive and the ground to frame.
It is 6 V by the way.

He is correct. It will be fine either way as long as the vibrator has not been replaced with a solid state one.

Oh, it's tempting, but I'm not going to make any kinky comments!

Jerry

HotRodLincoln,
I was searching one day on Ebay for some radio vibrators and it took a lot effort to get the search criteria right!

Well... I was searching the net for an erotic vibrator - ended up with a NOS vibrator for a 50's era Delco Radio.

Not wanting to waste it, I bought a 54 Chevy to restore.

Not sure if the wife would have preferred the former or not.

Don't know about human preferences, but as truckernix said, if you are upgrading to sold state, the polarity matters (it does not matter with the original-style vibrator).

http://www.antiqueautomobileradio.com/vibratorsF.htm

Tim

Cletis,
here is a more lengthy explanation.

The 6V is used in three places in the radio:
1) The vibrator supply
2) the tube heater supply
3) the dial light

So for instance, if you wanted to convert to a 12v operation, you would change the tubes to the 12v version, change the dial light and change the power transformer.

The truck radios did not use the 6v for bias supply. However, in the very early days of radio, they used two and sometimes three different battery voltages and in those early radios, sometimes the bias supply came from the lower voltage batteries.

It's been a LONG time since I did any vacuum-tube work, but the Air Force did spend a lot of time and money training me how to do it! As I remember, the vibrator is used to create a pulsating voltage that can be run through a transformer, and stepped up to a high enough voltage to be used as the B+ voltage on the tubes. It won't matter what polarity the low voltage is, as long as a mechanical vibrator is used. Solid-state oscillators do need correct-polarity voltage, but most of them are 12 volts, I believe.
Jerry

HotRod,
yes, that is how they work. The primary has a center tap which is connected to ground. The vibrator alternately applies 6 v to each side of the winding.

An interesting thought is how they did this before the mechanical vibrator. The early auto radios used a motor generator set to accomplish this. I can't recall the name for that arrangement.

I get the impression that the military provided some pretty good training while on the job.

So the vibrator makes AC for a transformer becauce transformers don't work on DC?

Right! it's actually a pulsating DC, almost a square wave, that can be stepped up with a transformer like AC. An ignition coil does essentially the same thing, stepping the 6 or 12 volts up to several thousand to fire the spark plugs. The buildup and collapse of the magnetic field causes voltage to be induced in a winding, and the more turns of wire in the coil, the higher the voltage will be.
Jerry

This is, oddly enough, still how many power supplies operate, but using electronic switches instead of a coil and a contactor.

Vacuum tube circuits use very high bias voltages (denoted by B+, usually at least 90 volts) to set up an electric field at the plate, which is controlled by a slightly negative voltage at the grid. The cathode in most radio tubes is a separate element that is more negative than the heater filament to keep noise from the electrical system from coupling into the radio by forming a reverse biased diode between the cathode and the filament. Some older types of tubes have directly heated cathodes that are not isolated. Since the filaments consume the most power in a tube circuit they are usually run directly off the battery (or electrical system) of the vehicle, and will pick up noise from the charging system and ignition system.

My old Fender guitar amp has a switch on the back that lets me flip the polarity of the heaters to select the less noisy mode.

If the radio was designed well it should work equally well with either polarity with the correct vibrator, but if it is not a good design it will be much noisier if run in reverse polarity. I don't have access to the schematic of your radio, so I couldn't tell you.

I've designed a couple of high gain tube circuits in my life and it can be pretty tough to make them quiet.

with a little work you can change the tubes to a series circuit, using two tubes in series across 12v works of find a 12v tube with the same specs as the original like a 6au6 becomes a 12au6

Will this be on the exam?

Actually I do think it makes a difference on polarity of connections. If radio were insulated from the vehicle ground( metal of the dash), then yes it could be used with either polarity. But since the radio bolts in to the metal dash it does have to be correct for the vehicle's polarity. '54 Chevy truck has a negative 6 volt ground.

I sent my radio to a shop to get it working and
the repairman told me it didn't make any difference how my truck was grounded. It works
with neg. or pos. ground.