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M1ECY - Sean Williams - 23cm Linear Amp Project



23cm Linear Amp

These pictures show my progress in the construction of a 23cm amplifier.

This has been based on a design by N2UO, and uses a Russian surplus triode called a Gi7b, an interesting device that can be used from HF to 1.3 Ghz, it has a 350w anode dissipation, and best of all is really cheap (even at Ebay prices). Cast your minds back to the last Picketts Lock Rally - there were four of these valves for sale on the bring and buy stall, both myself and Alan (M0DAL) bought them, and ever since have wanted to use them.

This is where the problems occurred, all the designs published on the web assumed that you had access to a massive machine shop and limitless sizes of copper tubing, in good old Blighty, we have problems with the supply of metric tubing - even with EU Meddling we still adhere to the practice of imperial sizes and standard gauge thickness for tube walls. Bryan (G8DKK) then mentioned this design that used mainly plumbing sizes found in the US, it seemed logical that this would be the way forward, and some enquiries were made.

23cm Linear Amp

So, now we have materials, some copper some brass, I needed to then fit all of this together.

I admit, that given a hacksaw and thin wall tubing my cutting skills are awful to say the least, and seeing as I had some 10mm thick copper busbar for the base of the cavity, I felt that it was time to avail myself of better workshop facilities. I made all of the grid, cathode and heater lines at home in my workshop, even turning the ptfe spacers up as well (I recommend everyone has a go at turning PTFE on a small lathe, the mess is fantastic!)

Then I looked at the problems of making the anode cavity up. I know of a government surplus dealer in a village not far from me, who owed me a favour or two, so I went and made use of a large lathe he had there, now this was an experience, the lathes I trained on at college had all their belts and motors in the cabinet below, this beast has a three inch wide belt driven from a cross - shaft that is 10 feet up (I made a comment that Noah may have used this lathe, but was told it was the 1915 model, a Sullivan apparently). Anyway, things worked well on this old and unwieldy machine, and sped up production immensely.

The results you see in the pictures, not complete yet, but not bad for three days work, and when in a nice box with some volts flying around should give me about 250w of rf to play with, now, about that microwaved sparrow.............