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    Active User New Member YU1OXB's Avatar
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    MF-1000 Home Made 1KW HF linear amplifier with two GI-7B

    The decision to make this linear amplifier came suddenly when a friend of mine (YT9M – Niksa) asked me to repair his old linear SOMMERKAMPFL-2500 modified to work with 5xPL519, that's when I saw the benefits and advantages of the higher power.

    It's nice to work with a small or medium power (5-10W or even up to 100 watts) but there are a lot of situations where it only brings frustration and annoyance and even the loss of a chance to do the rare country or rare prefix. Just a good antenna and power are solve the problem and there really is no alternative.

    Thinking about how to conceptualize, I decided to work with two small Russian ceramic triodes GI-7B since they are both easy to obtain and relatively cheap, they are physically small but very robust and durable and can provide up to 1KW PEP output power on a quite decent anode voltage which is not over 2500V. There are better and "more capable" tubes but they are more difficult to source and generally significantly more expensive and they demand higher voltages to work with them and that implies more specific and expensive passive elements to reliably work on such voltages which in turn bring more difficulties to source them and they are more expensive.

    All of this initiates a vicious circle leading to a larger, more expensive and more difficult device to work with. Thanks to the goodness of a few close friends, I came (in the form of gifts) in a possession of some key components, so it is very difficult to estimate a total invested funds in this building, but if I've just bought everything as normal material (all!) would certainly not be worth less than 5 - 600 Euros.

    My well intentioned advice is to first get all the key components -tubes, "plate" and "load" air variables, turbine or fans, the main transformer, electrolytics ... - and then very carefully deploy components on the table, seeking the most logical arrangement and layout with the shortest connections (it should not be overdone in order not to make it become so densely packed that components can no longer be approached!). Only when you are positive you have reached the best layout you can think of then you should start thinking about the design and manufacture of the case-housing.

    The reverse way is wrong, and that's what people often do ... they first find a box that they like and then start jostling components into it at all costs just to pack everything inside no matter what!
    Projects like this one demands a lot of will, effort and time, but if you are not sure that you will last to the end, you had better not start. Most of the time goes into thinking and planning the layout of components on a chassis, because the concept and schematic are just classic and there was no need to reinvent the wheel. As such we know similar units were worked out and “chewed” and tested who knows how many times, but the arrangement of components, to be with the shortest lines, in a most logical way ... that is what everyone has to solve in their own way and that aspect of how high power linear amplifier design works, DOES MATTER!

    Many advocate the theory: "... come on, that's HF... it doesn't matter if you run 25 cm wire from the output coil to the band-switch ...people have been working that way many times and it all works ..."but it is not exactly so. Maybe some people do it that way and may be it worked, but later you often hear somebody that “does not know what is going on say this oscillates, "how do I stop it"” and so and so on ...

    I have no intention of theorizing here, but will try, with a help of pictures and important remarks and explanations, to present this project so that other potential builders can repeat it with a greater chance of success. However, there are a few details that are done on my linear differently than usual, so I'll give them in a little more detail.

    As I said, the concept is pretty much classic, and there is not much room to do and say something radically new and different. Lamps are working in GG configuration with excitation in the cathode. At the output there is a more or less standard Pi filter which, to a certain extent, reduces higher harmonics in the output signal and also matches the output impedance of tubes to the standard impedance of coaxial cables i.e.50 ohms.

    An interesting detail is what I've done with a plate choke. In the beginning I started in quite a classical way ... as a body I used a15cm piece of fiberglass old telescopic fishing rod (it SHOULD NOT BE of the newer "carbon" or "graphite" rods ... IT MUST BE plain pure fiberglass) with an outside diameter of 22 mm on which I put *** turns of 0.6mm wire in four unequal sections. I didn't mention number of turns in sections because you will see later that it turned out it doesn't matter ... it may as well be closely wound without sections). Then followed the measurements of serial and parallel resonances until I got serial-resonances not falling close to the amateur bands. At the end it got around 135uH which is already sufficient inductance for correct operation of amplifier ... I have seen amps with a lot smaller chokes. Reactance of the plate choke should be at the lowest operating frequency at least 4 to 5 times higher than the impedance of the point at which it is connected, and this choke at 3.5MHz has a reactance of nearly 3Kohms which is not even a full two times of 1600 ohm of two tubes plate output impedance. Then it occurred to me to try to put inside of the choke former a 12cm long and 10mm thick ferrite rod used for medium-wave antenna from old portable receivers. Logic was: if people are using them in a broadband balloons for antennas in HF range and push 1-2KWthrough them, why could they not be used here.

    The Ferrite rod is positioned in the center of the coil former and to prevent movement, I made two small "toroidal" shaped pieces of hard foam and pulled them tightly over the both ends of the ferrite rod and shoved it all together in the hollow choke former. I did not touch the turns of the choke and the addition of ferrite raised inductance to 980uH and now there is nowhere any resonance throughout the entire HF range and reactance at 3.5MHz is 21.5 Kohms! At full power and on any band plate choke is cold as is the ferrite rod in it.
    The next interesting detail is the plate suppressors, which were(together with plate chokes) the most common pain in the a** when building tube linear. The most common mistake is the use of copper or even silver-plated wire or strip! Suppressor is the component that should thwart the onset of VHF parasitic oscillation, and whenever you want to reduce or disable some oscillations, you are reaching for some form of reduction in Q-factor in the circuit, and by use of copper or (even worse) silver wire or strip you are just doing the opposite – you are increasing the Q-factor and consequently increase the chances of self-oscillations. Probably the majority of people are afraid that if they don't put highest quality wiring, they will lose significant part of the power that will not be transferred from the anode to the output PI section. This is simply not true and the loss, if there is any, is in practice negligible.

    I used 1.2mm thick "constantan" resistant wire, although with the same success one can use Nichrome or Cantal. The difference is that neither the Nichrome not Cantal can be soldered (at least not by our standard way - there are special lots for these materials) while the constantan is perfectly solderable. I just made a loop in the shape of the letter "U" or “hairpin loop” with a total length of 10-12cm before folding. Wires are folded around in the middle over a body with diameter of 20-22 mm. It shows the same results if suppressor is as a classic coil wound with 3 turns on the body diameter 7-8mm but stretched so that length is about 20-25 mm. These things are not at all so critical so they don't have to be copied to the tenth of a millimeter exactly. Parallel to these coils I soldered the 3 carbon composite resistors of 150 ohms / 2W each to get about50 ohms 5-6W. That also is not critical because this value can range from 33 to 75 ohms without any noticeable difference in operation, still I rarely go over 50-56 ohms. I never put a resistor inside suppressors coil and certainly not wind the coil on them. Be sure that the suppressor coils are placed so that they are as far as possible from each other and in particular that they are not parallel.

    TO CONTINUE >>>

    Pictures of front panel...



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