06-19-2008, 04:16 PM
Since I could not get an answer either here or on ARF, and since this was driving me crazy (plus I wanted to finish the restoration), I decided to do some digging. Sure enough I found the answer in my Radiotron Designers Handbook (3rd edition).
It turns out that the 6A8G tube, which set uses, is VERY susceptable to frequency changes when the supply voltage changes. There are other tubes which are not as susceptable, such as the 6K8. The result is "flutter" or motorboating. To reduce this tendency, the book says that it is recommended that the anode-grid supply be taken directly from the rectifier cathode through a dropping resistor and high value capacitor to B-. This reduces "flutter" and reduces hum (modulation).
Since this is an AC/DC set, with low B+, I suppose Philco decided to use a choke instead of a high-value resistor.
I left the replacement low-resistance choke in place and increased the filter capacitor to 22uf. The radio works, but I cannot tell if "flutter" has been reduced.
It turns out that the 6A8G tube, which set uses, is VERY susceptable to frequency changes when the supply voltage changes. There are other tubes which are not as susceptable, such as the 6K8. The result is "flutter" or motorboating. To reduce this tendency, the book says that it is recommended that the anode-grid supply be taken directly from the rectifier cathode through a dropping resistor and high value capacitor to B-. This reduces "flutter" and reduces hum (modulation).
Since this is an AC/DC set, with low B+, I suppose Philco decided to use a choke instead of a high-value resistor.
I left the replacement low-resistance choke in place and increased the filter capacitor to 22uf. The radio works, but I cannot tell if "flutter" has been reduced.
