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Drake L4B - All In One Redesign
by: Ronald Baker / WB4HFN
Contents Page 1 Page 2 >Page 3< Page 4
Input Circuit Redesign
I never knew why Drake used this style of feed-through
capacitor but I knew they needed to go. Those capacitors along with
the large bifilar choke isolates the RF input power from the ground potential
through the filament power transformer. The bifilar choke is
attached to the first tube socket. The tube filaments are in parallel so I just
cut the leads going over to the second tube when I removed the rear tube socket.
Shown in the second picture to the left is a close-up of the remaining tube
socket. The original bifilar choke is attached to the first tube socket
which was left in place. You can see along the bottom of the
socket where the copper strapping was cut at the tube pins going to the second
tube socket that was removed. I also added that disc capacitor to
make sure the RF drive was equally distributed across both sides of the
I decided to remove the large feed through capacitors and add smaller disk capacitors of the same value to both sides of the circuit to ground. I also added the same value capacitor across both lines to ensure any stray RF signal was eliminated at this point. The basic function of this circuit is to pass the filament voltage to the tubes and completely isolate the RF input signal from the ground potential through the power transformer. The input power from the exciter is routed through the antenna relay directly to the tube filaments. The L4B design is "grounded grid", so in this case the input power is applied to the tube filaments since there is no separate cathode. Shown in the picture below is the feed-through capacitor resign.
Shown the pictures at the bottom, on the left, is the new antenna relay mounted and the watt meter board turned 90 degrees and remounted parallel with the chassis rear. Remounting this board this way gives easy access the the red watt meter calibration controls and puts it directly in front of the RF input and output connectors. The other picture at the bottom, on the right, shows the "before modification", how the original relay and watt meter board are mounted in the L4B Amplifier. Also, note the large feed through capacitors in the filament circuit were removed in the modified L4B design to make room for the new larger antenna switching relay.
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The picture above is the modification with the new sealed relay and repositioned watt meter board. |
The picture above is the original L4B design with the open frame relay and large feed through capacitors. |
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