[Boatanchors] When to use neutralizer capacitor

[John Saxon]

Excuse my ignorance here, but I am in need of some consulting.
I would like to build a 6AG7-6146 transmitter.  As I research around I see some 6146 PAs that have the neutralizing capacitor and some do not.  So I have not been able to figure out when a neutralizing cap must be used.
I am guessing that the following are possible factors:1. grid-plate capacitance of PA tube2. PA on same frequency of OSC, or not3. Whether or not there is a tank circuit in the OSC
So I would appreciate some comments from you gurus out there, or perhaps a pointer to a good description.  I have been doing reading, but am still confused (not the first time that has happened).

Any and all information greatly appreciated,JohnK5ENQ