By Dave Johnstone, GM4EVS

Linear Amplifier Switching
Even today, many older linear amplifiers with high-voltage keying lines are still in use. Though transceivers may have internal relays capable of keying such amplifiers, solid-state switching can offer better performance.

Your transceiver
Many radios are fitted with a relay for linear switching, with a pair of normally-open contacts presented on the rear panel. In addition, modern radios typically offer either an open-collector output which is switched low on transmit, or an output rail that goes from 0 to 12 volts on transmit.

Before using any of these connection options, note their voltage and current limitations by referring to your radio's manual. Many are intended for light-duty switching only.

Your linear amplifier
Modern amplifiers typically present low positive voltages (often <12 volts) on their keying lines, and during transmit, currents can be as low as 10-20mA. Such amplifiers can usually be keyed by direct connection to modern radios, but do check the voltages and currents carefully in advance.

In considering older amplifiers, these may present 100-150 volts on their keying lines, either positive or negative. For example, a Kenwood TL-922 presents some +120 volts on its keying line, while a Heathkit SB-200 presents some -135 volts. The linear-keying relay in your radio may be suitable for keying such amplifiers, but as noted before, check the voltages and currents.

Though relay switching may work, it can create additional noise. And it may be slow which risks hot-switching ie, the linear relays may not be fully-settled into their transmit position before RF arrives from the transceiver. This may lead to damaged relay contacts, as well as a distorted transmission.

A better way?
To overcome these issues, avoid using the keying relay in your radio altogether. And where possible, disable the relay so eliminating its clicking noise during changeover - some radios provide a switch for this purpose.

Instead, to switch your amplifier, use the low-duty keying rail provided by your radio coupled via a suitable high-voltage, solid-state, keying interface.

Two designs are presented here. One for high positive voltages, and one for high negative voltages. The schematics include design notes describing basic circuit operation. The transistors used are low-cost items readily available on the web.

NB To render the Scaleable Vector Graphics [SVG] files below, you will need the Mozilla Firefox browser. IE shows only the text in the file. I hope you find these circuits, or similar adaptations, useful for quickly and quietly switching your old amplifier.

Please remember, you use these designs at your own risk. And, as ever, take great care when working with high voltages.

73 and good luck
Dave, GM4EVS