[MrFixr55]

There IS some 120 Hz sine wave on the trace, therefore you do have some power supply hum. You can bump up the caps some. However, raising the capacity raises the voltage and raises the load on the rectifier tube. You can try up to 30 uF after the filter choke but don't go to high on the cap before the choke, especially if using a vintage (globe) rectifier. Keep it 20 uF or less.

I have 2 theories here. Others with more EE experience than I do, please chime in:

Electrolytic caps were a relatively new invention in the early 1930s and were of the "wet" type and making caps with values higher than about 10 uF was expensive. And the early 1930s was NOT a time for expensive. Newer sets, especially post war sets used caps as high as 30 uF or sometimes even higher, as high as 40 uF. Please note that although the RCA Receiving Tube Manual Ver. RC14 (1940) states that the type 80 has the same specs as the 5Y3 that replaced it and at the time of publication the 80 was a 5Y3 with a 4 pin base, this tube likely has ratings much higher than the original 80 which relied on caps no higher than 4uF and chokes 20 Henries or higher to keep peak current down. Therefore, although by 1940, for cap input filters, RCA was specifying a max input cap value of 40uF, I don't think that an 80 made before 1940 would withstand the current caused by a 40 uF cap.

The other theory is that the cap input filter that used a choke and 2 caps (The speaker field served as a choke in many sets) constituted a "Pi" filter and therefore functioned almost as a tuned circuit.