09-13-2013, 07:10 PM
Yes, it appears that after the pop, there is a loss of load current from the 42 output, which causes the B+ voltage to rise. The 75 plate voltage does not change because the loss of load causes the bias on the 75 grid to drop, compensating for the rise in B+ and keeping the plate voltage relatively constant.
Remember that the bias votages for the radio are obtained from the voltage drop across the wirewound bias resistor in the negative return of the supply. If the supply current drops, then the bias voltage will also will drop, causing a corresponding increase in plate current.
It appears that the 75 plate resistors are fine. The problem seems to be loss of plate current of the 42, which looks like a bad tube.
Remember that the bias votages for the radio are obtained from the voltage drop across the wirewound bias resistor in the negative return of the supply. If the supply current drops, then the bias voltage will also will drop, causing a corresponding increase in plate current.
It appears that the 75 plate resistors are fine. The problem seems to be loss of plate current of the 42, which looks like a bad tube.
