09-30-2012, 07:41 PM
Messing up the negative connection on the filter caps is a very common mistake that people make if they are used to working on sets newer then 1939, I know because I did it myself before. Most sets built before 1939 used discrete filter capacitors for the input and output filters so they would isolate the input cap from the chassis with a cardboard insulator and bolt or clamp the output filter cap directly to the chassis, so if you connect the negative of the input cap to the chassis it behaves like it's open circuit.
It was very common practice during the 1930s to use fixed bias on the tubes by installing a resistor network in series with the B- or center tap of the power transformer. It's kind of a cleaver way of achieving a negative bias for the tubes by making the common negative less negative then the center tap of the power transformer. The newer method, in the case of the power output tube, is to make the cathode more positive with respect to the grid by connecting a resistor in series between the cathode and common negative rather then making the grid more negative. I hope I didn't get mixed up here on a few points but that is what they did even if my grasp of the theory is wrong.
Regards
Arran
P.S If the power transformer is running warm make sure that someone didn't substitute a #42 output tube for a #41, a #42 draws a lot more filament current then a #41.
It was very common practice during the 1930s to use fixed bias on the tubes by installing a resistor network in series with the B- or center tap of the power transformer. It's kind of a cleaver way of achieving a negative bias for the tubes by making the common negative less negative then the center tap of the power transformer. The newer method, in the case of the power output tube, is to make the cathode more positive with respect to the grid by connecting a resistor in series between the cathode and common negative rather then making the grid more negative. I hope I didn't get mixed up here on a few points but that is what they did even if my grasp of the theory is wrong.
Regards
Arran
P.S If the power transformer is running warm make sure that someone didn't substitute a #42 output tube for a #41, a #42 draws a lot more filament current then a #41.
