Need and easy to build 100 watt 6 meter linear that can provide 10 dB or so gain? Have a 28 VDC supply available capable of about 6 amperes to run it? Then read on.
This design was originally published in the publication QEX in June 1982. It is still a valid design today, providing you can obtain an appropriate transistor. The drawing shows it using an S175-28 transistor from CTC. It can also be built with a Solid State Microwave SD-1450 or even an SD-1407. Neither company exists today but these devices are still available from the Ham community. You may just have to look around a bit. There are certainly devices being made today that might well fit this application though none of them have been tried by the author so you are on your own. Any currently made device may require circuit modifications in order to work properly.
This amplifier is easy to build and almost as easy to tune up. Good VHF building practices are very important for this amplifier. Construction of this amplifier may not be for the novice builder who may be unfamiliar with these practices. Short low impedance connections are a must. When I first designed and tested this amplifier it was found to be extremely stable into severe mis-matches but of course it was most happy when seeing a nice 50 ohm load. A fairly substantial heatsink is required for this amplifier, especially if you are contest oriented. It is designed strictly as a linear though with some minor modifications, and increased heatsinking, it could be set up as a Class C amplifier for FM (or even CW) use. The entire amplifier will easily fit on a 4" X 6" piece of G-10 epoxy circuit board. This can easily be mounted inside a standard 5" X 7" chassis with a cover.
A every day 50 volt 1 ampere diode temperature sensor is used to provide feedback to adjust the bias voltage to maintain linearity and is cemented directly onto on of the transistor mounting bolts. I suggest using brass hardware to mount the device. A nice upgrade to the described bias circuit would be the W3KM bias network shown elsewhere on this web site. A link is listed at the bottom to go to that circuit. Using Dave's circuit will provide slightly better results and be a little more fail safe. Though not at all required, you should look at his bias circuit and decide which way you want to go prior to starting construction.
Construction is done using point to point layout. No circuit board has been designed and it is not needed. You can use some of the larger components themselves as support for resistors, bypass capacitors, and wiring. A small piece of G-10 board can also be cut out and either soldered or glued to the main circuit board to provide wiring lands. I gave the amplifier away recently so no picture is available. You will also notice that I used a BNC on the input and a type N on the output. I do not like UHF connectors, especially at VHF and above frequencies. 50 MHz is near the upper limit for a "UHF" connector before they start to introduce SWR. However, you can use UHF connectors but I advise against it. In spite of the name they were given, they are terrible at UHF frequencies.
Tune up is fairly straight forward. Of course it is always great if you have a spectrum analyzer and an appropriate attenuator available but they are not required. When I first designed the amplifier I was lucky to be working for a company that had the equipment to tune this amplifier into a spectrum analyzer. But don't let the lack of this type equipment stop you from building and using this reliable amplifier.
Once you have your amplifier completed I strongly suggest you go over your wiring very carefully at least one more time. Make certain there are no left over solder bridges between components that can cause tune up problems or device failure. And of course, look for that one connection that may have been overlooked and left unsoldered. Next apply about 13 VDC to the amplifier through a meter capable of at least 250 mA. With no drive applied, quiescent (idling) current should be around 25-50 mA. If it is not, then adjust the value of the 110 ohm 2 watt resistor to get it into this range. Reducing the resistance will increase quiescent (idle) current. You could also put a potentiometer in place of the fixed resistor but that is one level of complication I choose to exclude. You would not want it to get bumped later on and kick you out of the linear range or worse yet, destroy the device. It is up to you though.
Terminate the output in a 50 ohm dummy load. Switch to a meter capable to measuring at least 10 amperes full scale. Now apply a very small amount of CW drive (maybe 1/2 to 1 watt) through an SWR meter rated for 6 meters and adjust C1 and C2 for minimum reflected power. This should also result in increased collector current on your ammeter. Now move the power meter to the output side of the amplifier, reapply drive and tune C4 and C5 for maximum power output. Now increase the drive signal to around 10 watts and readjust C4 and C5. You should also go back and retune C1 and C2 for minimum input VSWR. At this point you should have around 40 watts of output. Next apply 28 VDC and go through the same tuning procedure one last time. Now you should be seeing around 100 watts output with 10 watts of drive.
If you do have access to an inline attenuator and spectrum analyzer, you can adjust C4 and C5 to attenuate the harmonics as much as possible. What you should find is that all harmonics are greater then -60 dB down from the main carrier.
Remove the dummy load from the output and connect the amplifier into your active transmit circuit. You will need a pair of relays here to switch the amplifier in and out since it will not make a good receive preamp! Now you are ready for some on the air tests to make sure it is operating in the linear mode. (if you have the equipment, do a two tone test to be certain) But, if your initial idling current was as specified, there is no reason to suspect it will not be running linear. Of course there are different linear modes but most equipment winds up running at or close to Class AB. A true purist will only run Class A but 100 watts can not be obtained in Class A mode and is hardly needed for Amateur Radio operations.
Now make sure the amplifier is inside of a metal enclosure and you are ready to go. The previously mentioned standard 5" X 7" bow will work great; just keep the cover on it.
This has not been done by the author but:
To convert this amplifier to Class C for FM operation remove the 10 ohm resistor from the base to ground and ground the bottom of L2. That should get you close.
Good luck and good DXing on 6.
To check out Dave's (W3KM) bias circuit go here:
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