I ordered a matched pair of Svetlana 572B tubes. (Russian)  They are in the Heathkit SB-200 and I think I will keep them there for a while and see how they hold up.  So far they are doing fantastic.  I now have the pair that was in the amp, Sylvania (USA), the Shuguang 572B's  (China) and the Svetlanas (Russia).  So I should be set for a while all though I'm still weary about the Chinese tubes.

At some point in the future, I will attempt a homebrew Class C plate modulated AM transmitter with transmitting tubes.  I am doing a mental excersize about using 572B's for this.  If someone has a copy of the RCA Data Sheet for 572B's I would LOVE to see it!  I do have the RCA data sheet for 811A tube.  I could use those specs for a very conservative design using the 572B's at 811A specs.  Anyway who knows what I will actually do when the time comes (a few years or less maybe?).

73 from KX5JT