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

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, or from the W6PQL parts page in the form of an upgrade kit. In the case of the upgrade kit, the coax is already prepared for you. To prepare it yourself, here are the instructions:

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:

Bill of Materials (BOM)

RefDes Value Name
BALUN 4T, 50z rg402 FEP made with a Laird 28B1020-100 core
C1 .1u 100v capacitor_1206
C2 .01u 100v capacitor_1206
C3 .01u 100v capacitor_1206
C4 1u 100v capacitor_1206
C5 1000pf ceramic capacitor
C6 1000pf ceramic capacitor
C7 .01u 250v capacitor_1210
C8 .01u 250v capacitor_1210
C9 .01u 250v capacitor_1210
C10 .01u 250v capacitor_1210
C11 .01u 250v capacitor_1210
C12 .01u 250v capacitor_1210
C13 .01u 250v capacitor_1210
C14 .01u 250v capacitor_1210
C15 .01u 250v capacitor_1210
C16 .01u 250v capacitor_1210
C17 .01u 100v capacitor_1206
C18 1u 100v capacitor_1206
C19 100u 63v capacitor - electrolytic
C20 .01u 100v capacitor_1206
C21 1u 100v capacitor_1206
C22 100u 63v capacitor - electrolytic
C23 240p capacitor-MC18 mica
C24 240p capacitor-MC18 mica
C25 240p capacitor-MC18 mica
C26 240p capacitor-MC18 mica
C27 .047u 450v capacitor
C28 .047u 450v capacitor
DZ1 6.2v Zener Diode
R1 50 resistor_1206
R2 50 resistor_1206
R3 50 resistor_1206
R4 50 resistor_1206
R5 300 resistor_1206
R6 100 resistor_1206
R7 5k thermistor resistor_1206
R8 510 resistor_1206
R9 50 resistor_1206
R10 10 resistor_1206
R12 10 - 2w resistor_2512
R13 100 - 2w resistor_2512
R14 10 - 1w resistor, metal film
R15 10 - 1w resistor, metal film
R16 430 2w resistor, metal film
R17 430 2w resistor, metal film
T1 4 to 1, 2T/1T RG316 input transformer, made with 1 Laird 28B0562-100 core
T2 5T x2 #14 200C magnet wire BIFILAR drain choke, made with a Laird 28B1020-100 core
T3 4T, TC-18 coax coax transformer, made with a Laird 28B1020-100 core
T4 4T, TC-18 coax coax transformer, made with a Laird 28B1020-100 core
VR1 200 IDQ ADJ trimmer
PCB Circuit board, TC-350 material available through
TC18 18 ohm Teflon coax 29 inches
RG316 50 ohm Teflon coax 7.5 inches
RG402FEP insulated RG402 conformable coax 18 inches
magwire #14 magnet wire, 200C 36 inches