Rob's web

Variable end-linked coils

Home - Techniek - Electronica - Radiotechniek - Radio amateur bladen - QST - Variable end-linked coils

When high-power tetrodes are used in a single-ended amplifier circuit, it seems wasteful to use a split-stator tank system to obtain a variable link. Since no variable end-link coils are on the market, one was made in the following manner.

A standard B & W 500 watt TVL jack-bar was used to hold the plug-in coils. The associated split-center TVL coils were altered by removing one of the two identical windings on the 80 meter coil, and substituting in its place one of the windings from the 40TVL coil. The result is a coil that can be used on 80 meters when it is plugged in one way, or on 40 meters when it is plugged in after 180° rotation. A 20-10 meter coil was made in like manner from the 20 and 10 meter TVI, coils.

Fig. 1 shows the circuit connections necessary to permit this arrangement. The inoperative portion of the coil assembly has no effect on the performance of the amplifier, as it is not connected to the circuit in any way, and is isolated from thy operative portion by the swinging link. (In rare instances, the "floating" coil might be self-resonant at the operating frequency, resulting in a wavetrap action, but if this is the case, it can be detuned with a condenser or shorted to ground to eliminate the trouble. - Ed.)

Fig 1
Fig. 1. Method of obtaining variable-link coupling with single-ended circuits. Two bands are covered with, single coil assembly, as described in the text.

This arrangement is not applicable to single-ended triode stages where a balanced circuit is needed for neutralization.

Walter Zuckerman, W2LBF.