I believe that the parts layout into their functional sections on a circuit board is one of the most challenging parts of tube amp construction. I tend to put a great amount of thought into this camp, and I usually have a few kinks that have to be worked out in the end. The toughest part is when I get crosstalk that causes oscillation. Hopefully I have learned enough lessons over the years that this one actually works as planned.
The chassis and parts layout happen at about the same time for me. I used Inkscape to do the layout. Here’s the information that it takes to make this happen: part values, part dimensions, a schematic, and anything else that will help you draw a rough sketch of the layout.
In Inkscape I drew squares for the part sizes as looking down on the circuit board. I started with my turret board and then populated it with the parts as I followed them on the schematic. I started with the power supply on the right side, moved to the preamp, phase splitter, and finished with the power amp. Then I checked the schematic and rough bill of materials to make sure all the mounted parts were taken into account. Mounted parts include transformers, circuit board, pots, jacks, switches, tube sockets, and any fasteners.
While the layout was made I was sure to take into account any wires that crossed. I tried my best to keep in-phase signals close and out-of-phase signals far, far away. Anything out-of-phase would need to cross at 90 degree angles and preferably at as much distance as possible.
Once the Inkscape layout was created, I used Google SketchUp to model the chassis. It isn’t difficult to make a 3D model of the chassis. You make the metal part, and then you measure and draw holes where you’ll drill it. Piece of cake. This way I am able to use that model to punch and drill the chassis with accurate measurements.
There may be a way that would suit you better such as pencil and paper, but I like to use computers as much as I can. I understand that it might seem like a huge hurdle to try and learn a complicated CAD program to do layouts when you could easily do it on paper. Whatever method that you use, be sure to do a dry fit before drilling holes in a twenty dollar chassis. This is time consuming on a first run, but it’ll save you some heartache in the end.
Watch these dimensions when designing:
- Transformer mounting positions and shapes
- Potentiometer sizes and panel mount drill sizes
- Knob sizes (not too close)
- Spacing of panel mounted controls and jacks
- Size of external mounted parts and clearances
- Tube spacing and wire routes
- Chassis dimensions
- Component part sizes and circuit board spacing
- Watch your signal flow!