All 12AX7 Tubes are NOT Equal - Some are crap!

[Joseph Bento]

I've been ripping my hair out trying to get a circuit to perform 
properly.  Seems the culprit was brand new Chinese manufacture (J&J 
brand) tubes - 12AX7 and 12AU7.  My circuit uses both. Noise, feedback, 
microphonic... any undesirable characteristic was there!  Tried some old 
1950's RCA 12AX7 and 12AU7 and everything worked perfectly.  Tried a 
different manufacture Chinese 12AX7 that I've had for a few years, and 
all is well - a bit hotter than the old RCA.  I actually sub'd a 12AX7 
(another older Chinese manufacture) for the 12AU7 and am happy with the 
result.

Now why am I playing with tubes?  The answer will be revealed when we 
gather for Hamvention.

Joe, N6DGY