I've been asked to convert a Mopar 812 from 6 volt positive to 6 volt negative and if really possible to 12 volt negative but no problem if I can't as he has a drop down "device". Any suggestions would be appreciated.
Regards
Rudy

The simplest approach would be to totally insulate the radio chassis from the vehicle's metal, which would be at battery ground potential, add a new "hot" lead (with a 10 amp inline fuse, just in case you don't get the insulation 100% right the first time) from the radio chassis to the ignition switch, and tie the old radio's "hot" lead to vehicle ground.

You will also need to ensure that the antenna shield only grounds at the radio end, and not also at the car fender end. If you have a one-wire connection to a remote speaker, you will need to convert it to two-wire and float it above vehicle ground.

Insulating the radio chassis from the vehicle chassis may be easier said than done. Nylon washers and nylon screws may facilitate the operation.

If the set doesn't use raw battery power to set grid bias voltages (as did some earlier car radios) you might be able to almost use the set "as is", without needing to insulate it. Sorry, I wasn't able to figure this out from the low-res schematic that I found on this set. You might need to replace the electromechanical vibrator with a solid-state circuit, or rewire its socket to preserve the original battery polarity that it would "see". (I believe that the old vibrators were sensitive to polarity, but I am not sure exactly why- possibly the contacts were set up to minimize arcing in the direction of "normal" current flow and prolong vibrator life.)

Converting to 12 volts- same comments as above apply, but I think that I would replace the tubes with their 12V equivalents, then install a dropping resistor in series only with the vibrator input. Most of the current that the set draws is from the tube heaters, so this would reduce the dropping resistor to a manageable power rating.

And, if internal mods are required, paste a large label on the back of the set that says "CONVERTED TO NEGATIVE GROUND OPERATION" for the benefit of some future owner a generation or two removed from the present.

At a glancing look the radio seems to be all tube with an electromechanical vibrator. May not care if pos or neg ground.

Clifton

:And, if internal mods are required, paste a large label on the back of the set that says "CONVERTED TO NEGATIVE GROUND OPERATION" for the benefit of some future owner a generation or two removed from the present.

You only need the dropping resistor for 6 volt operation. the radio can be negative or positive ground. The vibrator power supply make its own internal ground.

:You only need the dropping resistor for 6 volt operation. the radio can be negative or positive ground. The vibrator power supply make its own internal ground.

He should be warned that the dropping resistor will get very warm or even too hot to touch. It should go into the engine compartment, in my opinion. Otherwise it will be like driving with the heater on all summer.

::You only need the dropping resistor for 6 volt operation. the radio can be negative or positive ground. The vibrator power supply make its own internal ground.
:
:
:He should be warned that the dropping resistor will get very warm or even too hot to touch. It should go into the engine compartment, in my opinion. Otherwise it will be like driving with the heater on all summer.

Watch out for electrolytic capacitors in the input circuit of the 6 Volts. Otherwise, the B+ will come off the cathode of the rectifier tube, and the radio really couldn't care less about the polarity of the input current. And, yes, use a BIG resistor to get the six Volts. It's gonna get hot.
Lewis

:::You only need the dropping resistor for 6 volt operation. the radio can be negative or positive ground. The vibrator power supply make its own internal ground.
::
::
::He should be warned that the dropping resistor will get very warm or even too hot to touch. It should go into the engine compartment, in my opinion. Otherwise it will be like driving with the heater on all summer.
:
:
:Watch out for electrolytic capacitors in the input circuit of the 6 Volts. Otherwise, the B+ will come off the cathode of the rectifier tube, and the radio really couldn't care less about the polarity of the input current. And, yes, use a BIG resistor to get the six Volts. It's gonna get hot.
:Lewi
How about putting a 6 volt, 50 watt zener in SERIES with the radio's A+ lead. You would have to use a suitable heat sink but it would be better than a resistor and would keep the radio voltage constant.

:How about putting a 6 volt, 50 watt zener in SERIES with the radio's A+ lead. You would have to use a suitable heat sink but it would be better than a resistor and would keep the radio voltage constant.

What about the rest of the cars 12 volt supply? That's a shunt to the whole system. The old time Drop-A-Volt resistor mounts to the firewall handles the 6 volt radio just fine.

:
::How about putting a 6 volt, 50 watt zener in SERIES with the radio's A+ lead. You would have to use a suitable heat sink but it would be better than a resistor and would keep the radio voltage constant.
:
:What about the rest of the cars 12 volt supply? That's a shunt to the whole system. The old time Drop-A-Volt resistor mounts to the firewall handles the 6 volt radio just fine.

