Linear Connections August 16th, 2013
We are getting a lot of enquiries about HF linear amplifiers and the correct way to connect them up. This topic is obviously causing headaches to some potential purchasers.
The first thing to be aware of is that an amplifier produces a lot more power than you may be used to. You need to take precautions. Make sure all coax cables are adequate, Don’t use thin RG58 and make sure that your VSWR meter can handle the power.
I suppose the main issue is what leads do I need to connect it to my transceiver? Well the answer to that is just two leads. One for the PTT line and the other for connecting to the RF output from the transceiver. And if you are using a solid state amplifier, do make sure that the output of your transceiver is connected to the input of your amplifier and NOT the output socket. Make this mistake and you could cause serious damage. So what about the ALC connection?
We don’t advocate making such a connection as it can cause more trouble than it solves. Most modern transceivers have good ALC control and most now have an independent power out control that is quite separate from the ALC circuit. In the old days, the ALC was designed to limit drive to a safe transceiver PA power level. It only operated when maximum power was approached on speech peaks. The Elecraft K3 is a good example of how a modern transceiver copes with power output.
You can set the K3 mic gain and compression up to a level which provides adequate ALC action and you can see this on the transceiver metering. That level of ALC holds good no matter what output power setting you make. So you can set the power output to say 10W and no matter how loud you speak, the power out will always limit at 10W. So with this arrangement there is absolutely no need for any ALC action from the linear. It will always be receiving a constant maximum power set by the transceiver, which in turn will dictate the maximum linear power output. It is that simple. But you do need to check your particular transceiver. That can easily be done by setting the mic gain for ALC action and just making sure that the ALC is working at different power output levels. Peter G3OJV