Converting a positive ground valve/tube car radio to negative ground ?

No user switchable option. I can see its possible to rewire the vibrator so you still get right polarity for the anode supplies. Heaters presumably happy enough powered the other way round but how to change the biasing of the valves ?

Reply to
N_Cook
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Unless it's a synchronous vibrator, it should run equally well on either polarity. The radio doesn't get anode voltage from the car's DC, it gets it from the radio's DC-DC converter (vibrator, transformer, rectifier, filtering).

Reply to
Brenda Ann

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With positive ground you automaticly get a negative voltage for biasing but what to do in the negative ground case ? In this case its a synchronous mechanism but its easy enough to get inside to rewire it for the opposite polarity, it is the biasing voltage I cannot fathom out.

Reply to
N_Cook

This must be an unusual radio (by old US car radio standards). No valve car radio I have ever seen has used the car's battery supply for any voltage other than the valve heaters. All DC voltages to run the valves come off the output of the transformer, with bias voltages developed by raising the cathode of the valve above B- (making the grid negative with respect to the cathode, but not with respect to ground).

Reply to
Brenda Ann

I'd like to see a schematic of that beast. I can see using the neg for bias in a 6V pos gnd system, but it would only be -6V, not much. Easy to get that off the xfmr center tap. Ken

Reply to
Ken

presumably

biasing

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This one also, on checking. I had not realised that the 12V to the radio is just for the heaters and the dial lamp and the on/off switch. Just leaves whether there is an added RFI noise problem from the "inverter".

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N_Cook

No need if it's a mechanical vibrator. That produces AC...

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*Procrastination is the art of keeping up with yesterday.

    Dave Plowman        dave@davenoise.co.uk           London SW
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Reply to
Dave Plowman (News)

not this one - synchronous - see recent thread on Ekco CR280. I had to rebuild the contacts to get it working. Needs smoothing yes, because its chopped but not ac. Does not require a rectifier thermionic, Se or Si. But feed it the wrong way and you get negative HT

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N_Cook

Why would the RFI issues change? If it is a synchronous vibrator, isn't it simply a matter of reversing two transformer connections to convert from positive ground to negative ground, how does this change affect RFI?

Regards,

John Byrns

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Regards,

John Byrns
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Reply to
John Byrns

If you're getting negative B voltage, reverse the two secondary leads going to the vibrator. Ken

Reply to
Ken

Mr. Cook, This was mentioned in the previous thread on synchronous vibrators. To enable running on reverse supply polarity, just reverse the two HV (HT) secondary leads that would normally go to the anodes of a tube rectifier. Bill(oc)

Reply to
oldcoot

aaah, this doesn't make sense - on every vibrator radio I ever used (only a few dozen), you could swap ground polarity and do nothing and it works. Now, if you put in an SS vibrator, then it's a different story. Why are you thinking yours is different?

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Reply to
William Noble

Check the vibrator socket. Some, but by no means all, synchronous vibrator sockets were symmetrical so that the vibrator could be plugged in two ways, one for positive ground and the other for negative ground. If it isn't that kind, then you will have to reverse the wiring as others have mentioned.

The reversible vibrators I have seen have + and - symbols on top to show which way to install them.

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Reply to
Jim Mueller

Can't seem to find that.

Right. Heard of them but never seen one.

Is this a UK market radio? It would be strange to produce a positive earth only radio given that even in the UK not all makes went to positive earth.

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    Dave Plowman        dave@davenoise.co.uk           London SW
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Reply to
Dave Plowman (News)

vibrator

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This one has 2 polarising pins so one way round only. Changing round the 2,

12V supply leads is easier, the can is earthed but not connected internally to the vibrator coil or contacts. a circuit for a synchronous one ( no rectifier required) I've put on
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-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N_Cook

2,

ly

OK, the schematic shows the hookup differs a bit from the old Delco synchronous-vibrator system I was referring to back in post # 11. But exactly the same principle applies:

To enable running on reverse supply polarity, simply reverse (swap) the two transformer secondary leads, i.e., the ones which have the

1500V buffer cap across them. Been there, done that. It works. Bill(oc)
Reply to
oldcoot

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There appears to be an error in this schematic. Someone else may confirm. The armature should be connected to common negative. Instead, the line from the armature is shown 'jumping over' the common negative line. As shown, this is an open circuit with no DC return path. Make the correction by tying the armature to common neg., and operation of the circuit becomes self-explanatory.

Reply to
oldcoot

Probably because it uses a synchronous vibrator to eliminate the need for a rectifier tube.

Regards,

John Byrns

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Reply to
John Byrns

Um, it IS connected to common negative were the line comes down from the armature terminal. From that point a line then goes to the bottom end of the .1u input filter cap.

Neil S.

Reply to
nesesu

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Sounds right. But as not in this posting, do recall at least one 12 volt European car radio of the 1950s that had a polarity plug that had to be placed in the correct position. But reading this now can't think why that was necessary! Maybe they were synchronous vibrators or something.

BTW not pertinent to this posting but have operate an item of 6 volt DC w. vibrator gear by feeding in 6.3 volts AC from a radio, removing the vibrator and strapping one half of the input winding of vibrator transf. to the supply. So instead of alternatec pulses of DC to each half of the CT input winding, we had 6.3v 60 hertz AC constantly to half the winding.

Don't understand the biasing question?

Reply to
terry

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