[R-390] 3TF4 Ballastube

[Drew Papanek]

Brian wrote:

>hello to everyone happy xmas. have just fitted atf4 ballast tube in 390a
>and it seems to be working fine.it glows a little more than a atf7. can
>anyone tell me will this give me any problems. many thanks brian

I believe you are referring to type 3TF4 and 3TF7 ballastubes.

Both are specified to regulate current to 300 mA plus or minus a few tens of 
milliamps.

The  3TF7 regulates with a voltage drop of from 8.6 to 16.6 volts.
The  3TF4 regulates with a voltage drop of from 4.3 to 8.3 volts.

In the R-390x application the supply voltage for the PTO and BFO tubes is 
nominally 25.2 VAC.  The two 6BA6 heaters fed with 300 mA drop 12.6 volts 
and the 3TF7 drops nominally 12.6 volts, operating somewhere in the center 
of its voltage range.

When installed in the R390x the 3TF4 would operate outside of its specified 
voltage range; it would pass more than 300 mA and its life and the life of 
the PTO and BFO tubes would be reduced.

I assume you are using the 3TF4 for lack of a 3TF7.  Many other ballast 
substitution schemes abound.
One entails jumpering the ballastube socket  (a short piece of wire inserted 
into pin contacts 2 and 7) and then using 12BA6's to replace the PTO and BFO 
6BA6's.   Another is to replace the ballastube with  a resistor of about 42 
ohms, 5 watts.

The possibilities are myriad.  Wei-i Li helps all of us by distilling the 
information coursing through this forum into his highly enjoyable work, 
"Pearls of Wisdom".  To receive these Pearls from Heaven, goto r-390a.net    
   Click on "References", "Pearls of Wisdom", "Ballast Tube".  Be amazed, be 
very amazed.

I love the smell of Dead Horse in the Morning!   It smells like.....Victory!

Drew