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

[N_Cook]

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 ?

 

[N_Cook]

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).

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.

 

[Ken]

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).

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

 

[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).

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".

 

[N_Cook]

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

--
*Procrastination is the art of keeping up with yesterday.

Dave Plowman [email protected] London SW
To e-mail, change noise into sound.

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

 

[John Byrns]

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".

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

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/

 

[Ken]

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

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

 

[oldcoot]

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)

 

[N_Cook]

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.

--
Jim Mueller [email protected]

To get my real email address, replace wrongname with eportiz. Then replace
nospam with sacbeemail.

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
http://www.diversed.fsnet.co.uk/vib2.gif

 

[oldcoot]

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 onhttp://www.diversed.fsnet.co.uk/vib2.gif

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)

 

[oldcoot]

http://www.diversed.fsnet.co.uk/vib2.gif

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.

 

[John Byrns]

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?

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


Regards,

John Byrns

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/

 

[nesesu]

 http://www.diversed.fsnet.co.uk/vib2.gif

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.

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.

 

[oldcoot]

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.

Maybe the image I'm looking at is different than yours (if that's
possible), but mine clearly shows the line from the armature 'jumping
over' the negative DC bus, and ending at the bottom of the .1uf cap
(the bottom of this cap should also be tied to common neg., but
isn't). On my print, the 'jump over' symbol is clearly an error. John
B., Halp!
Bill(oc)

 

[N_Cook]

Look to the left near the hash choke; it's connected to common there. It's
a strange way to draw it but it is correct.

--
Jim Mueller [email protected]

To get my real email address, replace wrongname with eportiz. Then replace
nospam with sacbeemail.



Maybe the image I'm looking at is different than yours (if that's
possible), but mine clearly shows the line from the armature 'jumping
over' the negative DC bus, and ending at the bottom of the .1uf cap
(the bottom of this cap should also be tied to common neg., but
isn't). On my print, the 'jump over' symbol is clearly an error. John
B., Halp!
Bill(oc)

Probably a bit difficult getting back to the author, G A Quarrington ,
printed 58 years ago

 

[Ken]

http://www.diversed.fsnet.co.uk/vib2.gif

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.

On diags that show the jump for no connection, lines crossing means
there is a connection. I like the connection represented by a dot, but
on some hard to read prints, this dot can disappear, leaving you
wondering, which is it. Ken