Hot Switching Linear Amplifiers MUST be avoided!


Linear Amplifiers for Ham Radio come in many size, shapes and flavors.


Although nearly all modern linear amplifiers will interface seamlessly with any transceiver for running Semi-Break-In CW, very few are suitable for fast QSK CW.


If you attempt to run QSK with a non-QSK amplifier and you operate a lot of CW, you will burn its T/R relay contacts, unless you address the issue by adjusting the timing.


Many transceivers have adjustable parameters for delaying the initial "first dit", giving the amp's slower T/R relay time to switch before applying RF power.  Some also have adjustable post-dit delay ("hang time"), which keeps the amp's TR relay in TX mode a little longer, thus prehibiting it from trying to follow the CW keying.  READ YOUR TRANSCEIVER'S MANUAL.


If your transceiver does not provision for these two features, or if you do not adjust them correctly, you can potentially experience substantial/expensive damage to your amplifier's T/R relay.  And in CW, you will have Key Clicks!


The Ten-Tec ORIONs, OMNI VII, and Eagle all have the pre-dit delay (TXD), but they are fixed values:

  • 15mS for the ORION & OMNI VII
  • 17mS for the EAGLE 

In addition, the ORION and OMNI VII also have post-dit delay (hang) adjustable in a software menu.


The Elecraft K3/K3S also have fixed pre-dit delay (TXD); 5mS.  This is marginal and should be increased to 6mS.


The EAGLE does not have a post-dit hang-delay feature as standard, but it is available as an option.  You will need to purchase the Model 318 Amp Keyer.  You may also purchase one of several 3rd party devices that solve this problem.


As can be seen in the pictures below, the T/R relays inside various amplifiers came in 3 different styles with significant size, power-handling and especially speeds!



The Open-Frame relay has been used in amplifiers dating back to the days of the Drake L4B, Henry Amps, and Heathkit amps.  It was big, bulky, noisy, and very  s-l-o-w, but it could handle a decent amount of power, even with slightly higher SWR.


Many modern amplifiers now use much smaller plastic-enclosed relays.  As you see, they have tiny solder pins as connections.  Their 7mS is fast enough for safe QSK operation with all transceivers that offer at least the pre-dit delay.  These are quieter than the open-frame relays, but they obivously are not as robust.  Never-the-less, they can easily handle medium amounts of power (say, 1kW), as long as the SWR is not too high.


The non-plus-ultra is the vacuum relay.  It is the fastest switching relay suitable for linear amp use, handles high voltage and current, as seen by the size of the solder pins.  Unfortunately they are not cheap and found only in a few (expensive) of the very high power amplifiers.  They are are also very quiet. 


A 4th method is of course PIN-DIODE SWITCHING, which like the name says, uses diodes for switching between transmit and receive.  This is the fastest method, completely silent, but it also has some draw backs.  Some contesters swear by them but many refuse to use them due to potential IMD problems (on RX) and reliability issues.  In a Multi/Multi operation, with 6 legal-limit transmitters transmitting all the time, there is a lot of RF in the air.  This RF tends to partially forward-bias the PIN-Diodes, which causes Intermodulation Distortion (IMD) in the receiver.


Finally, there is a 5th method - clearly the safest method, but very few transceivers  or amplifiers use this.  It requires two keying lines:  Request to Send, and Enable to Send.  When a dit is keyed, before sending RF, the transmitter emits a Request to Send.  The amp's internal relay(s) (usually Vacuum Relays) switch, then after they have switched, the amp sends the Enable signal back to the transmitter.  The transmitter then begins sending RF.  


My Opinion on Ten-Tec Timings:

IMO: 17mS pre-dit delay is adequate for keying the small plastic relays and vacuum relays - no problem.


BUT:  17mS delay is NOT long enough for keying older open-frame T/R relays.

  • If you are a casual CW operator, you will probably get many years of service out of your amp and not experience any problems.  That's because the burning only occurs occasionally and the damage process will occur very slowly.
  • If you are an active contester and pushing your rig hard, the damage may become apparent after 3 or 4 weekends of intensive contesting.









Initially you don't know.  Everything seems to be working just fine.  Eventually, the amplifier will begin to cause intermittant desensitizing of the receiver.  You will switch back to receive but the antenna connection to the receiver will be through burned relay contacts.  This creates a high resistance connection, causing desensitizing of the receiver.


RECOVERY:  The first few times it happens, recovery is usually no problem.  Just key a single dit, the relay cycles and the problem "seems" to be gone.  If you address the issue immediately and clean the relay contacts with a relay contact "burnishing tool", you can usually restore everything to normal.  However it will happen again... and again.  Eventually the relay contacts will become pitted and then you have permanent damage.  When that happens, you should replace the relay.


3rd Party (External) Solutions Enabling Clean (non-Hot Switching) and Home Brew Solutions:


The information discussed above was pertaining to interfacing TEN-TEC transceivers to non-QSK amplifiers, using their built-in time-sequencing features.


