You probably need power supply capacitors. Look for capacitors inside the radio with values around 8uF to 20uF. You'll probably find two or three; Replace them with polypropylene 250v caps and the hum will probably go away.I found a schematic that may be for your radio on Rider volume 14. It has 35Z5, 50L6, 12SA7, 12SK7 and 12SQ7 tubes. That was a very common tube set for RCA radios. I have a couple of chassis like those myself. One of mine was just like yours when I got it - Working but with a lot of hum. I replaced the power supply caps and it was much quieter.