Using External Amplifiers with a FLEX-6000 Transceiver – Part 1
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Using External Amplifiers with a FLEX-6000 Transceiver - Part 1

By Dan Quigley, N7HQ

The purpose of this article is to address some common conceptual errors amateur radio operators make when interfacing transceivers and external RF power amplifiers.  We will illustrate three of those misconceptions and provide basic amplifier interface guidance when the transceiver is a FLEX-6000 product. Interfaces to band data, ALC, matching, or antenna switching systems, and a discussion of power-backoff settings for splatter-free amplitude modulated digital signals like PSK-31 and CODEC2 are planned for Part 2.

Improperly interfaced external RF amplifiers can (and usually will):

  • Damage the keying circuit in the transceiver.
  • Damage the keying circuit in the amplifier or T/R relay contacts.
  • Damage amplifier electronics, filters, tube(s) or transistor(s).
  • Cause unwanted distortion products (IMD) and/or splatter.
  • Cause key clicks (when working CW).
  • Any combination of the above and more.

First Common Misconception:

You can drive any amplifier directly with a 100W transceiver. 

Second Common Misconception:

There is an amplifier interface standard. 

Third Common Misconception:

You are done interfacing when you can key your amplifier and transmit.

FlexRadio SmartSDRTM software provides features that simplify managing amplifier switching interface requirements effectively, so you can quickly and efficiently change them on the fly – fine-tuning your system to match how you are currently operating. 

The Basic Amplifier Interface

Getting the basic interface – the drive, amplifier TX-keying, and timing, right is the first step, no matter the complexity of the system. 

PGXL owners using FLEX-6000 transceivers need not worry about any of this since it is integrated and keyed over a networked or a direct Ethernet connection.


The output of a FLEX-6000 transceiver is calibrated at the factory or service lab to produce 100W when the output transistors are warmed to 45C.  Thus, when a FLEX-6000 transceiver is cold, you can see more than 100W when the SmartSDR RF (or Tune) Power control is set to 100(%). Similarly, when the output devices are warm, you can measure less than 100W.  The value of RF Power control on FLEX-6000 transceivers does not correlate to a precise output power level.  

Overdriving amplifiers will result in excessive IMD causing splatter and distortion in your signal and negative reports from other operators. Provided you keep an eye on the output power of the amplifier, and keep it below the maximum rated output, adjusting the drive level using SmartSDR is generally an easy task. 

Please keep in mind most amplifiers sold into the amateur radio market are designed to operate at their full power rating when driven by a 100W transceiver, however, the actual drive level required to deliver full amplifier output varies by model.

Some popular amplifiers intended for low-power (QRP) transceivers are not recommended for use with a FLEX-6000 transceiver unless an appropriate power attenuator (usually requiring additional T/R switching) is used between the transceiver and the amplifier.  Without an attenuator, you may be required to set your RF output control to a value of 1 or 2, providing no means for adjustment and greatly increasing the risk of damaging the amplifier. If you intend to interface one of these amplifiers, be sure to select or build an attenuator that can handle 100W. 

Currently, external RF amplifiers sold in the US for the amateur radio service are limited to 15dB gain. Doing the math, for a 1500W amplifier with 15dB gain (like the PGXL), results in an RF drive requirement of about 50W to achieve the legal limit. The actual drive level required for full rated output can also vary per band.  Solid-state amplifiers, for example, can require more drive at the low-end (160m) and high-end (6m) of their rated frequency range.

The Max Power setting on the TX tab (see Figure 1 below) of the Radio Settings dialog is a global setting, it is not persisted per TX profile. You can use that to ensure you don’t inadvertently raise FLEX-6000 output power beyond a level that exceeds your rated or legal limits. 


Most modern amplifiers use keying circuits that switch ~12vdc to ground. Some are very simple analog relay circuits but most have their own low-current internal switching circuitry which is typically switched at 12vdc and is directly compatible with FLEX-6000 radios.  

FLEX-6000 radios have multiple keying outputs, most have four. They are open-drain outputs suitable for driving relays or logic inputs directly. The load or logic to be driven should appear as a pull-up to some positive voltage source.

Open circuit voltage should not exceed +40 Volts DC. Closed-circuit current is 150 mA continuous max, and 500 mA peak max. There is the equivalent of a +45 Volt Zener across the output to ground that will clamp any positive-going transient peaks above +45 volts. 

Some older, and still popular TUBE amplifiers like Heathkits and the Kenwood TL-922, switch at a much higher voltage (on the order of -120vdc to -140vdc), some require a positive voltage and some even use 115vac with their relays. These types of amplifiers CANNOT be directly switched with FLEX-6000 radios and you will need to provide a keying buffer relay designed for the task. It is essential to check the manual of your amplifier to determine the keying voltage and current. 

