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SE-5a Chassis wiring - part 2


In this part of your build, you will add the wires that connect the turret board to the chassis and front panels controls, in addition to all of the mains-related connections. As always, check twice and cut once!

Wiring the front panel controls

There are six wires (five blue and one black) that run from the turret board to the pots on the front panel controls (see Wiring guide diagram). In addition, there are some extra wires to add between the controls; these are all detailed here. To start, there are several wires that you can do straightaway, so trim and solder:
* the blue wire from R4 to lug 1 of VR1 (Gain)
* the blue wire from C4 to lug 1 of VR2 (Treble)
* the blue wire from C6 to lug 2 of VR4 (Middle)
* the blue wire from R8 to lug 2 of VR5 (Master)
Next, there are some new wires to add and solder directly:
* blue wire to connect lug 2 of VR1 (Gain) to pin 7 of V1
* blue wire to connect lug 2 of VR2 (Treble) to lug 1 of VR5 (Master)
* blue wire to connect lug 1 of VR4 (Middle) to lug 3 of VR3 (Bass)
Finally, there are several solder lugs that take several connections:
* for lug 3 of VR1 (Gain), attach the black wire from C1+R3, but before soldering it in place, add a new black wire that's long enough to attach to lug 3 of VR4 (Middle); now solder lug 3 of VR1
* for lug 3 of VR4 (Middle) you have one wire already attached; now attach another black wire that's long enough to reach lug 3 of VR5 (Master). You can now solder lug 3 of VR4 and lug 3 of VR5
* for lug 3 of VR2 (Treble), attach the blue wire from C5, as well as a new blue wire that's long enough to reach lug 2 of VR3 (Bass). Solder both lugs.
The above list seems daunting, and it's one of those jobs that many amp builders hate, but take it step-by-step and it's not so bad. Once you're done, the diagram below shows how your front panel controls should look. (Note that I've used two blue colours in the diagram for clarity.) Check each connection before continuing.

Grounding the turret board

At this point, there are now just two turret board wires remaining unconnected. These two black wires ground the circuit to the chassis. The first is attached to C8; trim and solder it to the ground lug close to the input socket (J1). This grounds the preamp part of the circuit.
The other black wire is connected to C11 and must be trimmed and soldered to the grounding lug closest to the Standby switch (S2). This grounds the power section of the amplifier circuit.
At this point, your chassis will look like the photo below, with all of the wires that run from the turret board soldered into place.




Trim and solder the two black ground wires to the grounding lugs near the front panel

Wiring the IEC socket and fuse

First, use a short piece of green wire to connect the Earth lug of the IEC socket (J4) to the ground lug that's closest to the socket. Solder the wire at both ends. This is possibly the most important connection in the whole amp - it earths the chassis, the guitar and many of the amp's components.
For the Live supply, solder a short brown wire from the Live lug on the IEC socket to the rear lug on fuseholder (F1). Then solder about 20cm of brown wire to the fuseholder's other solder lug and run it across the chassis towards the S1 Power switch - but don't solder it to the switch yet. Take 30cm of blue wire and solder it to the IEC socket's Neutral terminal. Run this wire across to the fuseholder, and twist it with the brown wire as you take it across to the Power switch. Solder this pair of wires, one each to the centre pair of S1's solder lugs. The result will look like the second photo on the right.


Connect the mains input's Earth input to the grounding lug on the bottom of the chassis


Twist the brown and blue mains supply wires together as you run them across the chassis to the Power switch

Connecting the mains supply

Now take two pieces of red wire, each about 13cm long, and solder one to each of the top pair of solder lugs on the Power switch. Twist them together and run them back along the route of the blue+brown twisted pair, go about a third of the way across the chassis and then run them over to the power transformer, T1. Connect - but don't solder - these two red wires to the outer pair of lugs on the primary winding (these are the outer two lugs labelled "0V" and "120V").
Connect a pair of grey wires, each about 13cm, to the same pair of lugs on the power transformer primary, twist the wires together and run them back along the path of the red+red and brown+blue wire bundle and over to the neon indicator (L1) on the front panel. Trim and attach one grey wire to each of the two solder lugs on the neon indicator. Now solder all four solder lugs (two on the transformer primary and two on the neon indicator). Use one of the cable-ties to hold the three bundles of wire neatly together (see Keeping it tidy, right).
Finally, make a short connecting wire and connect the centre pair of lugs on the primary of the the power transformer (the ones labelled "120V" and "0V"). The result should look like that shown on the right.

Keeping it tidy
Most amp makers adopt a few simple measures to help keep their amp's wiring under control:
* Twisting related wires - such as the wires from each of the power transformer's windings. It's much easier to route them around the chassis, and the added stiffness makes them more likely to stay put
* Using cable ties - three or four carefully placed ties keep groups of wires together. It avoids 'rat-nest' wiring layouts which are a nightmare to follow fix if there are problems later


The Power switch and neon indicator connect to the mains side of the power transformer. Make sure to remember the short red wire connecting the centre pair of lugs on the two 0-120V windings!

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