When I checked the Tactic site "Tacticrc.com" it says that the TX850 can be powered by two LiPo or LiFe cells. That is putting 8.4 volts on the receiver, it that true? The power for all the other receivers is listed as "4 alkaline or 4 Nixx cells". Can they also use up to 8.4 volts? Now I know that most servos will not like that high a voltage, I'm only asking about the receiver max allowable voltage without damage. Thanks for the info.

A 2S LiPo battery pack puts out 7.4V, not the 8.4V you mention. A 2S LiFe pack is 6.6V. As long as your servos can accept the voltage, a 2S LiPo or LIFE pack is OK.

All my LiPo packs put out 4.2 volts per cell when fully charged.

That may be the peak, but not the nominal voltage. If you're using 2S LIthium battery packs, you'll be OK.