03-19-2023, 05:55 PM
Hi Burch,
Glad it worked out.
I have seen the choke or field coil in the B Supply return in several radios including some AC-DC modls. My theory of why they put the field coil in the return is 2 fold.
First may be safety related. Assuming that the speaker is not physically connected to teh chassis, if the coil developed a short to the speaker frame, the speaker frame is somewhere between 250VDC and 360VDC above chassis potential and now a shock hazard (few transformer powered radios had backs in that time period.
Second, in addition, by having the field in the B supply return, the Field was helpful in developing the "C-" Bias voltages needed to bias the Audio Amp and Power Output tubes, as the cathodes are often connected to Chassis ground which served as B- / C+.
This all exists in later radios, but differently, by placing cathode bias resistors between chassis ground and the cathode of the tubes in question. It is actually simpler.
As to the 2 high value resistors that parallel the field in the Delco, they are merely a voltage divider to get the desired bias voltages.
Glad it worked out.
I have seen the choke or field coil in the B Supply return in several radios including some AC-DC modls. My theory of why they put the field coil in the return is 2 fold.
First may be safety related. Assuming that the speaker is not physically connected to teh chassis, if the coil developed a short to the speaker frame, the speaker frame is somewhere between 250VDC and 360VDC above chassis potential and now a shock hazard (few transformer powered radios had backs in that time period.
Second, in addition, by having the field in the B supply return, the Field was helpful in developing the "C-" Bias voltages needed to bias the Audio Amp and Power Output tubes, as the cathodes are often connected to Chassis ground which served as B- / C+.
This all exists in later radios, but differently, by placing cathode bias resistors between chassis ground and the cathode of the tubes in question. It is actually simpler.
As to the 2 high value resistors that parallel the field in the Delco, they are merely a voltage divider to get the desired bias voltages.
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
Best Regards,
MrFixr55
