Assembly instructions for kits shipped after July 2017

The amplifier produced from this kit is essentially the same as before; I made some small changes to the board layout when it was time to re-order circuit boards, and incorporated all the improvements added to the original kit as those became necessary. These are the changes made to the kit:

Here's a link to a high resolution copy of the revised schematic diagram

Beginning with the output board, mount the surface mount capacitors as shown here. The smaller green mica is mounted so its forward edge is aligned with the trace transition (where it changes from wide to narrow).

Note the black mark near the end of the lower drain trace near the center of the board...this was done with a narrow sharpie pen to mark the 30mm distance from the beginning of the trace (more on this in a moment).

Add the drain chokes and the electrolytic capacitors. The electrolytics are polarized parts, so be certain to attach the negative leads to the ground foil, and positive leads to the VDD traces.

Attach the coaxial matching capacitor and the balun as shown here. The output board is finished.

Leaving the input coaxial balun for last, mount the input board parts. The small green trimmer must be mounted so it is not centered between the two gate traces, it must be offset so the round part in the center does not sit above the opposite trace. If centered, it will short the two traces together. Pre-align the adjustment slot on the trimmer to mid-range (it is set to mid-range in the photo).

To assist you with placement and component identification, see the hi-res photo here

In this last assembly step, slide the boards under the transistor tabs (transistor should have been previously flow-soldered to a copper spreader). The gate tabs were trimmed a bit before the transistor was mounted to provide clearance for C16 and C17.

Here is a video showing how to flow-solder your LDMOS to the spreader.

Here is the drilling template for the spreader.

Secure (but do not over-tighten) the boards to the spreader with 4-40 screws (2.5mm if you are using metric hardware with your own spreader).

Using a bit of liquid flux, solder the LDMOS tabs to the board.

Recommended tune-up procedure

  1. Fasten the spreader to a suitable heat sink using the guide on the right. Use a very thin coating of heat sink compound between spreader and heat sink (too much, and it will actually impair heat transfer).
  2. Attach input and output coax jumpers. Your driver should be limited to 5w max, and the output should be on a dummy load for the initial testing.
  3. Attach ground, bias (12v?) and VDD (50v) wires to the RF deck, but do not apply power yet.



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

  2. Turn on the bias and note the idling current drawn from the 50v supply. Adjust IDQ for 1 amp. Note: the current drawn by the bias supply (usually12v) is not what you are measuring must measure the idling current (IDQ) the LDMOS draws from the 50v supply.

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

  4. Drive the amp with about 1w, and adjust the input trimmer capacitor for max current drawn. This should be the same setting as lowest input vswr, and the input trimmer should be close to mid-range. If it isn't, spread or compress the turns on the input inductor until it is.
  5. The amplifier can now be driven to full output.