Tube Tracer Build 3 - Connecting the Tubes

Tube docking bay

25 x 25 mm aluminum profile was used to construct the docking bay for the tube adapters. Each tube socket type will have a simple PCB, which acts as the base for the tube socket. The electrical connections to the main unit are made using a nine pin D-sub connector. There are of course numerous possibilities for connecting the six wires, but considering the easy availability and low price, the D-sub connector was chosen. 

The heater cables are connected using pins 1 and 2 for one cable and 4+5 for the other. The use of two pins per cable increases the current carrying capacity of the connector.

The docking bay made of aluminum profile. Case front is on the right.
Small pieces of thinner profile are fixed inside to retain the adapter PCB's

The docking bay size is approximately 100*120 mm, but its body spans the whole depth of the case (front to back, appr. 240 mm) The body is fixed to the back profile with screws. There is a space of approximately 90*100 mm behind the docking bay, which was just suitable for a 80*80 mm cooling fan. I purchased the fan quite early and planned it to run from the 19 V supply. Therefore a 24 V fan was chosen although a 230 V AC fan would have been equally suitable now that AC input is available inside the case.

Tube adapters

As already mentioned, the tube adapters were constructed of single side PCB material. There is not much in terms of PCB design, since there are only the D-sub connector and two three-pin screw terminals soldered on the PCB. The tube sockets are mounted with screws and spacer blocks on the PCB and the necessary pin connections made with suitable lengths of connecting cable.

My first tube adapter PCB  for Noval socket tubes

If the tube uses top contacts (plate or grid), they can be connected to the screw terminals with top contact leads. The same leads can be used with any tube adapter.

Each adapter costs only about 2 € plus the price of the tube socket, so it is easy to make as many as needed. I made the original design using Eagle software. The CAM process used the Roland CNC router in the Oulu University FabLab. The input data uses simply PNG-images of the traces and outline of the PCB.

After my first prototype I noticed that it is easier to tweak the traces with an image manipulation software. Consequently, I made the final design using GIMP. I needed to add more insulation space around the high voltage traces (plate and screen grid) and broaden the heater traces to increase the current capacity.

The PCBs are essentially identical regardless of the tube socket type. I included a small pilot hole in the design to indicate the center of the tube socket, which helps to ensure that all tubes will be standing in the same center location. The tube sockets will be standing above the base PCB. The exact height is not critical since the adapter PCB sits about 39 mm below the face plate. The idea is that none of the sockets will protrude the face plate.

If I want to store tube adapters in the lid part of the case, the free height of the lid limits the total thickness of the adapter assembly.

The face plate has a rectangular opening for the installation of tube adapters. The opening is 100 * 120 mm with rounded corners. For cosmetic reasons I have docking bay cover plates cut from the Dilite stock. I plan to have several cover plates having different size holes for the tubes. The future will show how many I end up making.

The cover plates are used to cover the tube adapter opening when operating the uTracer. They are held in position by small strips of Dilite glued under the corners of the opening. Consequently, the covers are neatly flush with the face plate.