[R-390] NEED HELP R-390

[Mark Huss]

David is right.  I repeated Dallas Lankford's work and found that the 
receiver PTO is much more stable with a 42 Ohm resistor than the 
origional 3TF7.  Unless you have AC power that varies more than about 7 
volts, use a 25 watt 42 ohm resistor.  It generates some heat, but no 
more than the origional 3TF7.  And if you have a fetish about drift of a 
few hertz, use a Voltage Regulator and a good heat-sink.  You have to 
add a ground wire to the socket, but it was staple to the point that it 
was unmeasurable on my equipment.
I tried the 12BA6 with a 1 ohm resistor in series to measure current 
rush, and saw quite a spike in current on turn-on.  Makes sense as 
filiment resistance goes up as the filliment heats up.  Enough that I 
decided to stick with the resistor to keep the filliment surge down.  It 
helps, I noticed that the PTO takes about two seconds longer to come up 
than normal.
    If anybody is interested, I did the design work up to Breadboard for 
a plug-in Voltage Regulator.  it is designed for 40Volt Peak input 
voltage and 80 degrees C. ambiant at 20% overcurrent.

David C. Hallam wrote:

>Why replace the ballast tube at all?  Dallas Lankford's work showed that the
>ballast tube didn't do a very good job of stabilizing the PTO filament
>voltage anyway.  The resistor or 12BA6 substitutions require no
>modifications and do just as good or better job.  Besides they cost less
>than the $20-$30 you have to pay for a new ballast tube.  Unless a person is
>a fanatic about original condition, I don't see any reason to keep the
>ballast tube.
>
>David C. Hallam
>KC2JD
>
>  
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