Brenda Ann wrote:
"N_Cook" wrote in message
...
Brenda Ann wrote in message
...
"N_Cook" wrote in message
...
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 ?


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.




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