Amateur Radio Station K9DUR
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1kW Linear Amplifier On May 8, 2005, I purchased a used 1kW linear amplifier on E-Bay from an amateur in the Kankakee, IL, area. The price paid was only slightly more than the cost of acquiring the components necessary to construct a similar amplifier from scratch; besides I only live a couple of hours from Kankakee, so I would not have to pay the shipping charges. Therefore, I felt that I could not go wrong with the purchase.
The amplifier started out life as a B&W Model L-1000-A. However, it had been completely rebuilt by a previous owner. The modifications included:
The craftsmanship of the modifications was exceptional. However, the amplifier did have a couple of operational problems. The amplifier would load, but the power output would fade to almost nothing after a few seconds. Also, when trying to tune up the amplifier on 20m, an arcing sound was heard from inside the tuned input module. Rather than trying to troubleshoot the guts of the tuned input module, I decided to remove the module and restore the input circuit to its original direct-feed configuration. After all, I was going to be driving the amplifier through an automatic antenna tuner, so any mismatch would not be a problem. Another problem was that the output provided by my FT-847 to switch an amplifier was designed for more modern equipment and could not handle enough current to operate the relay in the amplifier. I constructed an interface box containing a small 12VDC relay and an amplifier control switch. The relay is switched by my PTT foot switch. This solved the current capacity problem. The control switch provides a convenient method of switching the amplifier in and out of line. After removing the tuned input module, wiring the input directly to the filaments of the 813's through a 0.01 mF 1kV capacitor, and installing a new pair of 813's, it was time to test the amp. I discovered that on 40 meters minimum plate current occurred at one end of the range of the plate tuning capacitor. That plus the lower output power led me to believe that the pi-network was not set up correctly on 40m. After checking, I found that the capacitor was at its minimum value. There is a mechanical arm attached to the shaft of the band switch that switches a 2nd section of the plate tuning capacitor in and out of the circuit. It was positioned to switch the higher capacitance in on 160m, 80m, & 40m. I repositioned the arm to only utilize the 2nd section of the capacitor on 160m & 80m. This solved the problem on 40m but created a new problem on 160m. This arm was also used to switch in additional fixed capacitors on 160m. A new contact assembly had been mounted to the side of the amplifier case so that it contacted the mechanical arm when the band switch was in the 160m position. When I repositioned the arm, it no longer made contact with the added switch contact. I was able to drill 2 mounting holes in the deck of the amplifier and remount the added switch contact assembly so that it again made contact in the 160m position. The LDG Electronics Model AT-100Pro automatic antenna tuner works like a champ matching the amplifier input to the 50-Ohm load required by transceiver. About 80 Watts of drive will produce 900-1000 Watts to the antenna on all bands, 160m to 10m. Since the original amplifier dates from the 1960's, the WARC bands were not a part of the design of the PI-network output circuit. Therefore, the amplifier does not cover 30m, 17m, or 12m. On-the-air tests with WA9WJJ & other members of the 1898 kHz gang indicated that the amplifier was operating properly with no detectable degradation in audio quality. I have since sold the amplifier to a local amateur who uses it constantly. In fact, he likes it so well that he sold his more modern amplifier & kept this one.
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