Following on from last month’s instalment, we now have a chassis that’s ready to accept all of the parts for the build.
The first step is to mount the valve sockets along with the retaining clips. These one-inch octal sockets from tubeampdoctor.com are a single-piece ceramic design with excellent two-hole solder lugs making our wire connections simple and neat. Mounted in the correct orientation with M3 hardware, they sit proudly in the chassis. Next up I install both of the transformers with M4 hardware and insert 10mm rubber grommets into the wire holes. This will prevent the sharp metal edges rubbing through our wires.
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Every amp builder is different, but I prefer to wire up as much of the chassis as I can before dropping in the board. This allows me to get to those hard-to-reach joints and ensure the neatest lead-dress possible. I start by mounting any components to the sockets, along with any inter-socket connections using vintage-style solid-core cloth wire. I like to stick to Fender’s original colour schemes of yellow, brown, and blue for signal wires, red for high-voltage, and black for ground/chassis connections. Once these are done, I can move onto the transformer wires.
With long runs of wire required on the outside of the chassis, keeping things neat is paramount. A useful trick is to use small sections of heat shrink to keep it all in order. The output transformer wires are run along the length of the chassis, before making a right-angle heading towards their destination. Once inside the chassis, we can make our connections to the sockets, jacks, and ground connections as required. The wires coming from the mains transformer enter the chassis almost directly below where they leave the unit itself. No messing around here, straight inside and soldered into place.
Moving on to the front panel, we install the potentiometers and input sockets and then add the mixer resistors to the guitar inputs, and the grid-leak resistor to the microphone side. Everything is mounted with lock washers to keep it all in place. With that done, the final wiring connections are made inside the chassis, including the mains input socket and power switch. We found a cool illuminating toggle unit that’ll eliminate the need for a separate jewel light. It’s also a DPST (double pole, single throw) so we can switch both the live and neutral wires at the same time. Wonderful.
Back to our eyelet board – it’s time to add some wires. We need wires that’ll connect the board to the inputs, valve sockets, potentiometers, and chassis ground. I like to run longer wires than I’ll need, just in case I’ve made a mistake in my layout diagram. Two wires are connecting our phase-inverter and preamp circuits to the high-voltage rail; running these over the top of the board rather than underneath allows for ease of servicing, should it be needed in the future. Any ground connections tying components together are done on the underside of the board. With all of these done, we can finally mate the chassis and board together.
Sitting on 6mm standoffs and secured with M3 hardware, the FR4 board sits rigidly inside the chassis. It won’t move or vibrate in sympathy with the speaker vibrations and won’t flex either, which would put extra strain on the components.
Dressing to impress
Starting at the bottom right-hand corner, I work around the board in a clockwise direction. The standard of your lead dress is imperative to a low-noise amplifier and the vintage cloth wire is a dream to work with. The cloth covering has a light wax coating which alongside the solid-core wire itself, allows for the wire to be manipulated into any position you want without it moving.
Always running the wires up against the chassis, rather than flapping in the breeze, has both a visual and operational benefit. Straight lines with radiused right-angles are the way to go when running your wires, with careful attention to which wires you can run parallel to each other and which you shouldn’t.
The three 6SQ7 sockets and the single 6N7 are wired up and the resulting look is pleasing to the eye. I follow the same principles as I make my way around the board finishing up at the top-right hand corner of the chassis with the microphone input. My last job before we can fire it up is to wire up the 6.3v heater circuit, connecting all of the valves (except the rectifier) together in parallel, and onwards to the 6.3v winding on the power transformer.
I use 18awg solid-core cloth green wire for this. It’s rated for at least double the current that’ll flow through it and, due to its thicker gauge, it’s easy to manipulate into position. There are many thoughts on the ‘best’ way to run your heater wiring and one day we’ll get into that. I prefer to float mine above the sockets to keep the noisy AC as far away from my signal wires as possible. With this job done the chassis is finally ready for testing. Come back next month to see how it sounds.
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