Car Radio Voltage/Polarity Conversion

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Car Radio Voltage/Polarity Conversion

Post by jsweinrich on Sun Jul 23, 2017 11:36 am

Thought I would share this and get anyone's thought on the the subject. I am currently restoring 8 Philco S4627 radios (https://mseriesradio.blogspot.com/) to be given to the Studebaker Truck Farmer community. As requests came in, so far, 5 out of 5 want 12 volt negative ground versions. I have been using the Aurora Design vibrators (http://www.tech-retro.com/Aurora_Design/Accessories.html) so switching to negative ground is no big deal, just a different version of vibrator to install.

The voltage is another story. To solve this problem I bought down converters off ebay. They convert the 12 volt negative ground power to 6 volt negative ground. Sounds simple, right? Here is/was my plan:

So I get the first radio running and test this setup. Powers up and everything seems to be functioning except this nasty tone every so often on the dial. I have not determined the base frequency or the harmonics that are made but it is pretty obvious that the down converter is spewing out some nasty RFI/EMI. I have tried the usual, more filtering on the leads leading to and from the converter and shielding around the converter to no avail. These radio are so sensitive to noise that a USB wall wart across the room will kill a station.

What to do? A series resistor in the power lead to the radio is sort a non-starter, maybe not in the Winter as one would be wasting about 30 watts in heat. So I look up the tubes to see if there are 12 volt filament versions. No cigar as these are those wonderful short lived "Locktal" tubes invented to not fall out of their sockets.

So now I'm pondering another solution. Since this radio has 8 tubes I'm thinking of splitting the filaments into two groups and connect the two groups in series so that the filament circuit would be 12 volts. This, of course assumes that the filaments have the same or close to the same current requirements. BTW here is a great tube specification site:

https://frank.pocnet.net/

This leaves the the low to high voltage conversion circuit to be converted. I have yet to determine what current that circuit needs, maybe use a resistor there? Of course more analysis is needed. Another thought would be to connect the low to high voltage conversion circuit in series with the 6 volt tube filaments but that circuit would get the brunt of the voltage until the tubes heat up.

Any thoughts?

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Re: Car Radio Voltage/Polarity Conversion

Post by jsweinrich on Sun Jul 23, 2017 4:03 pm

Did some fooling around with the circuit and found that connecting four tube filaments in parallel in series with the low to high voltage circuit works but the tubes take forever to heat up and as a result the low to high voltage circuit sings crazy until they do heat up. Once the tubes heat up the filament voltage is around 7 volts, a tad to high. So, maybe a few Zeners here and there might shorten the time the tubes heat up and prevent the low to high voltage circuit from getting clobbered so much. Here is my latest stab (don't have Zeners to try right now):



Back to back Zeners makes that part of the circuit polarity insensitive...

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Re: Car Radio Voltage/Polarity Conversion

Post by Dr. Radio on Mon Jul 24, 2017 5:05 pm

14 volt equivalent locktals exist. FYI.

6 volt systems see higher then 6 volts during charging, same with 12 volt systems. Hence the loctal design of 7xx and 14xx tubes.
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Re: Car Radio Voltage/Polarity Conversion

Post by jsweinrich on Mon Jul 24, 2017 10:59 pm

OMG! I didn't think to look for 14 series tubes!! Thanks!

Yes, I am aware that alternators charge at a higher voltage, that is why I picked Zener voltages to add up above that voltage ( 5.6+7.5+1.2+1.2=15.5). If I replaced the 7 series tubes with the 14 series then I would have to use a resistor or something for the HV circuit or change the transformer to a 12 volt version. All expensive for 8 give away radios...

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Re: Car Radio Voltage/Polarity Conversion

Post by jsweinrich on Sat Aug 12, 2017 6:20 pm

Decided to rewind transformer and go with 14 series tubes:

http://mseriesradio.blogspot.com/2017/08/update-on-6-to-12-volt-conversion.html

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