Modification (August 2016)

A modification to the input circuit was added to eliminate an instability discovered when operating above 20 MHz into certain narrow-band antennas. This instability became evident at power levels above 500w when driving these antennas, and were not present when operating into a dummy load or other broadband loads. The typical symptom was erratic reverse power readings, increasing at a non-linear rate as drive was increased. On a spectrum analyzer, these signals were as strong as -30 dbc, and numerous.

The problem was fixed by replacing the input transformer with a different type, and adding additional degenerative feedback components to the gate circuits. 4 components were removed, and different components were substituted for them. This modification takes about an hour to perform. It also reduces the overall gain of the amplifier a little; after the update, drive requirements will be approximately 2.5w (160m) to 5w (6m) for 1kw out.

These were the changes made:

  1. The three 75 ohm resistors (R14,15,16) were no longer necessary and were removed
  2. The dual-core input transformer was replaced with a single-core transformer
  3. A 10 ohm resistor and 1000pf capacitor, connected in series, were added from each gate to ground
  4. A 100 ohm resistor was added at R13
  5. The two 430 ohm 3w resistors (shown above), connected from the drains to the pad with the blue .047uf capacitors, are no longer required and can be removed. Their removal is recommended.
  6. And finally, as of May 2017, the bifilar drain choke was changed to two separate drain chokes.

Here are the parts you will need to get to make the change (kits and RF decks shipped after July 2016 include these parts):

All of the above parts are available from

To perform the modification to a previously assembled input board:

The modification is now complete. There is no need to re-adjust bias (IDQ) levels, as nothing in the D.C. bias path was changed. Future revisions of the PC board will allow the use of surface-mount parts to make assembly a bit easier.

A revised schematic diagram and bill of materials follow for the kits and RF decks as they are currently shipping. Some of the bias components have changed; these are minor adjustments to make IDQ settings a bit easier to do, and do not need to be changed if you are only performing the 08-2016 modification to an existing build.

If you are building a new input board, note these changes and use the photo here as a placement guide:


As of May 2017, the bifilar drain choke was changed to a pair of separate drain chokes. First, remove the original bifilar choke.

Now make the new drain chokes; locate the two #14 tinned magnet wires, straighten them out, and pass each through a ferrite core (Laird 28b1000-000) as shown here. One end should extend out the end of a core by about an inch, the other by almost 2 inches. The wire should pass through the center of the core 9 times.

This photo shows how and where to mount these chokes. Leave as much space as possible between them, about 1/8 inch is enough.