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The subject of positive ground vs. negative ground arises here often. My recommendation usually is to do the conversion to negative ground, but often the interest is in preserving originality, so the positive-ground configuration stays. A real problem, however, is finding a radio for a positive-ground car, and once found, what to do if the owner eventually converts the car to negative ground.
Recently I came across an interesting option for this. A friend asked me to look at the radio from his late-50s Mercedes, which turned out to be a Blaupunkt Frankfurt TR, US version, model no. 3780, from about 1961. I was quite surprised to see that it can be configured for either positive or negative ground, and can be set up for either 6V or 12V electrical systems as well. The instructions for conversion are on a card in the power module. The positive-negative ground conversion is just a few jumpers in one place, but the conversion between 6 and 12V is a bit more complicated. Of course, the 6-12V conversion is not of great interest to us (unless the unit you get is already set up for 6V!) since all our cars are 12V.
Below is a scan of the card I found in the power module, and a couple pictures of the radio. Fortunately, this one was in remarkably good condition, so I got it working easily. The separate module is the power module, which uses a transistor circuit to generate the high voltage (about 115VDC) for the tubes in the main part of the radio. It also has a two-transistor audio amplifier that drives the speaker. The power module fits on the back of the main unit and can be arranged so it hangs down or sticks out straight in the back, depending on how much room you have--which, of course, in Spridgets is not much!
Anyway, it seems to me that this is a neat option for someone with an early-60s car, who wants to keep the car original, wants a period-correct radio for an early car, and wants to keep the positive-negative conversion option available. Back in the 60s, most radios were installed by the dealer, so there really is no "right" radio for these cars.
I suspect that there are other models of tube radios that have the same feature. Certainly, it's practical to do. Later, all-transistor radios aren't really amenable to this, and since virtually all cars were negative-ground when all-transistor radios became available, I doubt that you can find fully transistor radios that can be used in either positive- or negative-ground configuration.
Recently I came across an interesting option for this. A friend asked me to look at the radio from his late-50s Mercedes, which turned out to be a Blaupunkt Frankfurt TR, US version, model no. 3780, from about 1961. I was quite surprised to see that it can be configured for either positive or negative ground, and can be set up for either 6V or 12V electrical systems as well. The instructions for conversion are on a card in the power module. The positive-negative ground conversion is just a few jumpers in one place, but the conversion between 6 and 12V is a bit more complicated. Of course, the 6-12V conversion is not of great interest to us (unless the unit you get is already set up for 6V!) since all our cars are 12V.
Below is a scan of the card I found in the power module, and a couple pictures of the radio. Fortunately, this one was in remarkably good condition, so I got it working easily. The separate module is the power module, which uses a transistor circuit to generate the high voltage (about 115VDC) for the tubes in the main part of the radio. It also has a two-transistor audio amplifier that drives the speaker. The power module fits on the back of the main unit and can be arranged so it hangs down or sticks out straight in the back, depending on how much room you have--which, of course, in Spridgets is not much!
Anyway, it seems to me that this is a neat option for someone with an early-60s car, who wants to keep the car original, wants a period-correct radio for an early car, and wants to keep the positive-negative conversion option available. Back in the 60s, most radios were installed by the dealer, so there really is no "right" radio for these cars.
I suspect that there are other models of tube radios that have the same feature. Certainly, it's practical to do. Later, all-transistor radios aren't really amenable to this, and since virtually all cars were negative-ground when all-transistor radios became available, I doubt that you can find fully transistor radios that can be used in either positive- or negative-ground configuration.
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