[Elecraft] Keying a Hercules from K2

[G3VVT at aol.com]

It would be possible to do this switching with the K2 Amplifier Keying  
Circuit in Elecraft application notes and adding a PNP transistor and  a resistor 
to provide the correct switching. This could be installed  external to the K2 
possibly at the TenTec Hercules amplifier keying input.
 
We used a similar circuit when constructing VHF/UHF repeaters from surplus  
two way radios which use a +ve PTT keying circuit. The NPN output  transistor 
in the K2 keying circuit would be connected to the base of  the extra PNP 
transistor via a 4.7K resistor, the emitter of which is connected  to the necessary 
+ve line and the collector goes to your +ve keying line in the  amplifier. 
The method of operation is when the keying output from the K2  goes to ground 
this biases on the PNP transistor that connects the keying  circuit in the 
TenTec amp to 12V +ve or whatever you use. This transistor  provides the isolation 
between the different voltages of the K2 (6V) and the  TenTec (12V) in use in 
the respective keying circuits.
 
If felt necessary you could add a pull up resistor of say 10K on the  K2 
keying output, though we never found this necessary and the prototype  repeater 
that used this circuit has been running on a hill top for many  years.
 
Regards,
Bob, G3VVT
K2 #4168