[R-390] bad tubes? test good

[wb3fau55 at neo.rr.com]

Here is my recent similar problem.  I had  just reworked an SP-600.  Working fine.  Then, after using it  for a few days,  it starts to drift badly.  Put my hand near the HF osc tube, a 6C4,  it walks  even more.  Tube tested  fine.  I pulled it, and put in a new 6C4, and added a black IERC tube shield,  been fine ever since.  As stated,  you cannot judge  a tube  by good action in a tube checker. [TV-7D]  Russ.