12-25-2021, 09:25 PM
Changing the resistor connected to the 47 grid from 240K to 4 meg will not change the actual bias, only make the bias unstable. The 47 bias is determined by the radio's DC current drain flowing through the 240 ohm section of the wirewound resistor 37. The voltage drop across this resistor creates the bias for the 47, not the grid resistor.
If you increase the value of the grid resistor, the tube bias is more affected by grid current due to gas in the tube or grid emission when the tube overheats. This can cause a runaway condition as the tube gets hotter and hotter, the current keeps increasing. That's why the tube data sheet specifies a max resistance for the grid resistor. It should be no higher than 240K.
If the grid voltage on the 47 is -20V, it means that the 47 or some other component is drawing excessive current through the 240 ohm bias resistor. Excessive current can create more hum as it loads down the power supply. Could be a leaky coupling capacitor, or a gassy 47 tube. Have you replaced capacitor 31?
Check the voltage across the 240 ohm resistor and see if it is in the neighborhood of 16 to 18 V.
If you increase the value of the grid resistor, the tube bias is more affected by grid current due to gas in the tube or grid emission when the tube overheats. This can cause a runaway condition as the tube gets hotter and hotter, the current keeps increasing. That's why the tube data sheet specifies a max resistance for the grid resistor. It should be no higher than 240K.
If the grid voltage on the 47 is -20V, it means that the 47 or some other component is drawing excessive current through the 240 ohm bias resistor. Excessive current can create more hum as it loads down the power supply. Could be a leaky coupling capacitor, or a gassy 47 tube. Have you replaced capacitor 31?
Check the voltage across the 240 ohm resistor and see if it is in the neighborhood of 16 to 18 V.
