Assembly Guide for the 70 CM 500W Amplifier Kit

First, locate the 35pf trimmer capacitor supplied in your kit. The photo to the right shows the back side of this capacitor, and the reason for this first note: the 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.

Mount it as shown in the left photo, with most of the capacitor body over the bottom trace. Solder into place.

Pre-set the slotted adjustment as shown, with the slot vertical. This will put the capacitor at mid-range, and very close to resonance with the rest of the input circuit.

Next, install all of the components on both boards except for the coax baluns.

The 10pf chip capacitor must be centered 8mm from the left end of the gate traces.

The 20pf metal mica capacitor shown in this photo is no longer used (see the next photo).

The two 33pf chip capacitors above and below the green trimmer capacitor have been replaced with disc ceramic capacitors.

The 58mm coaxial capacitor should be installed 30mm from the drain edge of the PC board. The open end should be formed with a gentle bend up and away from other components.

The installation of a 5nh inductor across the gate pads (and above the two 33pf capacitors) as shown here is a change made to reduce low frequency gain, which can cause instability and LDMOS failure under certain conditions. If you bought your kit before mid-January 2013, this part will not be in the kit. The inductor is easily made by winding 2 turns of #22 wire (or a cut-off component lead), 3mm inside diameter, space-wound as shown. The value isn’t critical, so if you miss the exact dimensions, you’ll be fine. This change does not affect input tuning or the performance of the amplifier at 70cm. In the case of the1kw amplifier, this needs to be done for each LDMOS device.

As shown in this photo, each drain bypass should consist of one each .001uf, .01uf, .1uf and 1uf chip capacitor as well as the 100uf electrolytic. The order isn’t important, so long as all capacitors are lined up where shown, with the electrolytic around the corner.

This next set of photos shows how to configure the bias feed resistors for various bias supply voltages. Most builders will probably want to use the 12v configuration, which is the bias output from the "ultimate" control board offered on the parts page, but the bias feed can be anything from 12 to 50v.

Install the coax baluns as shown on the right.

Slide the boards under the transistor tabs (transistor should have been previously flow-soldered to a copper spreader).

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.


The addition of the following components is recommended to ensure unconditional stability by applying degenerative feedback. This feedback is most effective at very low frequencies where these LDMOS devices have excessive gain. Without the degenerative feedback, and under certain rare conditions, self-oscillation can occur and destroy the LDMOS.
  1. 1.Remove the two 4-40 x 3/16 screws located nearest the gate pads, but leave the flat washers in place
  2. 2.Replace the screws with ¼” hex metal spacers; be careful not to over-tighten this spacer, it's screw can shear off.
  3. 3.Screw a ¼” nylon spacer into the metal spacer, finger-tight only.
  4. 4.Place a solder lug on top, and fasten in place using the 4-40 x 3/16 screws removed earlier.
  5. 5.Solder a 440 ohm 3w resistor to the drain pad and route the other end to the solder lug.
  6. 6.Solder a .047uf capacitor to the gate pad and route the other end to the solder lug.
  7. 7.Solder the resistor / capacitor connection at the solder lug.
  8. 8.Repeat for each LDMOS device (on both sides).

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 1/2w, 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.