11-01-2012, 11:09 AM
Arran:
You are quite correct in what you are saying if the field coil was wired as part of a Pi filter. However, since the original amplifier chassis is missing, I am adapting a jukebox amplifier that ran a 5K field coil from a + 300 V source with the other lead going to ground. ( 60 ma current draw ) The amp already uses filter resistors. I plan on using an additional filter resistor and capacitor to feed the voltage divider. This will keep hum out of the receiver, The voltage divider will load the +300V supply to keep that supply point at near +300V.
On another project, the restoration of a 1942 Freed-Eisman console, I redesigned the amp chassis when the field coil failed on its 15" woofer. In the original circuit, the whole receivers current draw was fed from the rectifier cathode through the 2500 ohm field coil. The output tubes were P-P 6Y6's. The tube manual stated that 6Y6's were designed to work at low plate voltages. No wonder-after the voltage drop through the field coil, there wasn't much left. That was a very poor design. To replace the field coil in the existing circuit with a resistor was impractical, as the resistor would dissipate a large amount of power. I replaced the output tubes with 6L6's with the plate supply coming directly off the rectifier cathodes. A suitable value filter resistor was chosen to provide B+ to the rest of the radio to supply the same voltage as before. The wattage required now was far less, as the output tube current no longer passed through this resistor. This is common practice in commercially made equipment, and the radio didn't hum after this mod. With your ear next to the speaker, hum was almost inaudible. The tone control circuits were also modified-the bass and treble controls had very little effect. The treble control was a pot wired in series with the tweeter, and the bass control was a pot in series with the volume loudness compensation network. This was replaced with a tone control circuit (12AX7 on an add-on sub-chassis) from the RCA tube manual. Even with the much increased bass boost, no hum could be heard. The set never sounded so good.
You are quite correct in what you are saying if the field coil was wired as part of a Pi filter. However, since the original amplifier chassis is missing, I am adapting a jukebox amplifier that ran a 5K field coil from a + 300 V source with the other lead going to ground. ( 60 ma current draw ) The amp already uses filter resistors. I plan on using an additional filter resistor and capacitor to feed the voltage divider. This will keep hum out of the receiver, The voltage divider will load the +300V supply to keep that supply point at near +300V.
On another project, the restoration of a 1942 Freed-Eisman console, I redesigned the amp chassis when the field coil failed on its 15" woofer. In the original circuit, the whole receivers current draw was fed from the rectifier cathode through the 2500 ohm field coil. The output tubes were P-P 6Y6's. The tube manual stated that 6Y6's were designed to work at low plate voltages. No wonder-after the voltage drop through the field coil, there wasn't much left. That was a very poor design. To replace the field coil in the existing circuit with a resistor was impractical, as the resistor would dissipate a large amount of power. I replaced the output tubes with 6L6's with the plate supply coming directly off the rectifier cathodes. A suitable value filter resistor was chosen to provide B+ to the rest of the radio to supply the same voltage as before. The wattage required now was far less, as the output tube current no longer passed through this resistor. This is common practice in commercially made equipment, and the radio didn't hum after this mod. With your ear next to the speaker, hum was almost inaudible. The tone control circuits were also modified-the bass and treble controls had very little effect. The treble control was a pot wired in series with the tweeter, and the bass control was a pot in series with the volume loudness compensation network. This was replaced with a tone control circuit (12AX7 on an add-on sub-chassis) from the RCA tube manual. Even with the much increased bass boost, no hum could be heard. The set never sounded so good.
