[RJMiranda]

Yes, I know it is AC. When I said the voltage from the transformer to the grid was negative, I meant that during the half-wave period that the plate is made positive, the grid gets the negative-going half-cycle from the transformer, so the tube does not fire without RF input from the control box. That´s why I wrote about a possible connection reversal somewhere that sent the positive-going half-cycle to the grid.
I know it is not a likely condition, but I have found several puzzling sets when every component seems to be OK but nothing works. 30 years back got a EV-300 Sony R2R VTR that didn´t play video. I swapped everything with a good VTR: heads, PB preamplifier PCB, PB booster PCB, demod PCB; and everything worked on the other machine, and the boards from the good one wouldn´t work in the defective VTR. Crazy! (Only thing I didn´t swap were the rotary transformers, because it is very difficult to do). Eventually I found that somebody had reversed the wires that connected the PB preamplifier to the playback booster, (in this VTR there were no connectors, everything was soldered), so the ch-A head signal came thru the ch-B circuit while it was being muted, and viceversa.
I think dtvmcdonald has a very good point, and it is really very unlikely that any cables are reversed. Just say, don´t overlook that possibility.