[R-390] A ballast tube primer/Tube Class 101 for beginners

[Llgpt at aol.com]

I wrote this over five years ago. Since that time many newcomers have asked  
about replacements for the 3TF7. The 3TF4 seems to always come up as a  
replacement. 
 
Rather than beat the deadest horse that has ever been beaten into even  
tinier shreds, I offer this tid bit for those who know not of what they  
speak.........
 
1. Ballast tubes have two ratings, a voltage range where current regulation  
takes place and the regulated voltage.
 
3TF7 8.6 - 16.6 volts, 200 - 300 milliamps
 
3TF4 4.3 - 8.3 volts   280 - 320 milliamps
 
2. If you substitute a 3TF4, it will be operated far beyond its recommended  
operating voltage 
    rating, and the two filaments it regulates will operate  beyond their 
recommended or  
    maximum voltage ratings.
 
3. Sure, it will work, but rather than replacing a 3TF7 with an improper  
tube, substitute one of
    the other modifications which all work quite well.
 
Les Locklear