09-13-2022, 07:37 PM
Hi JMeulemans,
+1 on what Terry Radioroslyn said. Definitely lift the ground on the 2.5V Center Tap and add the Cathode bias resistor and bypass capacitor.
Re the center tap not being grounded affecting the other tubes, probably not. If you have any weird oscillations, place capacitors (0.1 uF?) between each leg of the 2.5V secondary and ground.
The Audio Power Output stage of any radio employing a class A or AB power stage must be biased. On most radios, this is done one of 2 ways, essentially the same but different.
One technique is that the B- (center tap of the Rectifier secondary winding) is grounded to the chassis and the cathode of the output tube connects to chassis ground through a resistor. This resistor is the cathode bypass resistor and usually has a capacitor in parallel with it to prevent low frequency phase shift. Since the output tube is directly heated (the filament is the cathode), the cathode bias resistor connects to a center tap of the filament (heater) winding of the transformer. This is what should be done on your set.
The other technique is that the cathode is grounded to the chassis, but a resistor exists between chassis ground and the rectifier secondary center tap. this dreates a "C- at the junction of the resistor and the rectifier secondary center tap. a resistor form the Output tube grid to that point provides the negative bias.
I looked at a few schematics. The AK55 has separate windings for the 24s and 45s with the 45 filament having a voltage divider between the 2 legs (transformer did not have a center tap; this resister served the purpose. from the center tap of this resistor the cathode bias resistor and bypass cap tied to ground. the AK82 has a single end 47 Output tube. the filament (cathode is powered by the same 2.5V secondary as the other tubes but the secondary is "center tapped" by 2 resistors like the 55. the center tapmis to ground because bias voltage is through a resistor between ground and the center tap of the B+ which is not grounded.
My RCA R7A has push pull 47s fed by the same 2.5V secondary as the other 5 tubes but this set has a ]n interstage transformer and the center tap is to a "C- developed in the same why as the AK82.
Bottom line is if the single 2.5V secondary is hefty enough to handle all 4 tubes, and since the original schematic shows no grounding at all for the 24As, I think that Terry's recommendation should cause no trouble at all. However, if Terry's recommendation is not performed, failure to bias the tube will cause the 47 tube, the output transformer, rectifier or something else expensive to burn up.
One other word of warning, make sure that the speaker is connected and that the output transformer is good before firing up the radio. If you see a bright glow coming from the screen grid, shut it down! 47s are somewhat expensive and somewhat rare. they were only used for about 2 years before the 2A5 was released.
Good luck. This looks like a nice simple 3 circuit TRF with biased "power" detector and while not HiFi, should work rather well unless you have a whole bunch of AM Stations close to each other and to you. This is a good representation of an early 1930s "Screen Grid" radio and is a historically important circuit. I restored a 1931 Gloritone 26 which is similar to your radio except for a 45 Triode instead of the 47 Pentode.
+1 on what Terry Radioroslyn said. Definitely lift the ground on the 2.5V Center Tap and add the Cathode bias resistor and bypass capacitor.
Re the center tap not being grounded affecting the other tubes, probably not. If you have any weird oscillations, place capacitors (0.1 uF?) between each leg of the 2.5V secondary and ground.
The Audio Power Output stage of any radio employing a class A or AB power stage must be biased. On most radios, this is done one of 2 ways, essentially the same but different.
One technique is that the B- (center tap of the Rectifier secondary winding) is grounded to the chassis and the cathode of the output tube connects to chassis ground through a resistor. This resistor is the cathode bypass resistor and usually has a capacitor in parallel with it to prevent low frequency phase shift. Since the output tube is directly heated (the filament is the cathode), the cathode bias resistor connects to a center tap of the filament (heater) winding of the transformer. This is what should be done on your set.
The other technique is that the cathode is grounded to the chassis, but a resistor exists between chassis ground and the rectifier secondary center tap. this dreates a "C- at the junction of the resistor and the rectifier secondary center tap. a resistor form the Output tube grid to that point provides the negative bias.
I looked at a few schematics. The AK55 has separate windings for the 24s and 45s with the 45 filament having a voltage divider between the 2 legs (transformer did not have a center tap; this resister served the purpose. from the center tap of this resistor the cathode bias resistor and bypass cap tied to ground. the AK82 has a single end 47 Output tube. the filament (cathode is powered by the same 2.5V secondary as the other tubes but the secondary is "center tapped" by 2 resistors like the 55. the center tapmis to ground because bias voltage is through a resistor between ground and the center tap of the B+ which is not grounded.
My RCA R7A has push pull 47s fed by the same 2.5V secondary as the other 5 tubes but this set has a ]n interstage transformer and the center tap is to a "C- developed in the same why as the AK82.
Bottom line is if the single 2.5V secondary is hefty enough to handle all 4 tubes, and since the original schematic shows no grounding at all for the 24As, I think that Terry's recommendation should cause no trouble at all. However, if Terry's recommendation is not performed, failure to bias the tube will cause the 47 tube, the output transformer, rectifier or something else expensive to burn up.
One other word of warning, make sure that the speaker is connected and that the output transformer is good before firing up the radio. If you see a bright glow coming from the screen grid, shut it down! 47s are somewhat expensive and somewhat rare. they were only used for about 2 years before the 2A5 was released.
Good luck. This looks like a nice simple 3 circuit TRF with biased "power" detector and while not HiFi, should work rather well unless you have a whole bunch of AM Stations close to each other and to you. This is a good representation of an early 1930s "Screen Grid" radio and is a historically important circuit. I restored a 1931 Gloritone 26 which is similar to your radio except for a 45 Triode instead of the 47 Pentode.
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
Best Regards,
MrFixr55
