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The power amplifier build part 4, power supplies

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Part 4 – Power Supplies De de de At first glance, power supplies may seem straightforward: transformers, rectifiers, filter capacitors and you’re done. In this project, that’s true for some of the low-voltage rails, but the bias and especially the screen supply needed considerably more thought. High Voltage Supply The high-voltage (HV) supply is built around a 3 kVA UI-core transformer , followed by a full-wave rectifier, an oil-filled capacitor, and bleeder resistors. Since Swedish homes typically have 3-phase service, I wired the two 230 V primaries in series across 400 V . This helps balance the load between phases and reduces light flicker during key-down. The secondary winding has multiple taps; I selected a tap that provides about 3.2 kV DC under load. Screen Supply This was the most time-consuming part of the project. Several designs were tried, and quite a few MOSFETs gave their lives before the regulator was stable. I eventually settled on a series-regulated design . Why no...
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The power amplifier build part 3 - input circuit

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Finished board, waiting to be populated with filter components  In part 3 we have a look at the input circuit used. Sine this is a grid driven linear, we have to feed rf exitacion to the control grid. If we look at the schematic, we can see the load - or swamping resistors - which basically sets the input impedance the transmitter will see, sans stray capacitances and inductances. The resistors sits in series with the bias supply, and are groun ochded at RF by a ceramic disk capacitor. Input circuit shown, My build is heavily influenced by EX8A. Also load resistors (2 k) shown. I ended up 1 k load resistors. There are designs using a broadband transformer for this kind of input circuits, but I wanted to wet my feet in filter making and tuning. On my 2x813 linear, I used an LDG Z100 autotuner, which works fine, but there is always the possibility to tune to a harmonic, so for this, I settled for a design using conventional fixed and switched PI filters. Everything is switched from a...
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Modifying coax connectors

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  Custom N-Adapter for the Amplifier Build Yesterday during the amplifier build, I needed a coax adapter – an N male to N female – but a bit longer than usual, to connect the amplifier output from the directional coupler to the T/R board. It turned out I hadn’t paid attention to the minimum bend radius of the coax, so I had to get creative. At first, I planned to just order an adapter, but I couldn’t find exactly what I needed. Instead, I decided to modify some existing connectors. Interestingly, the backshell thread on many clamp-type N connectors is the same as the retaining ring thread – 5/8"-24 TPI – meaning you can actually thread a female connector into the back of a male RG213 clamp connector. Unfortunately, that combination would have been too long. I own an early 1940’s Blomqvist lathe, a close copy of the famous South Bend 9". It’s fully equipped with all the gears for cutting threads, so my solution was to set up for 24 TPI and cut the thread directly onto the...
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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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