[R-390] RE: More BallasTube Noise

[Drew Papanek]

Brian wrote:

>hi, just as a matter of interest about 7 weeks ago i purchased on uk ebay
>2 3tf4 ballast tubes both are nos for $18 the pair. i put one in my 390a
>whicth i use most days and so far it is fine, sure it glows alittle more
>red but is this another and cheaper way to keep the 390a running.

The 3TF4 used in that way will operate above its upper voltage limit with 
attendant shortened life.
One could insert resistance in series to reduce dissipation;  the value 
would range from 15 ohms to 28 ohms.  Dissipation would range from about a 
watt and a half to two and half watts.

Alternatively, a diode could be inserted in series with the 3TF4.  The 25.2v 
secondary when half wave rectified  would have an RMS value of 17.8v, 12.6V 
would be dropped by the 6BA6 heaters and the 3TF4 would see 5.2v.

The BallasTube can be dispensed with entirely.  Substitution schemes abound. 
  Goto r-390a.net    click on References, Pearls of Wisdom, Ballast Tube.  
There you will find out more than you ever wanted to know.

Drew