Hi, guys. Every solid state amplifier I have ever owned (CB/10, 6 and 2 meters, powered by 12VDC) have all been the RF sensing type...just key the mic. and the amplifier engages. I have noticed that the vast majority of amateur radio amplifiers like Ameritron, Henry, Heathkit, etc., don't have RF sensing, but require a keying line to activate the amp., usually with a foot pedal. Why don't they make solid state amplifiers with a keying line (when using them at home, NOT driving, haha)? I have heard that RF sensing is bad on your relay. How easy is it to modify a solid state amp. to use a keying line? If I have used certain terms correctly, my apologies. Thanks for your help.
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Amplifier keying line on solid state mobile amplifiers?
- Thread starter ahh
- Start date
cb amp builders use an rf sensing circuit because its cb radio and most end users dont know a from b. in reality its hard on the transceiver and relay due to whats called hot switching. a radio will key and send rf a few milliseconds before the amplifier keys thus the radio is transmitting into an extremely high vswr due to a no load situation because of the slow relay reaction time. this is very hard on solid state rf components and relays. the switching delay is very noticeable on ssb and cw modes and somewhat on modes like am/fm. a dedicated switching line allows the radio to key the amp a few milliseconds before the rf is sent thus eliminating hot switching. rf sensing sucks and a keying line is the correct way to operate the amp keying.
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Some old timers here in town used to use sewing machine foot pedals to key their old tube amplifiers. They had to key the mic on the radio and as they started to talk they would put the pedal down if they needed mopowa.
As an example the one guy who passed away several years ago ran a Johnson Viking Valiant radio and then his big tube amp was on a foot pedal.
As an example the one guy who passed away several years ago ran a Johnson Viking Valiant radio and then his big tube amp was on a foot pedal.
There are two dividing lines here. First is that a multiband ham radio includes the external-amp keying feature because it's legal, and handy.
A legal CB can't be used with an external amplifier, so including a way to key it in the radio would never get out the door with FCC approval. We sell a kit to add this feature to your (cough) 10-meter ham radio on Ebay. It's a far smoother setup if you fancy sideband.
The other dividing line is wattage. The receive-side contact points on your amplifier's internal relay will have RF power on them as the points draw apart. This draws an arc. A low-power radio won't draw much of an arc. A 100-Watt radio draws a bigger one. And a bigger drive level than that draws a bigger arc. This leads to problems with the receive signal dropping out unless you worry the key a time or two. Arc damage to the receive-side contact points will prompt complaints that the amplifier's relay is "sticking". Seems the only word people apply to this when it happens.
The bigger the drive level the more you'll gain from a hard-key relay in the radio. Gives the relay a head start before the RF drive fully cranks up.
A foot switch is a kluge. Some folks just like their kluge.
Had one customer who was diabetic and his "keying" foot would get numb and slip off the switch. As soon as he spotted the wattmeter fall, he'd get right back on it and key the amplifier. Every time he showed up to get a "sticking" relay fixed I would offer to install a relay in the radio, and prevent this nuisance. Said he would rather hire me to replace the linear's relay once a year, just didn't want to give up his foot switch.
73
A legal CB can't be used with an external amplifier, so including a way to key it in the radio would never get out the door with FCC approval. We sell a kit to add this feature to your (cough) 10-meter ham radio on Ebay. It's a far smoother setup if you fancy sideband.
The other dividing line is wattage. The receive-side contact points on your amplifier's internal relay will have RF power on them as the points draw apart. This draws an arc. A low-power radio won't draw much of an arc. A 100-Watt radio draws a bigger one. And a bigger drive level than that draws a bigger arc. This leads to problems with the receive signal dropping out unless you worry the key a time or two. Arc damage to the receive-side contact points will prompt complaints that the amplifier's relay is "sticking". Seems the only word people apply to this when it happens.
The bigger the drive level the more you'll gain from a hard-key relay in the radio. Gives the relay a head start before the RF drive fully cranks up.
A foot switch is a kluge. Some folks just like their kluge.
Had one customer who was diabetic and his "keying" foot would get numb and slip off the switch. As soon as he spotted the wattmeter fall, he'd get right back on it and key the amplifier. Every time he showed up to get a "sticking" relay fixed I would offer to install a relay in the radio, and prevent this nuisance. Said he would rather hire me to replace the linear's relay once a year, just didn't want to give up his foot switch.
73
I think the previous answers cover the reason, in my case, I do mods to amplifiers without RF sensors and add to them a switch with the RF sensor, instead controlling the main relay, the installed RF sensor controls the ground needed in the key circuit of the ham amplifier, works fine, I seen people with radios with broken key circuit that cost more to repair than to add the circuit on the amplifier, but I honestly prefer the key circuit, at least you have an RF option for those radios without key circuit.