Flex 6400 keying an Alpha 76a

This is a copy/paste directly from the 6x00 hardware manual:
Δ – The TX Relay outputs are designed to handle signaling levels of up to +40 VDC @ 140 mA maximum. Some amplifiers do not have circuits to prevent keying voltage transients and older amplifiers may exceed the maximum voltage level resulting in damage to the radio if directly connected. Verify the voltage/current on your external device before connecting to TX Relay 1-3. FlexRadio highly recommends the use of a use a buffer/isolator box between the radio and the external device as a best operating practice regardless of the keying voltage.

So, yes, you can key it directly if after looking at the keying pulse directly with an oscilloscope you determine there are no keying spikes from a relay coil field collapsing. 

73,

Mack
W4AX
Alpha Team