How the rectifier works: The voltage of the power transformer high-voltage secondary is applied to the plates of the 5Y3 rectifier tube. This tube has a directly heated cathode (the cathode and filament are the same element), so the pulsating DC is taken off of pin 8, which is one side of the filament. The plates act as a one-way gate to allow current to flow only when the plate is positive with respect to the cathode. Because the high-voltage winding center tap is grounded and is the zero reference point, each plate is positive only on one-half of the AC cycle. During one-half of the cycle, plate #1 will be positive with respect to the cathode, and during the second half of the AC cycle, plate #2 will be positive with respect to the cathode. This causes the voltage on the cathode of the rectifier to be a pulsating DC voltage.
You also need to measure the DC after the standby switch to see if that is working correctly.
check out this article for some good information on the 5E3 circuit
https://robrobinette.com/How_The_5E3_Deluxe_Works.htm