A common solution that has been used by many FlexRadio users to the aforementioned issue with high voltages is to use a keying buffer relay like the ARB-704 amp to transceiver interface. It should be noted that some operators have opted to use a keying buffer with amps that have lower keying voltage lines as “insurance” to prevent any possible damage to the exciter due to possible transient voltage spikes or RF ingress.


The simple truth is transceivers should not begin sending RF before the amplifier has completely switched into TX mode, no matter how fast or slow the amplifier can switch and settle. Likewise, the relay should not return to RX mode until the transceiver has ceased sending RF to it. FLEX-6000 series radios provide several adjustments that are managed by our TX Profiles feature accessible on the TX tab of the Radio Setup dialog. 

Settle (Bounce) Time

Figure 1. Smart SDR Radio Setup TX Tab.

Different amps use different types of relays with very different switching speeds.  Without accounting for the complete switching time, you cannot begin transmitting RF before the amplifier’s relay has had time enough to switch and settle (bounce time).  Bounce time is the intermittent switching phenomenon that occurs due to the collision between the movable metal parts or contacts when a relay is operated or released.

Typical switching time values are about 7ms for vacuum relays, 10ms-12ms for fast non-vacuum relays, and 20ms for open-frame relays (still used in many popular amplifiers unless a QSK option is installed). Bounce time can be as much as 2ms – again depending on the type of relay used. So if you are using an amplifier with an open-frame relay, your TX Delay should be at least 22 ms.


ModelOutputsRCA TXACC TX
63002TXPin 11
6400/67004TX1, TX2, TX3Pin 11
6400/6600/M4Out1, Out2, Out3Pin 11
Figure 2. FLEX-6000 PTT Output Table

Figure 2. FLEX-6000 PTT Output Table

  • All outputs follow the FLEX-6000 open-drain output specification (see text above).
  • ACC TX output is accessible on the DB15 connector at the back of the radio. 
  • The RCA (TX1-3/Out1-3) outputs are RCA jacks on the back of the radio.
  • TX Delay defines the total amount of time between PTT depress and RF being sent out the antenna port to your amp. TX Delay is always set long enough to allow for safe switching.  How long that is, depends on the amplifier, and other switching requirements. It is VERY IMPORTANT to understand your switching requirements.


You can use the FLEX-6000 timings capability to define a switching sequence for your devices. 

Consider a 6-meter operation scenario, where you might want to engage a relay that shuts off a preamp on a receiving antenna a few ms before it engages the amp. So you might connect the preamp bypass keying circuit to TX1 with a timing value of 0 and connect the amp PTT to TX2 with a value of 20ms. The TX Delay is set to 30ms here to allow 10ms for the amplifier to switch.

In this scenario, once PTT is pressed, the preamp would shut off right away, the amplifier receives it’s PTT signal 20ms later and 10ms after that the radio emits RF. Finally, the PTT release is automatic, FLEX-6000 radios will always stop transmitting before the output assertions are dropped.

Transmission modes, operating conditions, preferences, and objectives can also challenge your interface timing requirements. For example, if you are competing in a contest or not, operating a remote station, using VOX in a voice mode, or desire semi or full (QSK)  in CW, interface timing and related configuration parameters can be (and is often) very important. 

Many hams deploy complex systems with multiple amplifiers to cover different bands. They often require separate keying (with different timing) and drive circuits to control them.

Since all of these timing values are stored in each TX Profile, SmartSDR provides an easy way to set, save and recall them as you change up your operation.

TX Band Settings

FLEX-6000 radios allow you to enable keying outputs per band.  This is useful to avoid sending PTT signals to equipment that is not currently being used, or when the switching requirements change for the current band. The above scenario is enabled in the graphic below and would allow the operator to switch an external HF amplifier connected to TX3 or the ACC TX output using a separate TX profile.

Figure 3. SmartSDR TX Band Settings Dialog


At the most basic level, a proper amplifier interface prevents damage, spurious signals, and optimizes system performance. FLEX-6000 radios, unlike many transceivers, are designed for easy integration and provide a robust set of features that protect your investments in equipment and allow you to easily manage their use.

The FLEX Power Genius XL (PGXL) amplifier is by far the easiest of all amplifiers to interface, requiring only AC power, an ethernet cable and coax from a FLEX-6000 radio for seamless, plug-and-play operation.

Please take the time to research the switching requirements of all equipment that interfaces with your FLEX-6000 radios and, for amplifiers, apply the guidance in these articles. Hams with expertise are available to answer your questions in the FlexRadio community forum and our helpdesk and service departments.

Category:FLEX-6000 Series The Flex Insider
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