1.8 to 54 MHz 1 KW  Amplifier Assembly Guide

Begin with the input (gate) board.

Install all of the parts you see here except for R14,15 and 16. If your kit was purchased after July 2016, the following parts have been changed (see the update notes and the photos there as noted below).



Assembly of the gate board has been revised as of August 2016, so do not install R14, R15, R16, or the input transformer. These parts have been changed, please see the update notes here. Then skip the next 5 photos and resume assembly with the output board components here.

Assemble the 4 to 1 input transformer; locate the trimmed piece of TC24 coax, and solder the exposed shield at the center to the trace at the right side of R10. Tilt the coax ends away from you at 45 degrees.

Slide the two smaller ferrite cores over ends of the wires and down near the board.

Solder the shields at the open ends to the gate pads.

Pass the wire on the left through the core on the right and solder the end to the ground pad at the edge of the board.

Route the remaining wire through the other core and solder to the pad at R12/R13. Trim excess length from this wire if necessary.

The input board is finished.

Locate the two #14 tinned magnet wires, straighten them out, and pass them through one of the larger ferrite cores as shown here. One should extend out the end of the core by 2 inches, the other by 1.5 inches.

Bend these wires coming out of the top away from you over the top of the core to secure their length.

Pull the other end of the wires toward you, and fold up against the side of the core. Pass the ends of these wires through the top of the core and through the bottom. Place the remaining turns to the right of the first one.

Fold the ends of these wires in the same direction as the other ends, and the choke is finished.

Construct the two matching transformers.

Locate another large ferrite core and one of the prepared TC18 coax pieces. Pass the end of the coax through the core, leaving 1.5 inches extending out the left end (see photo).

Fold the end away from you over the edge of the core to keep it in place while you wind the remaining 3 turns.

Pull the coax toward you and up against the side of the core. Pass it through the core to the right of the previous turn and repeat until all turns are in place. Fold the end over the edge of the core away from you (see photo).

Make the second transformer the same way you made the first.

The output board is next.

Install the 240pf mica capacitors first (green). The first one of each pair should be placed 5mm from the board edge nearest to you (see photo), and centered a bit closer to the drain trace than the ground trace. This placement will keep it out of the way of the transistor drain tabs, and the #4 washer that will be used at the mounting hole next to it. The second capacitor of these pairs is placed right up next to the first, on the side away from you.

The .01uf ceramic bypass capacitors (C17,C20) are placed to the right and left of these as shown. C18, C19, C21 and C22 are placed at the other end.

Also located near that far end are the .01uf 250v capacitors (10 of them), two sets of 5 each.

Place one of the transformers as shown in the photo, and solder the coax ends in place.

Place the second transformer opposite the first and solder the coax ends in place (see photo).

Install the bifilar drain choke between the two transformers. The short wires go to the inner drain traces at the center, and the long wires to the outer VDD traces.

Inspect the other side of the board; if the magnet wire turns are resting on the board traces, or near the 10 .01uf capacitors, pull the magnet wire turns up away from the board and position the turns across the top of the core (see photo).

Place the pre-formed and prepared output balun standing up as shown. Bend the coax if necessary so the balun does not block access to any of the mounting holes in the board. The center conductor at the output end will need to be bent as shown to align with the output trace.

Solder the outer shields in place, then the center conductors.

It's a bit crowded near the 100uf capacitor, but leave a bit of space between the balun core and that part if possible. Move the capacitor a bit if necessary.

The output board is finished, and there will be 4 parts left over at this time (the degenerative feedback components, which are the .047 uf 450v capacitors, and the 430 ohm metal film resistors). These will be placed after the boards are mounted with the LDMOS.

Prepare to mount the boards, LDMOS and feedback components. Shown in this photo are the r/c feedback components, board spacers, LDMOS mounted onto a copper heat spreader, and a pre-drilled/tapped heat sink. Here is the drilling template for the heat sink.

All of these other items are available on the W6PQL parts page.

If you will be using your own heat sink and copper spreader, the recommended parameters are:

For the heat sink, a weight of at least 6 pounds (about 2.7kg) with a .375 base under the fins. Suitable material can be purchased from www.heatsinkusa.com, their p/n 10-080 (specify a 6 inch length). The surface of this material is usually not flat enough to mate well with the flat surface of the copper spreader, and may need to be fly-cut for flatness.

For the copper spreader, you'll need a 3 x 5 inch piece of 1/2 inch C110 copper plate. If you have it cut to 4 x 6.5 inches you can eliminate the board spacers. I use the same 3 x 5 spreaders I stock for the VHF amps (smaller PC board). The spreader surface that mounts against the heat sink may also need to be fly-cut for flatness. The copper plate material can be purchased (cut to size) from www.onlinemetals.com.

Apply thermal paste to the back of the copper spreader; then mount the LDMOS/spreader to the heat sink with two 8/32 screws, but leave them loose for now. Position the board spacers in alignment with the mounting holes in the heat sink.

Slide the boards under the LDMOS tabs and center them under the tabs. Fasten the boards to the spreader using 4-40 x 3/16 machine screws and flat washers at the 6 locations indicated by the red arrows. Do not over-tighten these screws.

If you will be using the spreader I offer on the parts page, note that 4 of the 4-40 tapped holes on the spreader are not used with this board. Only the 4 innermost locations near the LDMOS are used, the 4 outer ones do not have corresponding holes located in the HF PC board, they are for the VHF boards only.

Using 4-40 x 3/4 machine screws and flat washers, secure the remaining 14 mounting locations. Do not over-tighten.

Now tighten the two 8-32 screws near the LDMOS. These two screws should be very firm.

Place a bit of liquid flux at the edge of each transistor tab and solder the tabs to the board. This will require considerable heat (I use a 50w iron with a medium-to-large tip).

Finally, install the two .047 uf capacitors from the gate pads to the smaller pads at their sides. Then install the metal film resistors from the drain pads to those same small pads.


Assembly is finished.

Instructions for testing the amplifier are listed below.

Recommended testing procedure

There is no tuning required with this amplifier, and only one adjustment to make (setting idling current).

  1. Attach input and output coax jumpers. Your driver should be limited to 4w max, and the output should be on a dummy load for the initial testing.
  2. Attach ground, bias (12v?) and VDD (50v) wires to the RF deck, but do not apply power yet.



  3. Turn on the 50v main supply voltage, but not the bias; there should be no current drawn

  4. Turn on the bias and note the idling current drawn from the 50v supply. Adjust IDQ for 2 amps. Note: the current drawn by the bias supply (usually12v) is not what you are measuring here...you must measure the idling current (IDQ) the LDMOS draws from the 50v supply.

  5. Shut off the power supply, and remove current limiting.

  6. Turn the power supply back on; drive the amp with about 1/2w, and verify there is output power.
  7. If all OK in the previous step, the amplifier can now be driven to full output. Current should not be driven higher than 35 amps on any band. Check the performance graph in the main technical article for typical performance.