[Elecraft] K3 Two amplifiers 1 key line

[Don Wilhelm]

Lee,

You should be able to provide control lines for HexFETs or some other 
kind of switch with just a little bit of logic and the signals from the 
ACC connector.

Look at the Band Data outputs - the only time *both* Band3 and Band1 are 
at a high impedance state is on 6 meters.  A NAND gate (with pullup 
resistors on the inputs) will give you a LOW output on 6 meters and HIGH 
on all other bands - call this 'partial band decode'.
Now add an inverter to the KEYOUT-LP signal (another pullup resistor is 
needed at its input) and feed its output to one input each of 2 more 
NAND gates - the 'partial band decode' NAND gate output feeds the 
remaining input of one of these NAND gates and that one will have a low 
output when HF is selected *and* Keyout is active.
Invert the output of the 'partial band decode' and feed the inverted 
output to the input of the other NAND gate that you connected the 
inverted Keyout signal to - that NAND will have a LOW output when 6 
meters is selected *and* keyout is active.
I don't know how much voltage and current your amps PTT inputs require, 
so I leave the final keying of the amps up to you.  A pair of the K2 Amp 
Keying Circuit on the Elecraft website or one of Tom Hammond's K2 Amp 
Keying kits can be driven by the circuit I described and be used to key 
your amplifiers.  A 74AC00 dip and a 74AC04 will do this task with one 
NAND gate and 4 inverters to spare - it takes longer to describe in 
words than to draw it.

To be safe, I would not consider letting the RF path determine which amp 
transmits - if you ever find the ANT1/ANT2 selection changed (by an 
inadvertent button tap), you could send RF to the wrong amp.  You may 
not be able to simply run the KEYOUT to both amplifiers - that will only 
work if both amplifiers have the same voltage on their PTT IN lines 
(measure with your DMM with the amplifier on and nothing connected to 
the PTT IN jack).

73,
Don W3FPR

Lee (WW2DX) wrote:
> I have a HF amp on ANT1 and (soon to be) 6M amp on ANT2. ANT2 is 
> dedicated 6M antenna.
>
> Has anyone come up with a way to PTT each amp when on desired band? Is 
> it possible to just hook both AMPS to the same PTT key line so both 
> amps  Key at the same time but the RF path would then take over?
>