[MrFixr55]

Hi Richard,

Thx for posting the schematic.  As I remember as a kid, these were great performers for the time, likely about 5W RMS output to the 2 6X9 speakers (one in front, one in back) that Chevy used, vs Ford's SE 12AQ5 of the period (they did have one or 2 deluxe units with 2 12AQ5 (or 2 6AQ5 in 55 and earlier), but as I remember, they were mostly Single End.

The 235hz freq of the noise makes sense as the advertised freq of the  PeKo solid state vibrator is 115hz and this is a full wave rectifier.  The issue being similar to the 120 hz hum of a full wave rectifier fed by 60Hz AC when the filter caps go bad.

I agree with the earlier statement to pull the 12AV6 and listen for the static.  If it exists at the same level, then the noise is definitely coming through the B+, and specifically, right off the rectifier, as the Plate and Screen for the 12V6s come right from the junction of the rectifier cathode, 20 uF cap and 1.2K resistor.  It does seem that they protect the filaments from switching spikes with the inductor to the radio filaments (interestingly, not to the 12V6 filaments), and the inductor feeding the primary circuit.  However, these do not filter the output of the power supply system.

In reviewing the schematics for some of the earlier Philco car radios, including the one for the 1937 Chrysler Airflow, filter chokes existed, that would filter the output tube plate and screen circuits. These did disappear by the 1950s.

The bottom line is that it is a puzzlement that a mechanical vibrator does not cause noise, but an electronic one does.  I mean, the points on these actually arc like the ignition points do.  However, putting a choke with a rather low resistance (no more than about 300 Ohm) between the rectifier plate and the filter network should help.  Maybe upping the value of the filter capacitors could help.

Attached are the specs for the 0Z4 rectifier from the RCA Receiving Tube Manual, RC14 (1940):
   

Interesting that this tube is pushed to its current limits, as even the idle current for 2 6V6s at 250V is 70mA.  Max current is 79mA.  Max output of a 0Z4 is 75 mA.  Note that there is a 30mA min current required for this tube to actually fire.  Also note the statement that "The use of filters may be necessary to eliminate objectionable noise".  The main use of 0Z4 as a rectifier is in car radios (and I haven't found a radio from the 1940s that use one), the main reason being that there is no filament, eliminating about 300-600mA of current draw from the battery.  This was an issue, as car radios typically draw 5-15 Amps (depending on whether 6 or 12V), and most cars had a generator capacity of no more than 30A back in the day.  Nothing like the 90A alternators of today, but back then, most cars did not have fuel injection, electronic ignition, electric fuel pumps, A/C, electric windows, rear window defoggers, load leveling suspensions, electric wipers (they were vacuum), etc.