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Amplifier Control Board

This is the Swiss Army Knife of control boards. While building the kilowatt 2 meter LDMOS amplifier, I thought it would be nice to add a couple of extra functions I usually didn't include in my own amplifiers...a high-temperature lockout in case the amp ever gets too hot, as well as a way to block RF drive while the antenna relay(s) switch over (this protects the input relay). My regular control board (version 5) didn't have these extra features, so I made a couple smaller boards to go along with it to add these things in. These extras worked very well, so it seemed logical to add them into the next revision (this is version 6). Here's the complete feature set for this rev:

  • On-board sequencer for control of antenna relays, amplifier bias and power supply feeds, and driver RF hold-off
  • High VSWR lockout that will accept either a negative or a positive trigger signal
  • High temperature lockout that will bypass an overheated amplifier until it cools down to a safe temperature
  • Temperature-sensitive fan control for normal cooling
  • Driver RF blocking during antenna relay changeovers
  • Internal regulator, which allows operation from power sources ranging from 12v to 50v
  • LED indicator feeds for power, transmit, SWR lock and High Temperature lock
  • Can be used as a controller for a mast-mounted LNA

There's a lot going on there, so let's examine how these features are implemented, one function at a time:

The Sequencer

The sequencer controls the timing of almost all control board functions; the typical timing for an amplifier control circuit goes like this: upon receiving the transmit command from the driver (PTT), do the following:

  1. Event 1 - While holding off RF output from the driver, switch the antenna relays over, and turn on the cooling fan(s)
  2. Event 2 - 50 milliseconds after event 1, turn on amplifier bias and/or enable main power to the amplifier
  3. Event 3 - 50 milliseconds after event 2, enable RF output from the driver

Upon switching back to receive, these events are performed in reverse order, spaced apart briefly in time (50ms).

The circuit that does this is fairly straightforward; the PTT signal turns on a voltage that charges capacitor C10 through resistor VR3, creating a voltage ramp. A comparator IC reads this ramp-up, and the individual gates switch on each event at a different voltage. When PTT is released, C10 discharges, allowing each event to switch off at it's preset ramp voltage. The time interval between events is adjustable via the setting of VR3, the timing trimmer. The circuit board LEDs (D14,15 and 17) give a visual indication of event timing (they switch on or off with each event).

Q7, the event 1 switch, is designed to pull the return line of a relay (or relays) to ground, and can switch up to 100v at several amps.

Q8, the event 2 switch, also designed to pull a control line to ground, can handle up to 28v at up to 300ma. I use this event to operate a solid-state FET switch, which gates VDD to the amplifier. This FET switch can be omitted, but it's safer to use one to more completely disable the amplifier during the receive cycle.

Q9, the bias switch, also on event 2, provides up to 200ma at 13.5v for biasing circuits. Since the typical LDMOS bias circuit requires only about 20ma, this port can accommodate about 10 bias feeds, more than enough for most of us.

Q6, the driver hold-off gate, is operated by event 3. RF from the driver is held off by placing a negative voltage on the ALC connection to the driving radio. Q6 gates this blocking voltage.

High VSWR Lockout


To use this feature, a directional coupler and a diode detector should be used to provide the input signal for the SWR lockout switch. The sensitivity of the circuit is less than 1 volt, and is adjusted with VR1, the sensitivity trimmer. I usually set the lockout to trigger at 2 to 1 SWR.

Here's how it works...the comparator monitors the sensor port, and if there is sufficient signal there, illuminates D4. Once D4 conducts, regenerative feedback through D1 and R6 cause the comparator to latch on and stay that way until the circuit is reset. To reset, main power must be switched off for a few seconds, allowing the comparator to unlatch. This persistent lockout is intended to encourage an investigation of the SWR problem.

The actual lockout is accomplished by switching on Q1, which blocks the PTT signal from the driver. When the PTT signal is blocked by Q1, the sequencer takes the amplifier offline, one event at a time.

Fan Control and High Temperature Lockout

The fan port is designed to sink the power return from the cooling fan(s) to ground, and can handle voltages as high as 100v at several amps.

The fan will run whenever PTT is engaged. It will also run under these other conditions:

A 5k negative coefficient thermistor (fan sensor), mounted to the amplifier's heat sink, is monitored by two comparators. The first (U1.3) switches on the fan whenever the temperature at the sensor rises to about 110F; it switches the fan off when the temperature drops about 5 degrees. This small differential in temperature is introduced by R25, in order to allow enough heat to be drawn away from the heat sink so the fan will not stutter on and off as the heat stored in the heat sink core makes it's way to the sensor, which is mounted on it's surface.

The second comparator (U1.2) also monitors the sensor, and if the temperature rises too high (about 135F), assumes the normal cooling circuit to be inadequate for the current operating conditions, and locks the amplifier into bypass mode until the temperature drops to safer levels (about 5 degrees).

It accomplishes this by turning on Q1 (previous schematic) through R13, which blocks the PTT signal. There is no manual reset for this function; it will stay active until the temperature at the sensor drops to about 130F.

