The 6146B amplifier uses a tuned link to feed the input signal to the grid of the tube, as shown in the schematic at right. Capacitor C1 tunes the secondary L2 to resonance.

Input link L1 consists of 3 turns of insulated hookup wire wound around the 10th turn from the grid end of L2, as shown in the photo at right.

The link must be installed temporarily to determine the tap points on L2, as detailed below. Once the tap points are determined, a drop of epoxy glue is then used to hold the link in place.

The secondary coil L2 consists of 37 turns of #20 wire, 1 inch in diameter, spaced 16 turns to the inch (B&W/Miniductor/Airdux #3015). According to the schematic for a double 6146B amplifier in the 1967 ARRL Handbook ( from which the design of the input circuit was taken), taps were to be placed at 3, 6, 9, and 25 turns from the grid end of the coil. However, since this amplifier used a single 6146B rather than a pair, and since it would also cover the new WARC bands, such as 30m and 17m, the original tap points were only used for starting estimates. The actual tap points were determined by experiment, as explained below.

To determine the actual tap points, a special type of alligator clip was used to make a temporary connection. These alligator clips can be purchased at Radio Shack and they are wonderful for making temporary connections in tight places, since they have narrow, flat jaws. They are also excellent for use as gluing clamps in many other applications. (They were used as clamps when building the high voltage cage, as shown in the photo at right.)

Once the appropriate tap point for a given band was determined by experiment, the alligator clip was removed and the tap permanently soldered into place. The process was repeated for each band.

The final input coil assembly can be seen in the photo at right. The grid end of the coil is to the left. From left to right the taps are for 10m, 15m, 20m (between the turns of L1), 30m, and 40m. The entire coil is used on 80m.