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The Power Amplifier build. Part 2

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Power Amplifier Saga – Part 2: The RF Deck So, back in 2020, I was working for an old friend of mine who runs a machine shop. I was operating the CNC laser cutter – a 4 kW fiber laser – and spending my days around a machine that could turn sheet metal into precision cut panels I started planning how to use this in my amplifier build. Then, as we all remember, COVID hit . I got sick, along with the rest of the family, and ended up spending two weeks at home in the middle of winter. Tired, restless, but with just enough energy to sit at a screen, I decided to finally learn Fusion 360 – to take advantage of the situation and make plans for the amplifier. From Components to CAD During those two weeks, I modeled the entire RF deck from scratch. That meant creating accurate models of: Variable capacitors  Roller inductor and fixed coils GU81 tubes and sockets Vacuum relays Stepper motors (luckily, existing models helped here) It was a lot of work – but worth it . On...
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The Power Amplifier Build – Prologue, Part 1

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The Power Amplifier Build – Prologue, Part 1 Big Tubes, Tank Circuits and Design Philosophy This project started brewing back in 2019 (actually, even before that), and by 2020 I had begun collecting the last pieces and putting the plan together. I had previously built an amplifier using 813s, but I wanted to take things up a notch — both in terms of scale and concept. One of the builds that really caught my eye was from EX8A. Although my final design diverges significantly from his, his approach showed a lot of clever problem-solving. In particular, his plate choke switching network intrigued me. Clarifying EX8A's Choke Design EX8A uses a conventional DC feed with the plate choke placed directly at the tube anodes. What makes his approach interesting is the use of relays to switch different segments of the plate choke. This, in combination with a bypass capacitor and the tubes' internal capacitance, creates a parallel resonant circuit. The purpose is twofold: To avoid destruc...
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Etching PCB's at home

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 Etching PCB's at home, why bother? Because let's face it, It's cheap beyond ridicoulus to have 5 professional boards fabricated, and if you don't mind waiting, shipping is not too bad either.  There are however moments when you just want something done, and it's simple enough to do at home, or when in a pinch, and you dont feel like waiting for the chinese board houses to deliver. Cons are lack of thru plated holes, and generally only single sided boards, the way I make them is possible. I usually turn to home etching for obvious one-offs, like high voltage power supply capacitor banks in small linears and transmitters, small relay boards, and simpler things like that, but I've also made an esp32-SI5351 vfo, using lot of smd components, with great success. I begin by making a design, in my case in EasyEDA (yes I know Kicad is a better choice, but havent had the courage to make the transition). When etching at home, I make the minimum track-to-stuff distance big...
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High voltage lab power supply

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  The time has come to wake up this old gem I've had sitting in the basement for a few years. It's an Oltronix LS107, a Swedish made, regulated power supply with an adjustable 0-500 volts at 200mA, 2 separate 6,3 volt windings, and a 30mA 0-150 volts negative bias supply. This is built up with vacuum tubes (who could have guessed) with 3(!) EL34's, a couple UL84's, UF80's and an 85A2 regulator tube. The circuit is a series-shunt regulated supply, with the EL34's acting as pass elements, while an UF80 shunts the control grid voltage from the EL34's to ground, using feedback from the output to it's control grid, and the 85A2 as reference at it's cathode. Before power up the unit, I begun by opening it, and had a good look at all the components. Nothing looked burnt or discolored. Not even the inside of the cabinet that sit close to the EL34's shows any signs of heat. A good sign. This has probably not been used much.  My initial plan was to replace...
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