- by W4LK
Recently, I modified an old Yaesu FL-2000B amplifier that had been sitting around my shop for years. It had a burned bandswitch and I was unable to find a suitable replacement. I have always loved six meters, so I undertook the task of converting it to six meters. The project went well, but when testing time arrived, I found that the pair of 572B output tubes were pretty much useless. Their emission was extremely low. I didn't have another pair, and I wasn't willing to invest in a pair of new ones.
An old friend of mine, W4EDS, told me about the GI-7B Russian triode that sounded like a good substitute. They are good up into the microwave regions, have 350 watts of plate dissipation and best of all, he had a pair for me to try. He also had a pair of sockets that he had homebrewed. These are ceramic tubes with a big cast finned anode coolers. They must be forced air cooled up through the socket through the fins and then exhausted to outside the cabinet. They need a chimney around the anode coolers to direct the airflow. A big plus for these tubes is their extreme ruggedness. They were designed for the Russian military for use in tanks and mobile equipment. Best of all, they can be found for approximately $25 each, new.
The first step in the project involved stripping out all the components in the RF deck, including tuning capacitors, inductors, the old cooling fans, and the tube sockets and their vertical mounting bracket. The bracket was re-used to support the two muffin fans which I used for cooling. The bracket was moved to the rear lip of the RF deck. The two large holes where the original cooling fans were mounted were the correct size and spacing for the new GI-7B tube sockets. The original plate tuning capacitor was relocated to be used as the new loading capacitor, and a plate tuning capacitor of smaller capacitance was installed. A silver plated inductor was fabricated to use in the pi network output circuit. A new plate RF choke was make and installed. The original plate coupling capacitor was re-used.
A new tuned input circuit was built. I used the YU1AW design which is a "T" circuit and used a slug tuned coil, a piston trimmer and a ceramic trimmer all mounted on a right angle bracket.
The only changes to the power supply required that the original 6.3 VAC filament transformer be replaced with a 12.6 VAC one. I didn't like the original bias circuit so W4EDS and I plagerized the W4ZT design. I also mounted a second relay to short a 100K ohm resistor that is used to completely cut off the tubes when in standby. I built a two-transistor "soft key" circuit on a pc board for keying the amp. I found room to mount a small 12 VAC transformer to power a 12 volt DC regulator circuit to supply the control circuit and the fans. Also, be sure to check the original AC line cord and replace if frayed - I had to do so. You will need a good one especially if you wire the plate transformer for 220 VAC - which I highly recommend.
I spent a lot of time sealing the upper RF deck to insure all the airflow came up through the tube sockets, and not through extra holes in the RF chamber. A top cover plate of solid sheet aluminum was fabricated to replace the original perforated cover. The chimneys were made from 1/8" sheet hi-temp silicon rubber. They are wrapped around the tube and glued where they overlap. Here's a picture of the bottom side of the RF deck:
