[R-390] in rush current limiters

[ToddRoberts2001@aol.com]

<PRE>Some interesting points were brought up about unequal voltages occurring 
across two tubes with their filaments wired in series, either 2 - 12BA6's or 
2 - 6BA6's when  used in the R-390A BFO and PTO circuits. Unless the tube 
filaments have exactly the same cold resistance and the same hot resistance 
when running this unequal heating can occur - using a ballast tube or even a 
voltage or current regulator will not correct this problem of two tube 
filaments wired in series. I wonder if anyone else has tried the modification 
for the R-390A where the filament wiring for each of the BFO and PTO tubes is 
re-routed in parallel to the 6.3VAC filament buss used in the rest of the IF 
strip? It is not complicated to do and only involves unsoldering, re-routing  
and re-soldering a few leads in the underside of the IF strip.  No extra 
wiring leads are needed. I have done this mod. on an R-390A to eliminate the 
need for the 3TF7. The 3TF7 socket is left empty and is non-functional after 
this mod. The receiver works fine and this insures the exact same 6.3VAC 
across the BFO and PTO tubes and retains the original 6BA6's for both. I 
confirmed the receiver stability by varying the line voltage + and - several 
volts with a variac. When tuned in to an AM broadcast station with the BFO 
turned on I could not detect any change in pitch when varying the line 
voltage several volts up and down to simulate line-voltage fluctuations. 73 
Todd Roberts WD4NGG.