Hi Squirrel Guy ,
When the manufacturers were first attempting transformer less radios , the tubes such as 50L6 and 35Z5 didn't exist yet . They had mostly 25 volt heater types such as the type 43 which was an output tube and the 25Z6 for a voltage doubling rectifier . Now , in the later "All American Five" sets , using 50L6 , 35Z5 , 12SA7 , 12SK7 , and 12SQ7 , the sum of the voltage drawn by the series connected heaters was 121 volts , just perfect for our 120 volt wall outlets . But the older , 25 volt heater tubes , usually used with 6 volt heater types such as 6A7 , 6K7 , 6Q7 , and then 43 and the 25Z6 only added up to 67 volts in series and thus needed some means of dropping the additional line voltage . Some sets used ballast resistors that looked somewhat like tubes , and some used a piece of resistance wire in the line cord to drop the difference and allow the tubes to draw their 67 volts . The cord would have three wires , one all of it's own to be one side of the line , one on the other side of the line that fed directly to the rectifier's plate , and the third one which was the resistance wire coming from the same plug blade as the one for the rectifier's plate . To most old radio guys , the term "curtain burner" refers ONLY to sets with the dropping resistance in the line cord , but some folks mistakenly seem to call ANY "AA5" set a curtain burner even the later ones with no dropping resistance needed at all .
Funny thing about the tube type ballasts was , that certain manufacturers got caught lying and calling them tubes (as in active receiving elements , not just glorified "dim bulbs" which the tubular ballasts were) and got in legal trouble over it . When tube count reigned supreme in the choice of which radio to buy , and one could easily up the tube count by a few by using some glass encased resistors , whose gonna notice , right ? Well , the other manufacturers who weren't falsely inflating their tube counts let the Govt in on the secret and a few went out of business .
Ya mean our generation DIDN'T invent corporate greed ?