In the case of the Eagle, you need to add post-dit delay (hang delay) for any amplifier using open-frame relays. Otherwise the amp's relay will attempt to follow the CW keyer and quickly burn its contacts.


  • IMO, if you are a heavy CW user, you will need an additional pre-dit delay of about 10mS for ALL TEN-TEC transceivers.
  • Until TEN-TEC increases this delay time in the firmware, this must be accomplished with an external circuit.

You have three options:

  1. Home Brew your own time-sequence keying circuit. You will find more information on this on the web site:
  2. Purchase an external keyer which incorporates these features and ALWAYS key your transceiver with the external keyer when running the amplifier.
    • See Below.
  3. Add a "Relay Accelerator" to your amplifier.  A relay accellerator is a simple device which momentarily incresase the switching voltage applied to your amp's relay.  It consists of 1 transistor and about 4 resistors and capacitors.   After it switches, it returns to the nominal voltages.




Unfortunately there are not a lot of keyers on the market to choose from.

I know of only two:

  • K1EL WinKeyer.  See: .  This unit is best managed by a computer (USB connection).  Parameters may be easily adjusted by a software menu.  You may also enter parameters via the paddle.  Advantages:  Interfaces with most contest logging software packages.  Assures best cw keying (timing) when keying via the logging software/computer.  Disadvantage:  difficult to make changes without the computer, except the speed is a front panel knob adjustment.
  • MFJ-495 Memory Keyer.  See: .  Comfortable stand-alone keyer.  Does not require a computer to controll its parameters.  Parameters are a front panel adjustment.  Disadvantage: its keying characteristics are not controlled by the logging software, which is usually OK when running "unassisted" class in ocntests.  When running "assisted" class, CW keying from the computer can get choppy in the presence of heaving spotting from the CW skimmers.


3rd Party Solutions Enabling FULL-QSK


(with non-QSK linear amplifiers)


Currently I am only aware of TWO:


1)  Ameritron QSK-5 QSK Switch; the QSK-5 switch comes in 2 flavors:

  1. Circuit board (only) for installing inside of Ameritron linear amplifiers
  2. External Accessory (with enclosure) for use with almost any linear amplifier

Both versions are electrically identical and use PIN-Diode switching technology, enabling almost any amplifier to be used together with a QSK-enabled transceiver to produce true, high power QSK.


The unit is fast and completely silent, but in my 20 years of awareness of the external accessory version, and having owned and used it with several different amplifiers, IMO it is not without issues.  Mine broke often; always the same thing.  It was easy enough to fix but breaking in the middle of a contest is very annoying.


Others I know who have used this or the internal version have had similar experience.


On the other hand, lots of people have reported good experience with these units.

Unfortunately they are not cheap; the external accessory version currently retails for $400.


2)  QSK-2500


This is a new design that interfaces any transceiver to any amplifier and performs all of the timing and switching including antenna switching inside of the QSK box. The only down side I see to this box is its $400 price tag, but it does what it is supposed to do and appears to be more reliable than the QSK-5 option above.





Buyer Beware!


ALL of the stuff described above (except the QSK-5) was assuming the linear amplifier uses a T/R relay that runs on 12 v.d.c.  That "can be" a very dangerous assumption.


OLDER LINEAR AMPLIFIERS often deployed T/R relays which ran on higher DC voltage or even on AC voltage.

These can damage some transceivers.

REASON:  Some transceivers key an external amp with a switching transistor (open collector circuit), others use a relay, and some have both options (switchable).  


Transceivers using the transistorized switching method, or running in the mode with this method selectod, can generally only key positive voltage with a maximum which may be anywhere from 24v, up to 60 or 70v.  READ THE MANUAL.

If you connect these to an older amp using AC voltage or higher DC voltage, you will kill the switching transistor in your transceiver.



Several companies offer "Amp Keyers" of one flavor or another.  Most of them address danger #1 above, but almost all of them do nothing to address the keying timing issues described in the top portion of this page.  Yet their marketing hype leads you to believe they can key anything.  

Well they can key anything but that does not mean that they can key it without hot-switching!  

In fact most of them CANNOT.



  • Ameritron ARB-704
  • TEN-TEC Model 318*
  • There is another one but I cannot remember who sells it.  I'll update this later.

 *The Model 318 completely addresses the "hang" issue, but it has no means of addressing the lead (pre-dit) delay.  Whether this will initially key the relay without hot-switching or not will depend on the pre-dit delay coming from the transceiver.  All late model TEN-TEC transceivers beginning with the Orion had 15mS of lead delay (Eagle has 17ms).  This may not be enough for many amplifiers. 




HOT SWITCHING is very dangerous for any amplifier.

Occasionally it even damages your transceiver.


If your amplifier is not a QSK amplifier, I suggest you operate Semi-BK and focus on assuring it operates without hot switching.


If you feel you must run QSK and your amp is not QSK-capable, I suggest you sell it and purchase a QSK-enabled amplifier.


IMO, gimmicks to make it run QSK are an expensive and potentially dangerous experiment.

Your mileage may vary.  (mine didn't).