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Re: radio polarity
#91529 Sun Sep 19 2004 09:15 PM
Joined: Feb 2004
Posts: 21,019
H
Boltergeist
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


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

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Re: radio polarity
#91530 Mon Sep 20 2004 11:32 PM
Joined: Oct 2001
Posts: 3,459
Extreme Gabster
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.


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A lone amateur built the Ark. A large group of professionals built the Titanic.
Re: radio polarity
#91531 Thu Sep 30 2004 01:14 AM
Joined: Jan 2001
Posts: 198
A
Member
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

Re: radio polarity
#91532 Sat Oct 23 2004 05:11 PM
Joined: May 2001
Posts: 1,524
W
Shop Shark
Will this be on the exam?


1948 3/4-Ton 5-Window Flatbed Chevrolet [sandeace.com]

28 Years of Daily Driving but now on hiatus. With a '61 261, 848 head, Rochester Monojet carb, SM420 4-speed, 4.10 rear, dual reservoir MC, Bendix up front, 235/85R16 tires, 12-volt w/alternator, electric wipers and a modern radio in the glove box.
Re: radio polarity
#91534 Sun Oct 24 2004 03:36 AM
Joined: Dec 2001
Posts: 1,249
W
Shop Shark
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.

Re: radio polarity
#91535 Sun Oct 24 2004 05:17 AM
Joined: Jul 2003
Posts: 586
5
Shop Shark
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.

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