[R-390] Ballast Tube Thread

[JamesMiller]

>From page 15 section 4.2.2.1 AC Power Units of Final Engineering Report on
Radio Receivers R-389 and R-390 Sept. 15, 1953:

http://www.r-390a.net/faq-eng-r3.pdf

"The filament regulation circuit for the oscillator filaments should also be
covered here although this ballast tube is mounted on the IF unit.
Considerable work was done with Amperite Corp. in designing this special
ballast tube which feeds a constant 300 mils to the two 6 volt 300 mil
oscillator tubes used in the VFO and BFO. These three tubes are connected in
series across the 26 volt filament supply. The ballast tube (3TF7) operates
on a current of 290 to 330 mils and holds this current within ±10 mils for
input voltage variations of ±15%. This reduced the 15% variation to
approximately 3%. There is some question if a filament regulator is
necessary in these receivers since the oscillators are very good even
without regulation. However, since the stability was a big factor in this
design and since the factor of tube aging was not known, the regulator was
included."