[MrFixr55]

Dan, an interesting issue.  A couple of things:

• I assume that this is not happening to any other AA5 type radios in your collection, therefore it is not environmental.
  
• Are all tube shields in place?  Considering the proximity of the RF stage to the Convertor stage I would think that the designers would have shielded one of those tubes.
  
• Lead dress is important here. 
  
• I have often wondered about the disappearance of the markings on caps indicating "outside foil".  I have heard many reasons such as "The design of the cap eliminates the need" to "There is an outside foil but they are too cheap to mark it".  It may or may not be important in B+ bypass caps, but likely is important for coupling caps. In this radio, the only caps to worry about are C10, the one across the 50L6 plate to screen grid (outer foil to screen); C9, the audio interstage coupling cap (outer foil to plate of 1st AF); C5, the AVC filter cap (Outside foil to B- / Common Return) and C4 from the B- / Common Return to chassis (outside foil to chassis).  NOTE:  Ensure that C5 connects between AVC and B- / Common and not between AVC and Chassis!
  
• Does the frequency of the squeal change as you get closer to the desired station, especially does the frequency lower until you are dead on the frequency of the desired station?  Does the squeal ever go away or does it "waver" in low frequency when the station is properly tuned in?
  
• It is common for the pilot light in many AA5 or AA6 radios to be very bright upon first powerup.  That's why so many of them burn out.  This phenomenon is due to the fact that the tube heaters have a much lower resistance when cold than when hot.  For example, in a 150mA heater string which this, to get a 12V drop across a tube such as the 12AV6, 12BA6, or 12BE6, the hot resistance would be 80 Ohm.  You will find the cold resistance to be much lower than that.
  
• The NTC (Negative Temperature Coefficient, sometimes referred to as a "Globar" resistor) resistor, R13 on the schematic, is not used in many radios of this type. It was likely Stromberg Carlson's idea of protecting the tubes, pilot light bulb or both.  The pilot in many AA5 sets is across the tap of the heater of the 35Z5 or 35W4 rectifier.  The section of the 35Z5 / 35W4 heater that is in parallel with pilot lamp exhibits the same cold resistance that other heaters do, so the bulb is less likely to burn out than if the bulb is bridged by a regular resistor like in your set.  Therefore, the NTC resistor is needed even more so in this set to prevent burnout of the pilot.  These resistors do run hot.
  
• The placement of the NTC resistor in the middle of the string puzzles me.  It is always best to keep heater to cathode voltage differential as low as possible.  In its current location, the 6BA6 heater is at least 20 V higher than it could be if the NTC resistor was located "at the top" pf the string, closest to the "hot" AC lead, where the 35Z5 / 35W4 would be.  If I designed the set (and maybe thank God I didn't), I would have located the NTC resistor at "the top" of the heater string.
  
• Check the voltage across the filter caps.  If the voltage across the second filter is too high, this may affect the gain of the various stages and may initiate oscillation.  Try putting the selenium rectifier back in circuit, if it is good, to see if the issue goes away.  
  
• It is interesting that you state that when the tube voltages were low, that the squeal was not there.  Do you have a "Variac"?  If so, bring the voltage up to the point where the radio starts to work.  Note whether the squeal exists.  Continue raising the voltage till the squeal starts, note the voltage, then back down till the squeal stops.  Rinse and repeat.
  

Nowadays, AM reception is subject to noise that did not exist years ago.  This includes:

• Compact Fluorescent and LED lights
  
• Switching power supplies
  
• Computers, Wi-Fi, the "Internet of Things" (Wi-Fi thermostats, home control modules, smart speakers, etc.)
  
• Cellular service towers
  
• Remote read electric, gas and water meters
  
• Etc., etc., etc.