Both of those testers are garbage. Get a good Beltron or Sencore CR-70 or CR-7000. I use both, the Sencore when I need to thoroughly test a tube, the Beltron for a quick check (I leave it in my car) or for a gentle rejuv cycle.
Assuming you have trashed what may have been left of the 17GP4's cathode (which is very likely) you'll need to find another. The Early Television Museum lists one for sale from their inventory of NOS and good CRTs:
http://www.earlytelevision.org/crts_for_sale_b-w.html
In any case, you simply cannot rely on a CRT tester. Commit this to memory: the best CRT tester is a working set. I've gotten good pictures out of tubes with supposedly low emission, bad cutoff, imbalanced guns (in a tricolor kinescope), etc. The 17GP4 isn't too terribly rare, so little harm was done. Before you smoke something harder to find though, PLEASE replace that Hickok and the Lectrotech.....