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. Once everything existed in CAD, laying it out was a breeze. I placed all components with short lead lengths in mind, and tried to keep things flexible in case I ever want to swap tube types. (A QBL 5-3500 is still sitting in the back of my mind.)



Laser Time

Once the layout was done, I created flat patterns of all panels and brackets and prepared them for cutting. One Saturday was spent running parts through the CNC laser – everything was cut in 2 mm aluminum, except for the vacuum capacitor mounts, which were done in 12 mm aluminum for extra rigidity.

Another Saturday was spent at the press brake, bending everything: side panels, angle brackets, top covers. If you’ve ever used a press brake, you know how easy it is to destroy perfectly good sheet metal. Sure, you can calculate bend allowance and K-factors, but I find it much more reliable to use scrap strips to dial in the settings before committing to real parts.


Assembly

For the top parts of the enclosure – where the vacuum capacitors mount – I wanted proper alignment. So instead of bent flanges, I used threaded square aluminum bars to join the panels. That meant a lot of drilling and tapping, but thanks to Mike (SJ2W) and his CNC mill, it was straightforward: we drilled all holes on the mill, then tapped them manually.





Once all parts were ready, it was time to assemble. The bottom chassis is held together with countersunk screws and internal brackets. It’s divided into sections to keep input and output circuits isolated, and the stepper motor compartment is fully shielded. All control lines pass through feedthrough capacitors, minimizing the chance of RF noise getting in when the motors are running.

A few parts were turned on my lathe – including custom standoffs for the stepper mounts. 


That First Fit

The joy of putting together something you’ve designed, cut, and machined yourself – and seeing it fit perfectly – is hard to describe. It’s extremely satisfying.



Top view of the RF deck


Detail of C1 cold end mount



Side view showing the hole where HV will enter and RF will exit



HV feedthrough


HV choke and blocking capacitor



80 and 160M padding relays


C2 hot end, bracket detail


GU81 sockets


C2 capacitor


10M bypass relay


160-30M tank coil with B2B vacuum relays





To be continued

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