[R-390] 3TF7 Tube Shield?

[Norman Ryan]

 Hi, all,
David has the right idea  --  the 3TF7 shield provides physical protection.  

Avoid placing a heat dissipating shield over the ballast tube as said shield's cooling effect slightly lowers the ballast's resistance (and the desired voltage drop), thus causing filament voltage to increase.  Naturally, excessive filament voltage will shorten tube life.  

As things stand, it's probable that many R-390*s run on higher line voltage than the nominal 115 VAC.
73Norman, KG4SWM

    On Friday, May 31, 2019, 11:28:51 AM EDT, David Wise <David_Wise at Phoenix.com> wrote:  
 
 The retainer with cutouts pictured in the auction is what my R-390A came with.  There is no need to shield it, just physical retention.

Dave Wise
SWL in Hillsboro Oregon