The exact trip temperatures can be set by the temperature trimmer (VR2).
Blocking RF from the Driver

To prevent damage to an amplifier's antenna relay while it changes state from rcv to xmit, RF output from the driver should be held back until the relay(s) finish switching over. This became apparent to me when I lost an input relay on one of my own amps; I was using a low-power microwave relay at the 50w level, and did not remember that even though the relay could handle 50w, it could only handle about 10w while switching state. Keying up in FM mode during testing, it lasted about 5 cycles.

While some radios like the FT817/857/897 have a connection for an RF hold-off signal, many do not. The one thing that almost all of them do have is an ALC input, and that seemed like the best approach to take. If you hold the ALC line high while the relay(s) switch, there will be no RF to damage them. Once the switching is over, remove the ALC voltage and the radio recovers to full power.

To do this, a negative voltage has to be placed on the ALC connection to the radio to reduce RF output to a safe level. U2.4 is an audio oscillator, and D10/D11 rectify this signal and produce the negative voltage needed, which appears across C16. This voltage is present whenever the amplifier's amplify/bypass switch is in the amplify position (in bypass mode, we shouldn't be blocking RF output from the radio).

Q6 removes this ALC signal on event 3 (after the relay(s) switch and the amplifier is enabled), and this allows the radio to recover to full output in a few hundred milliseconds. Some radios recover faster, some slower. If yours recovers too slow, an adjustment of the ALC level trimmer (VR4) will help. You don't need to completely block all RF; just enough to reduce it to a safe level for the input relay(s) to handle.

Q5 was added to handle a unique situation... and we all do this sooner or later when we demo the difference the amp makes to our friends; without Q5, if the operator were to switch the amplifier into bypass mode while transmitting, the input relay would be hot-switched. This happens because the supply voltage to U2 is switched off in this situation, turning off the ALC blocking voltage and disabling the sequencer. But Q5 keeps power to U2 on when event 1 is active, making certain the driver RF block is there and the sequencer maintains control until event 1 releases the relay(s), Shortly after event 1 terminates, the driver is back to full power; the amplifier is now in complete bypass mode, and driver RF control is no longer active.

The Voltage Regulator (reducer) and LED Indicator Feeds

The control board was designed to run on a 13.5 volt supply, and there is a pad on the board for this input. For those of us controlling a 28v or a 50v amplifier, it's good to have the option to supply the board from one power source, but it has to be stepped down to 13.5v.

If you are limited to a 50v supply, you can't do this with a 3-terminal regulator (they are limited to 35v max), so the reducer here uses a zener diode and a high-power Darlington current amplifier to make the transition.

The 13.5v output can be tapped to run other devices, but current drawn should be limited so the transistor isn't dissipating more than about 35w. At 50v, this means a max load of about 800ma; at 28v supply, you can use more (about 2 amps).

Caution: It is wise to use a current limiting resistor in series with the input if you will be using the voltage reducer with a 28v or 50v supply, otherwise a temporary short of the 12v output (we all goof sooner or later) will cause both Q10 and D18 to fail, and place either 28v or 50v on the 12v rail. Not good. For a 28v supply, a 5 ohm 10 watt resistor is fine; for a 50v supply, use a 25 ohm 25 watt resistor (Dale type RH10 or RH25).

LED indicators, which we all love to put on our amplifiers, can be fed from the control board. There are feeds for power, transmit, high-temp and SWR leds. The SWR led requires 2 connections due to its special use as part of the latch circuit. The other 3 require only one connection, and the other side of those LEDs can be connected to ground.

Here is the complete schematic for the control board


If you are building this project from a kit I supplied, here is an assembly guide to help with placing parts, and below are the mounting instructions for the transistor and PC board:

Shown below is a block diagram showing typical connections, and below that is a list of all connections available.



The block diagram below shows a typical connection scheme (this is the one I currently use in the VHF KW amplifiers)


If your control board version is 6.1 or 6.2, there are some minor labeling changes described here:

Version 6.0 to version 6.1 minor changes explained

•SWR pad renamed from SWR to “Load Fail Sig In”

•SWR trimmer renamed from SWR to “Load Fail Lock Adj”

•ALC pad renamed from ALC to “block”

•ALC trimmer renamed from ALC to “block adj”

•“KILL” pad added to allow external control circuits to disable PTT. Pulling this pad low will “kill” the PTT connection to the sequencer.

Version 6.1 to version 6.2 minor changes explained

  • diodes D20 and 21 added in series with LED indicators to keep large open-circuit load voltages from illuminating the LEDs

  • diode D19 added between Q1 and Q2 to prevent Q2 from affecting circuits connected to the 'kill' pad.

The kill pad can now be used to immediately disable a FET switch upon fault detection, avoiding the normal 1/10 second delay of the on-board sequencer's back-up routine (connect the kill pad on the control board to the disable pad on the FET switch board to enable this feature).

Here is the schematic for version 6.2: