The CM-1.5Ki

3-1000Z HF Linear Amplifier Project


In the late 1980s, I built a 5-band, linear amplifier using a 3-1000Z. The amp served me well for several years. While it worked well, it was NOT pretty. See it on the left of the photo in The Shack page. The working environment changed and the amp was stored. I had always wanted to rebuild it, making it more attractive. Now retired, I started this project in March, 2014 and finished six months later (no work was done in June – vacation). The CM-1.5Ki covers eight bands from 80-10, no 160 meters. It produces 1500 watts out on 80-20 meters and 1200 on all others.

Overall Design

The Eimac 3-1000Z is designed for zero-bias, direct-grounded-grid operation. As with most common 3-1000Z designs, a small amount of bias lowers the quiescent current, lowering the zero-signal power dissipation while still allowing good linearity. I use six, 1N5408 diodes in series from the filament transformer center tap to B- (standard design), biasing the grid at approximately -4.2 volts with respect to the cathode. The plate voltage is ~3500 volts produced by an old, restored, Heathkit Chippewa amplifier power supply. The quiescent current is ~80 mA. The amplifier uses a WD7S TU-6B, six-band (five used), relay-switched, low-Q tuned-input board to match the exciter to the approximately 62-ohm impedance of the cathode and to minimize third-order distortion. Tube control and protection comprises two circuit boards: Control Board One and Control Board Two. A simple pi-network couples RF from the plate of the tube to the antenna port. See schematics and the following paragraphs for detail.


I borrowed heavily from designs I found on the internet: most notably those on the WD7S Triode Controller site. I have, however, made my own modifications as required for this particular project.

Wiring Diagram PDF



Control Architecture

Two fault busses govern amplifier operation: hard-fault and soft-fault. Both busses unkey the amplifier. Hard-faults require power cycling the amplifier. Soft-faults are temporary conditions that unkey the amplifier but do not necessitate power cycling. Controller circuits use both fault lines interactively as described in this section.

Soft Faults are:

  • High-voltage step-start timeout
  • High-voltage low
  • Grid current trip
  • Operate switch off
  • Key line off

Hard Faults are:

  • Plate Current Trip
  • Tube base temperature high

Amplifier Control

Two circuit boards, Control Board One and Two, provide control and protection of the tube. These two boards are prototyping boards with holes spaced 100 mils. Connections on these bords are made using a special older bridging technique I learned while working at Texas Instruments. They are located in a shielded compartment I call the Control Enclosure.

Together these two control boards perform the following functions:

filament voltage step-start—On power up, a 27-ohm resistor in series with the filament transformer primary limits the filament in-rush current and allows the cold filament to warm slowly. After approximately 3 seconds the resistor is shorted by a relay, applying the full voltage (7.5 vac) to the filament. The timer also drives the Filament On LED.

high voltage step-start—Once the filament step-start completes, its active-low control line keys a relay in the HV power supply. This relay applies ac power to the high-voltage transformer. As in the filament transformer step-start, a 47-ohm resistor in series with the HV transformer primary limits the capacitor charging current. After approximately 5 seconds after Filament On, the high-voltage timer pulls an active-low control line that keys another relay, which shorts the limiting resistor. It also drives the HV On LED.

grid over-current warning—Grid current flowing through a 10-ohm resistor (R1 in the RF deck and located in the Control Enclosure) from the B- line to ground develops a voltage that is sampled by a potentiometer and applied to a transistor switch that drives the Grid Warning LED. It is set to illuminate at 250 mA. The LED will flicker as the current approaches this value.

grid over-current trip—This same voltage across the 10-ohm resistor is applied to a comparator (U2d) the output of which is applied to a latch (Control Board Two U2c). The latch pulls down the soft-fault buss, which prevents the amplifier from being keyed and drives the Grid Trip LED. The latch is automatically reset within two seconds by the auto reset line in the keyer circuit. The trip point is 300mA.

Plate over-current trip—Plate current passes through R2, a 1-ohm, 10-watt resistor located in the power supply B- line, developing a voltage that is applied to both the plate-current meter and Control Board Two on J3-1 (-Ip) and J3-2 (+Ip). The voltage is applied to a comparator, he output of which is latched by U1c, which pulls low on the hard fault buss through D7, unkeying the amplifier. The latch also drives the Plate Trip LED. The trip point is set to 800 mA.

high voltage low—The high-voltage sample line from the power supply is applied to a latch (Control Board Two U2d) that drives the soft-fault buss, preventing the amplifier from keying. It also drives the HV Low LED. It is set to 50% of the no-load high voltage value. The LED will illuminate on power up until the high voltage comes up.

high tube temperature—A thermal switch that closes at 130 degrees F mounted at the base of the tube is applied to a Schmitt-trigger (Control Board Two U1a and U1b), which pulls down the hard-fault buss and drives the High Temp LED.

keyer and bias—Antenna changeover and tube bias switching is located on Control Board Two, Four MOSFET transistors compose the keyer and bias switching control. Keying transistor, Q4, pulls the relay coils to ground, affecting the antenna changeover. Q4 also discharges and holds low a 100-uF capacitor. When the amplifier is unkeyed, the capacitor charges through a 75-kohm resistor, generating the two-second auto reset for the grid-trip latch. Operating simultaneously with the keying transistor is the bias relay switch, Q5. When this relay is energized, it shorts the 27-kohm “bias-off” resistor, biasing on the tube. Six conditions must be clear for the amplifier to key up and operate:

  • Grid-trip fault clear
  • HV step-start timed out
  • HV Low clear (HV power supply at full output)
  • Hard Fault buss high
  • Operate Switch On
  • Key Line low (key down)


Control Board One

Control Board One is one of two circuit boards, located in a shielded section of the amplifier. It performs step-start of filament voltage and high voltage with two timers. Both are set for approximately 3 seconds. It also develops the control operating voltages: Regulated 24VDC for control and unregulated ~33 VDC for antenna changeover relays.

Control Board One PDF

Power Supply

AC power from the HV PS (120vac) is brought to the board on J1-1 (AC common) and J1-2 (AC – 120v). Transformer T1 converts the voltage to 20 vac and BR1, a full wave bridge rectifier, and C2 convert it to 33 VDC. The 33 volts is applied to voltage regulator, VR1 (LM7824CV), which provides 24 volts to the control circuits. The unregulated 33 volts is brought off the board on J2-8 and the 24-volt Vcc on J2-3.

Filament Step-Start

When power is applied to Control Board One, and provided there are no hard faults, relay K1 is energized by switch Q1. The switch is biased into saturation by voltage divider R1 and R2. If a hard fault occurs, the base of Q1 is pulled low through D1, and Q1 is turned off. Diodes D2 and D3 raise the bias on the base of Q1 to 2.1v. This allows the D1 to pull bias to well below zero volts, insuring that Q1 is completely turned off.

Note: This biasing is used in several circuits on both control boards, and henceforth, will not be described in this detail for each occurrence.

The 120vac is switched by K1 to the contacts of K2 and R14, which is in series with the filament primary winding. R14 limits the current in the primary winding and causes a voltage drop on the loaded secondary, limiting the in-rush current to the filament. When the filament timer times out, K2 closes, shorting R14 and allowing full operating filament voltage. If a hard fault occurs, Q1 is switched off, and the operating voltages are removed from the tube.

The filament timer comprises comparator, U1a, and transistor switch, Q2. On power up, C1 is in a discharged state, U1 pin 5 is at 0 volts, pin 4 is at 5.6 volts, and pin 2 is at 0 volts. C1 and R4 compose an RC timing circuit. C1 begins charging to ~12 volts. As the voltage on C1, and therefore pin5, reaches the voltage on pin 4 (5.6v), the output on pin 2 switches to a high value (~22v), biasing on Q2, which energizes K2, shorting K14. Q2 also drives the Filament On LED. The 5.6 volt reference for both comparators on Control Board One is developed by R3 and D6, a 5.6-volt zener.

HV Step-Start

The HV step-start timer is identical to the filament step-start timer, except that the RC time constant is ~8 seconds. The collector of Q2 in the filament timer is brought out to J2-2, so that as soon as the filament is at full voltage, it keys the HV On relay in the power supply. On power up, the HV step-start timer is started (providing, of course, no hard fault exists).

The HV timer comprises comparator U1c and transistor switch Q3. The output of the comparator, pin 1, is brought out on the soft-fault buss. The collector of Q3 is brought out to J2-5 to energize the HV step-start relay in the power supply. It also drives the HV On LED.

LED indicators are located on a circuit strip (status board) behind the front panel. All LEDs anodes are connected to Vcc (24v). Each LED has a series 2.7-kohm resistor located on the associated control board.

Control Board One

LED Status Board


Control Board Two

Located in the Control Enclosure of the amplifier, Control Board Two performs fault protection and keyer functions. Faults include: grid current high warning (no fault set), grid current trip (soft), plate current high trip (hard), high voltage low (soft), and tube temperature high (hard). The amplifier keying circuits are also located on Control Board Two.

Control Board Two 1 of 2
Control Board Two 2 of 2

Control Board Two

Grid Current Warning (no fault)

Grid current passes through R1, a 10-ohm, 10-watt resistor located in the Control Enclosure (See photo of Control Board One) that shunts the B- line to ground, developing a voltage that is applied to both the grid-current meter and Control Board Two on J3-5. This voltage runs to the two grid control circuits: grid warning and grid trip. The grid warning consists of a simple transistor switch that drives the Grid Warning LED. The 10-kohm potentiometer, R21, sets the grid current warning level at the base of Q2. When the grid current reaches 250 mA, Q2 turns on, Because of the nature of the bipolar 2N2219A transistor, total saturation is reached over a small bias range instead of a single point. This means that the Grid Warning LED will increase in intensity as the collector current reaches saturation. This provides the operator an easy visual indicator while operating.

Grid Current Trip (soft fault)

The voltage sampled across R1 in the Control Enclosure is also applied to comparator, U2d pin 10. It is limited by D4, a 5.6v zener diode. Capacitor, C5, provides filtering. The 10-kohm pot sets the trip current level to 300mA.The output of the comparator is latched by U2c. The output of the latch is connected to the soft-fault buss through D6, and it also drives the Grid Trip LED. A 5.6-volt reference for the comparator comprises R17 (20 k-ohm) and D3 (5.6-volt zener). The Auto Reset line from the keyer circuit resets the latch within 2 seconds. This feature allows the operator to continue transmitting without the amplifier until the latch is reset. It is assumed that the over current causing the trip was short term, such a voice peak.

Plate Current Trip (hard fault)

Plate current passes through R2, a 1-ohm, 20-watt resistor located in the power supply B- line (physically located in the Control Enclosure), developing a voltage that is applied to both the plate-current meter and Control Board Two on J3-1 (-Ip) and J3-2 (+Ip). This voltage is filtered and applied to a 4N38 optoisolator. The output on pin 4 is proportional to plate current. The isolator provides a level of protection for the control board. Potentiometer R13, 10-kohm, provides adjustment for setting the grid-trip point (set to 300 mA) and is applied to comparator, U1d. The reference voltage for the comparator is provided by R5, a 20-kohm limiting resistor, and D1, a 5.6-volt zener. The output of the comparator is latched by U1c which pulls low the hard fault buss through D7, unkeying the amplifier. The latch also drives the Plate Trip LED.

High Voltage Low (soft fault)

The power supply provides a sample of the HV via a voltage divider. It is connected to J3-4 and applied to comparator plus input, U2b pin5. Potentiometer, R12, 10-kohms, provides adjustment to the comparator negative input, pin4. The comparator is set to switch at 50 percent of the full HV. The output of the comparator, pin 2, pulls low the soft-fault buss through D2, and drives the HV Low LED.

High Temp (hard fault)

A thermal switch that closes at 130 degrees Fahrenheit mounted at the base of the tube provides protection from excessive temperature. It is connected to J3-3 and brought to the base of Q1, a 2N2222A transistor switch. The base of Q1 is held high by resistor R10, 10-kohms, connected to Vcc. This saturates Q1 and keeps low pin 6 of comparator, U1. C2 provides filtering to help eliminate contact bounce. When the base of the tube exceeds 130 degrees, the switch closes, turning off Q1, bringing the comparator minus input to ~18 volts, This voltage exceeds the reference of 5.6 volts on pin 7, causing the comparator output, pin 1, to switch to 0 volts. Latch, U1b, then pulls low the hard-fault bus, causing the amplifier to unkey. It also drives the High Temp LED on. The amp remains unkeyed until it is power cycled and the tube base cools down.



A dedicated 1-mA F.S. meter monitors grid current. The grid current flows through R1, a 10-ohm resistor, between B- and ground, and develops a corresponding voltage across it. The voltage is applied to the Grid Meter through a 5-kohm calibration potentiometer.

Another 1-mA F.S. meter provides monitoring of plate current, high voltage, and RF output. A proto board attached to the multimeter switch, which selects one of these parameters. A 5-kohm potentiometer located on the board provides plate current calibration. Another 5-kohm pot in series with the meter provides a set point for RF sense. The RF indication is NOT absolute. It is meant only to help in tuning the amplifier. The RF voltage is provided by a rectifier circuit located near the amplifier output. I bought these two meters from MFJ. MFJ uses them in their higher-powered amps.

Metering PDF

Note: Two additional RF detectors located on the same proto board are not currently used. I originally intended to provide tuning fault protection but decided not to.

The scaled HV indication from the power supply needs no calibration pot on the board, as that function is in the power supply.



My goal was to build the amplifier enclosure small enough to fit comfortably on a desk top. The unit measures 10.5-inches high, 17-inches wide, and 15-inches deep. From the picture below, one can see that little space is left. All sides, top, and bottom are separate aluminum sheets. I did not have a decent brake to make right angles, so I used angled aluminum strips to join them. Most screws are secured by tapped holes rather than nuts. The control boards, meters, and support circuitry are housed on their own RF shielded enclosure.

There are two front panels: one is part of the "inside" cabinet (I call it the "faux panel"), and the other is attached to this (the real front panel). This allows me to mount the plate tuning reduction mechanism, status LEDs and meters so that only the tips of the LEDs show and only that portion of the meters within their “ridge” are exposed. This makes for a cleaner, more professional looking amp. The two control boards, multimeter switch, and meters are enclosed in an RF shield.

Cabinet front panel or "faux" panel

Showing space between cabinet and real front panel

The tube is mounted on a 7-inch by 17-inch by 2-inch chassis that is pressurized by a squirrel cage blower. The chassis runs the width of the unit. I decked it first and planned the construction of the amp based on its dimensions. The filament transformer is mounted on top of the chassis behind the shielded Control Enclosure. The filament voltage leads pass through the chassis to the filament choke via ceramic pass-throughs. Incidentally, I used the large pass-throughs not for voltage protection but for strength to hold the large-gauge, stiff leads and to block the holes for air leakage.

Tube Chassis While Under Construction

Under the chassis is mounted the WD7S TU-6B, six-band, relay-switched, low-Q, tuned-input board. The 12-volt relays are switched by a wafer on the bandswitch. Located near the board is the LM7812 regulator. The 12-volt line and each of the band select lines pass through feedthroughs and on to the bandswitch to help eliminate RF on board.

The chassis construction process leaves a small space in all four corners. I filled the holes with silicone to prevent air leaks. The tube chassis attaches to the bottom plate by #8 sheet metal screws. As can be seen in the picture below, the thermal switch is mounted near the tube socket. The pins are bent so that the switch is as close to the tube base as possible.

Interesting Note: One evening while running the amplifier on AM, the amplifier shut down with a High Temp fault. I discovered that the blower motor runs very hot. It has its own thermal cut-off switch. The heat generated by the tube (dissipating ~600 watts) coupled with the heat of the motor shut down the blower, causing the tube base temperature to raise high enough to trip its thermal switch, which saved the tube. I’m sure glad I employed that safety feature! I put some foam between the tube and the blower to alleviate the problem and have had none since.

A Look Under the Tube Chasis


A standard pi-network couples RF energy from the plate (~2600-ohm impedance) to the 50-phm load. The plate tuning capacitor I purchased from Ameritron. It is used in its high-powered amplifiers, (i.e. AL-82, AL-1500, etc.). The loading capacitor I purchased off eBay. It has the tuning range to cover 80 through 10 meters without switching in additional capacitance. The coil I used in the first incarnation of the 3-100Z amplifier, a Barker-Williamson PI-DUX 195-2. I discovered during testing that on higher frequencies, an arc would flow from the high inductance end of the coil to the shielded enclosure. To fix the problem, I shortened the coil assembly by removing the 10-meter portion and moving it away from the enclosure. The tubing portion of the assembly is more than sufficient enough to cover 10 meters. A few turns of 14-gauge airdux were added to the 80-meter side to make up removing the 10-meter strap. One-quarter inch brass strap is used to switch the 10-20 meter taps on the bandswitch. The bandswitch I purchased from Ameritron. The switch has three configurations. One is a single-pole-six-throw used to select the appropriate relays on the input tuning board.



Finishing Touches

The faceplate and cabinet cover are what “meets the eye” first, so I took pains to make them look as good as I could. To make the faceplate, I carefully measured all holes and then generated a file using Photoshop. I printed it out on plain paper and taped it to the panel, making adjustments until I was confident all holes lined up. I wanted the faceplate to be black but not jet black. I wanted the color of my Yaesu FT-950, a sort of “off” black. I took a picture of the transceiver and used Photoshop to capture the color. It turned out to be VERY close. I took a JPEG file to a local graphics shop in Garland. They printed it on a 2-mm thick Lexan sheet. The basic color, of course, was black, but the markings were clear instead of white. So I painted the back side of the Lexan white. I affixed the faceplate to the aluminum panel using spray contact cement.

The cover was made at a local sheet metal shop in Rowlett. Careful measurement and bending was required for a tight fit. It turns out I had to get it done twice. It’s not a perfect fit but close enough. I had the cover powder coated at a shop in Garland, not far from the graphics shop. The final tough were the knobs. I had to order them from China off eBay. They are meant for high-end stereo units. They are heavy and feel good. I painted the narrow lines on those that needed them.


High Voltage Power Supply

High voltage is provided externally to amplifier by a 1950s vintage Heathkit KW-1 Chippewa power supply. The HV transformer is rated at 7000 volts, center-tapped, at 500 mA. This seems to be a very conservative rating, because I have often times loaded the amplifier to 750 mA or more without problems. The transformer runs relatively cool, especially because there is no rectifier filament current being pulled. The primary consists of two 120-vac windings connected in series. The power supply requires 240 vac.

High-Voltage Power Supply PDF

Two solid-state rectifiers are used in a full-wave configuration. The anodes connect to a bank of 10 each, 330-uF, 450-volt electrolytic capacitors, providing a total of 33 uF at 4500 volts.

Capacitor Bank

Note: The original power supply used an 8-uF, 4000-volt, oil-filled capacitor, but during testing it failed causing much distress!

Each capacitor has a 47-kohm, 10-watt resistor to equalize its voltage. Bleeder resistance comprises four 15-kohm, 50-watt resistors in series and enclosed in a perforated aluminum enclosure for safety.

A tuned, swinging choke from the HV transformer secondary center tap to B- provides additional regulation. The choke is rated at 8-15 Henries and a .01-uF capacitor tunes it. The capacitor-choke combination provides high impedance at low current draw, which tends to lower the output voltage. As more current is pulled and the choke inductance drops toward zero, the series impedance drops, which tends to increase the voltage.

High voltage is taken out of the supply by an insulated PL-259 connector, painted red for safety. The voltage connects to the amplifier by RG-213 coax cable with the outer shield removed.

A screw terminal block provides control and amplifier ac power connections. An 8-conductor cable runs from inside the shielded control compartment in the amplifier, through feedthrus, and through a conduit to this terminal block on the power supply.

Power Supply Terminals

Control lines enter Control Enclosure via metal conduit and feed-thrus

Back of Amplifier

Two relays provide control the powering up and step-starting of the power supply. 24 VDC from the amplifier is applied to both relays and the coils are energized by the HV On and HV Step-start (also referred to as soft start) control lines. See section Control Board One.



I am very happy with the project. The amplifier works great. I have near full-legal-limit output on all bands. As of this writing, I have used it almost daily for over a year. It is stable. All in all, it was a rewarding experience.

Amplifier Project