07-16-2022, 10:23 AM
Hi Mike,
I know that you don't wanna disassemble your 82 again, but my thoughts:
Most of these early radios have rather small (8-10 uF filter caps), because I don't think that they made much bigger. By the 1940s, Filter Caps increased to 20 - 30 uF or higher., and higher still when they went to permanent magnet speakers
The RCA Manual states that the maximum input filter (between cathode and B-) for an '80 or 5Y3 is 40 uF. You state that you bridged C2 with a 20 uF Cap to no avail. That would increase the current through the field coil and may add to the hum. Try bridging the input filter cap "Filter Condenser 1" on the Rider's Diagram).
The other contributor is the '47 Output tube being directly heated (The fiament is the cathode). Try subbing (by making an adaptor) a 2A5 which was the successor to the '47. It has the same filament voltage, plate load impedance and grid bias as the 47 but has the "separate heater and cathode" design instead of the direct heated cathode. The only addition is the cathode terminal which should be grounded. If this greatly reduces hum, this will prove that the output tube is the major contributor.
The hum level is probably a lot lower than the AK36, 40, RCA Radiolas, etc. which used 71As and worse yet, 26s. 26s were the first "AC" Tubes, used in in the Radiola 17, 18, AK36, 40, etc., and were a compromise by using a heavy filament and 1.5V. They still hummed. The Philco 20 had all tubes as being true AC Tubes (24A, 27), except the 71As but these were push-pull, and the pie plate speaker had very poor frequency response due to the very hard suspension.
I think that the AK 82 had a much better suspension, similar to the Rolas of the time, and agree that very likely did not have a hum bucking coil.
I have not worked on an 82 but did restore a 165, which except for the choke / cap IF coupling to the detector, was a thoroughly modern transformer powered 2.5V version of the AA5. The field was in the B+ side of the power supply, instead of the B- leg like the 82 did, and does have the hum bucking coil. If I remember, it had a Rola speaker. It performed very well with negligible hum. However, I used a 33 uF Caps for the input filter (C13).
BTW, a lovely job on a true icon of the "cathedral" style radio!
I know that you don't wanna disassemble your 82 again, but my thoughts:
Most of these early radios have rather small (8-10 uF filter caps), because I don't think that they made much bigger. By the 1940s, Filter Caps increased to 20 - 30 uF or higher., and higher still when they went to permanent magnet speakers
The RCA Manual states that the maximum input filter (between cathode and B-) for an '80 or 5Y3 is 40 uF. You state that you bridged C2 with a 20 uF Cap to no avail. That would increase the current through the field coil and may add to the hum. Try bridging the input filter cap "Filter Condenser 1" on the Rider's Diagram).
The other contributor is the '47 Output tube being directly heated (The fiament is the cathode). Try subbing (by making an adaptor) a 2A5 which was the successor to the '47. It has the same filament voltage, plate load impedance and grid bias as the 47 but has the "separate heater and cathode" design instead of the direct heated cathode. The only addition is the cathode terminal which should be grounded. If this greatly reduces hum, this will prove that the output tube is the major contributor.
The hum level is probably a lot lower than the AK36, 40, RCA Radiolas, etc. which used 71As and worse yet, 26s. 26s were the first "AC" Tubes, used in in the Radiola 17, 18, AK36, 40, etc., and were a compromise by using a heavy filament and 1.5V. They still hummed. The Philco 20 had all tubes as being true AC Tubes (24A, 27), except the 71As but these were push-pull, and the pie plate speaker had very poor frequency response due to the very hard suspension.
I think that the AK 82 had a much better suspension, similar to the Rolas of the time, and agree that very likely did not have a hum bucking coil.
I have not worked on an 82 but did restore a 165, which except for the choke / cap IF coupling to the detector, was a thoroughly modern transformer powered 2.5V version of the AA5. The field was in the B+ side of the power supply, instead of the B- leg like the 82 did, and does have the hum bucking coil. If I remember, it had a Rola speaker. It performed very well with negligible hum. However, I used a 33 uF Caps for the input filter (C13).
BTW, a lovely job on a true icon of the "cathedral" style radio!
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
Best Regards,
MrFixr55
