Motorola 401A
Needs new vibrator, 4 pin,
Is it + or - ground?

If you are going to use a mechanical vibrator, it won't matter. If you are going to use a solid-state vibrator, you should check to see how the car is wired. Many 6V cars were + ground, but have been converted, often to 12V - ground, and a dropping resistor for the radio.

Brit vehicles are + ground. bass ackwards

Just because a vehicle was manufactured as a + ground doesn't mean that it still is.

Actually most cars were up until the mid 50s. Negative ground was more the exception since electric starters were installed. GM probably had the most cars with negative ground. Ford and most independents were positive ground from the beginning. But as stated, it really did not matter until semiconductors came along.

Some cars reversed themselves. Dead Battery, jump start, get the leads mixed up, start the car and
the generator output polarity follows what the battery is. I know from experience it ain't an urban legend .

So how can I tell if a radio is + or - ground?
I now have two car radio which may or may not get used in the future and would love to know the polarity, The diagram does not seem to say.
Peter

As I, and others have intimated, there is no polarity as long as the tube radio still uses a mechanical vibrator. It is when you introduce solid-state devices (other than rectifiers or detector diodes) that you have to worry about polarity.

the battery should tell. pos side connected to chassis is + ground neg side is - ground. more accurate pos or neg is common not really a ground. hear say was problems with electrolysis from pos ground. never did follow up on that one. phone company uses pos ground so it cant be that bad

Positive ground actually minimizes or eliminates battery terminal corrosion.

Carl

Nit-picker alert!

This statement is true (but not always) if the mechanical vibrator has only 3 or 4 pins. Some exceptions are mid 50’s Mopar signal seeking radios (running the motor driven tuner in reverse will cause damage).

More than 4 pins and it might be using a synchronous type vibrator and polarity does matter. First clue here would be the lack of a rectifier tube as the synchronous vibrator is self-rectifying.

As Meade pointed out, his caveats are duly noted, BUT, as for that 401A, it does not care as long as it physically fits the car it is going in and is 6V. IF THE VIBRATOR HAS BEEN REPLACED with a solid state after market vibrator, then the markings on the vibrator will tell you what polarity the battery must be in order to use it. If the car does not agree with the vibrator, then you will need to purchase a mechanical vibrator or else a solid state vibrator that is the same polarity as the car. The rest of the radio does not care.

Now what is the second radio in question?

The second radio in question I have the original installation manual for and there is no polarity given, It is a Silvertone.
I will get to that one in a couple weeks,.
Thanks all for clearing up my questions.
Peter