I think you are both wrong...a Zener would drop a constant 6 Volts, but the battery Voltage would change from about 12 to 14 as the alternator charged the battery, so the Voltage to the radio would go from 6 to 8 Volts. But, it wouldn't be a shunt as it would be in series with the radio. The radio probably uses about 35 Watts of power, or about 6 Amperes of current. 50 Watts sounds about right as does about one Ohm, which shouldn't be too hard to wind with hardware store iron wire. These are ball park figures, I have just pulled them out of my head with about three beers in my stomach, so don't trust me to be accurate.
Lewis
P. S. All right, four or five beers.

::
:::How about putting a 6 volt, 50 watt zener in SERIES with the radio's A+ lead. You would have to use a suitable heat sink but it would be better than a resistor and would keep the radio voltage constant.
::
::What about the rest of the cars 12 volt supply? That's a shunt to the whole system. The old time Drop-A-Volt resistor mounts to the firewall handles the 6 volt radio just fine.
:
:
:I think you are both wrong...a Zener would drop a constant 6 Volts, but the battery Voltage would change from about 12 to 14 as the alternator charged the battery, so the Voltage to the radio would go from 6 to 8 Volts. But, it wouldn't be a shunt as it would be in series with the radio. The radio probably uses about 35 Watts of power, or about 6 Amperes of current. 50 Watts sounds about right as does about one Ohm, which shouldn't be too hard to wind with hardware store iron wire. These are ball park figures, I have just pulled them out of my head with about three beers in my stomach, so don't trust me to be accurate.
:Lewis
:P. S. All right, four or five beers.

Thanks for all your replies. I was of the opinion that it wasn't polarity conscious as I have connected a so called +ve earth car radio to negative by accident and it worked fine. With regard to the drop down resistor, in my apprenticeship days one could buy one, which as mentioned, was attached to the firewall. Someone once suggested an old jug element which is about 1 ohm.
Lewis, it's winter over here at the moment so I'm having my first red wine. By winter we mean between 12-25 Celsius.
Regards
Rudy
PS It must be US car radio time, as I'll be posting another one next.

:::
::::How about putting a 6 volt, 50 watt zener in SERIES with the radio's A+ lead. You would have to use a suitable heat sink but it would be better than a resistor and would keep the radio voltage constant.
:::
:::What about the rest of the cars 12 volt supply? That's a shunt to the whole system. The old time Drop-A-Volt resistor mounts to the firewall handles the 6 volt radio just fine.
::
::
::I think you are both wrong...a Zener would drop a constant 6 Volts, but the battery Voltage would change from about 12 to 14 as the alternator charged the battery, so the Voltage to the radio would go from 6 to 8 Volts. But, it wouldn't be a shunt as it would be in series with the radio. The radio probably uses about 35 Watts of power, or about 6 Amperes of current. 50 Watts sounds about right as does about one Ohm, which shouldn't be too hard to wind with hardware store iron wire. These are ball park figures, I have just pulled them out of my head with about three beers in my stomach, so don't trust me to be accurate.
::Lewis
::P. S. All right, four or five beers.
:
:
:Thanks for all your replies. I was of the opinion that it wasn't polarity conscious as I have connected a so called +ve earth car radio to negative by accident and it worked fine. With regard to the drop down resistor, in my apprenticeship days one could buy one, which as mentioned, was attached to the firewall. Someone once suggested an old jug element which is about 1 ohm.
:Lewis, it's winter over here at the moment so I'm having my first red wine. By winter we mean between 12-25 Celsius.
:Regards
:Rudy
:PS It must be US car radio time, as I'll be posting another one next.

Well, Rudy, it has been raining here (Atlanta, GA area) for the past few days, but today it is sunny and mild. By mild, I mean about 30C, but with a humidity around 200%, if that is possible. Quite miserable for those who rely on perspiration for cooling. Cold beer is the only answer. Probably costs less than running the air conditioner, and feels a lot better, too.
Lewis
Lewis
:

::::
:::::How about putting a 6 volt, 50 watt zener in SERIES with the radio's A+ lead. You would have to use a suitable heat sink but it would be better than a resistor and would keep the radio voltage constant.
::::
::::What about the rest of the cars 12 volt supply? That's a shunt to the whole system. The old time Drop-A-Volt resistor mounts to the firewall handles the 6 volt radio just fine.
:::
:::
:::I think you are both wrong...a Zener would drop a constant 6 Volts, but the battery Voltage would change from about 12 to 14 as the alternator charged the battery, so the Voltage to the radio would go from 6 to 8 Volts. But, it wouldn't be a shunt as it would be in series with the radio. The radio probably uses about 35 Watts of power, or about 6 Amperes of current. 50 Watts sounds about right as does about one Ohm, which shouldn't be too hard to wind with hardware store iron wire. These are ball park figures, I have just pulled them out of my head with about three beers in my stomach, so don't trust me to be accurate.
:::Lewis
:::P. S. All right, four or five beers.
::
::
::Thanks for all your replies. I was of the opinion that it wasn't polarity conscious as I have connected a so called +ve earth car radio to negative by accident and it worked fine. With regard to the drop down resistor, in my apprenticeship days one could buy one, which as mentioned, was attached to the firewall. Someone once suggested an old jug element which is about 1 ohm.
::Lewis, it's winter over here at the moment so I'm having my first red wine. By winter we mean between 12-25 Celsius.
::Regards
::Rudy
::PS It must be US car radio time, as I'll be posting another one next.
:
:Well, Rudy, it has been raining here (Atlanta, GA area) for the past few days, but today it is sunny and mild. By mild, I mean about 30C, but with a humidity around 200%, if that is possible. Quite miserable for those who rely on perspiration for cooling. Cold beer is the only answer. Probably costs less than running the air conditioner, and feels a lot better, too.
:Lewis
:Lewis
::

It's raining today for the first time in 6 weeks and winter has finally come with a maximum of 16c.
With regards to the drop down resistor, I found 2 on e-bay one is a 1 ohm 100 watt metal cover and the other is a 1 ohm 150 watt wire wound. That one should be OK if mounted on a heat sink with bracket so that it can be attached to the fire wall, any thoughts ?
Rudy

Yes this has been discussed here before but all the old posts disappear at the end of each month (sniff).

The resistor will work in a brute force kind of way.
More elegant? The 12 volt car battery is 2 volt cells joined with bus bars. Find the middle bar and drive a screw into it. There's your source for 6 volts.

:It's raining today for the first time in 6 weeks and winter has finally come with a maximum of 16c.
:With regards to the drop down resistor, I found 2 on e-bay one is a 1 ohm 100 watt metal cover and the other is a 1 ohm 150 watt wire wound. That one should be OK if mounted on a heat sink with bracket so that it can be attached to the fire wall, any thoughts ?
:Rudy

:Yes this has been discussed here before but all the old posts disappear at the end of each month (sniff).
:
:The resistor will work in a brute force kind of way.
:More elegant? The 12 volt car battery is 2 volt cells joined with bus bars. Find the middle bar and drive a screw into it. There's your source for 6 volts.
:
:
:
:
:
:
::It's raining today for the first time in 6 weeks and winter has finally come with a maximum of 16c.
::With regards to the drop down resistor, I found 2 on e-bay one is a 1 ohm 100 watt metal cover and the other is a 1 ohm 150 watt wire wound. That one should be OK if mounted on a heat sink with bracket so that it can be attached to the fire wall, any thoughts ?
::Rudy

You wouldn't believe, this I was thinking back to my apprenticeship days when car batteries had external lead links and we used to drill a hole for a 6 volt tap connection. I don't think you can do this with a sealed battery.
Regards
Rudy

:You wouldn't believe, this I was thinking back to my apprenticeship days when car batteries had external lead links and we used to drill a hole for a 6 volt tap connection. I don't think you can do this with a sealed battery.
:Regards
:Rudy

I think it can still be done. The hole would go where you remember the middle link used to be. Then the screw would need some caulking to maintain the seal. No, I haven't tried it.

:Yes this has been discussed here before but all the old posts disappear at the end of each month (sniff).

Well, actually... they don't! If you click on "previous month" on the Forum start page you can navigate all the way back to 1997, with a few lapses due to server crashes, etc. Plus, if there is a specific topic for which you want to search, you can Google the general description and tack Nostalgia Air on to the string- for example, if you wanted to look for "car radio dropping resistor" just type in "car radio dropping resistor Nostalgia Air".

::Yes this has been discussed here before but all the old posts disappear at the end of each month (sniff).
:
:Well, actually... they don't! If you click on "previous month" on the Forum start page you can navigate all the way back to 1997, with a few lapses due to server crashes, etc.

Maybe you can but I get an error message. I can go back one month only. Maybe I need a different browser to go back farther.

But yes, I have found Nos Air threads with Google. But attachments such as shematics are gone, lost in the ozone like old radio waves.

just type in "car radio dropping resistor Nostalgia Air". Yeah that works good, Google search.

Do y'all type your e-mail address in the second line to post? I skip that line, it's not needed.