[Tubesforme]

One has to tell the hum from the noise reception and home wiring fom AC hum.

Also, 20 same as 70 or 90 is susceptible to the bad aluminum can shields connection to chassis, where the shield is floating rather than shielding.

All my baby grands, as well as other tube radios have at least a little hum, most of it being AC modulated RF, due to home wiring.  A pocket transistor radio has the same but then it allows to orient it away from this noise. So is my 46-250 that has a loop type antenna, it also can be oriented out of it.

Grounding may reduce noise also.

Another thing to look for is bad grounding due to oxidized riveting when riveted contacts are used as grounding points.

The model 20 that I have does not use the aluminum caps. It has a single large square cap the contains only the original paper capacitors, potted, not the later electrolytic version. I have replaced both of the 1uf's with NOS 1UF @ 630VDC and replaced the .13 with a standard .1uf @ 630VDC and the 1.5 uf which is used to filter the bias supply I replaced with a 10uf @ 450VDC. That has lowered the hum major. The voltages are still too high on the plates of the 71A's. Yes, I have tried replacing all of the tubes except for the 80 rectifier. I have been told by many people and know from experience that in a large city, like I live in, all I need is an antenna consisting of about 20' of hook up wire. Thanks