Part 9 - Chassis wiring guide III



Wiring the mains supply

First, use a short piece of green wire to connect the Earth lug of the IEC socket (J5) 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 25cm 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. The back panel will look like the first picture (right).
      Solder these brown and blue wires, one each to the centre pair of S1's solder lugs. Then take two 18cm lengths of red wire and solder them to the upper pair of lugs. The result will look like the second photo on the right.


The IEC socket's Earth connector must be connected to chassis ground


The mains input is connected to the Power switch's centre lugs and the red wires connect to the upper pair of lugs


Power transformer connections I

Take the two red wires from the Power switch and 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 20cm, 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).
      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.


From the front panel, a twisted pair of red wires run (from the Power switch) and another pair of twisted grey wires (from the neon indicator) run to the power transformer primary winding. Don't forget the short red wire!


Power transformer connections II

The final connections on the power transformer are for the high voltage winding, Sec1. Take 20cm of white wire and solder it to the '0' lug on the Sec1 winding. Then take 20cm of brown wire and connect it to the '275' lug on the same winding. (Note that that '190' lug is not used.).
      Twist these two wires together and run them across towards the turret board. At this point the power transformer should now look like the photo on the right. Use cable ties to help direct the cable runs (see Keeping it tidy, right).
      Take the brown and white twisted bundle of wire, and run it over to the turret board. These wires connect to the centre pair of turrets that hold the four rectifier diodes. Trim each wire to length and solder one to each turret, as shown below (either way around is fine).



The brown and white wires are the final connections for the power transformer

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 and fix if there are problems later


Output transformer connections I

Start by soldering the wires to the output transformer's secondary winding - the lower row of three solder lugs for the speaker connections. Tip: when soldering wires to this bottom row of lugs on the output transformer, you may find it easier to temporarily dismount it from the chassis.
First, solder about 22cm of orange wire to the 0 lug (the lug on the right of the row). Then solder 22cm of green wire to the 8-ohm lug and 22cm of grey wire to the 16-ohm lug. Twist all three wires together and thread them through the rubber grommet.
      Inside the chassis, take this bundle of wires and run it to the speaker sockets, J3 (16-ohm) and J4 (8-ohm). Trim and solder the grey wire to lug 1 of J3, and the green wire to lug 1 of J4. Trim and connect the orange wire to lug 3 or 4 of either socket, but don't solder it yet.
      Use a piece of the uninsulated solid-core wire to run through lugs 3 and 4 of both sockets. Solder some black wire to the grounding lug that's close to the 8-ohm socket and then connect the other end of this wire to lug 3 of J4. Now solder lugs 3 and 4 on both sockets. The speaker socket wiring should now look like the second photo on the right.


Add three wires to the output transformer secondary, pass them through the rubber grommet...


...and solder them to the speaker output sockets. Add bare wire and a black wire to ground the sockets


Output transformer connections II

It's important to remember that when the amp is running, the top row of lugs on the output transformer carry a very high DC voltage - the only part of the amp outside the chassis that does. So this set of lugs must be properly insulated against accidental touch. Using the insulating sleeve and heatshrink can be fiddly, but it's a necessary safety precaution.
      First, connect 22cm of white wire to the left lug, 22cm of brown wire to the centre lug and 22cm of red wire to the right lug. Solder and insulate these wires as shown in Insulating sleeves, right). Then twist these wires together and push them through the rubber grommet. The fully wired-up transformer will look like the one shown below.



Insulating sleeves
When soldering to the top row of lugs on the output transformer, don't make a fat ball of solder. Keep the joint small, so that there will be room for the heatshrink and the insulating sleeve will be able to slide over it (below).


Slide an 8mm piece of heatshrink up each wire and over the soldered joint. Apply heat so that the tubing shrinks around the joint (below). Add an 8mm piece of heatshrink to the unused centre lug, and apply heat to insulate that lug, too. Now slide each wire through the appropriate hole in the insulating sleeve (making sure you have it the right way round!). Finally, slide the insulating sleeve up to cover the row of solder lugs.




Final turret board connections

The final connections for your PP-18 are the three wires from the output transformer primary. Take the white, brown and red wires that come through the rubber grommet and run them over to the row of three turrets between C11 and C12 on the turret board. Connect them to the tops of the turrets as shown here:
* white to the turret closest to the valves
* brown to the centre turret
* red to the turret closest to the front panel


Connect the white, brown and red wires from the output transformer to the top of the turrets in the centre of the turret board


Get ready for testing!

This completes the wiring. Your completed chassis wiring should look like the image below. Before you power up your amplifier, it's vitally important that you move on to testing your chassis wiring and soldering work (see Part 10).


  PP-18 construction guide
  1 - Kit contents
  2 - Block diagram
  3 - Reading your schematic
  4 - Chassis assembly
  5 - Turret board assembly
  6 - Turret board wiring
  7 - Chassis wiring guide I
  8 - Chassis wiring guide II
  9 - Chassis wiring guide III
  10 - Initial testing
  11 - Final test

  Main diagrams:
  Schematic
  Input wiring
  Control wiring
  Turret board layout
  Turret board wiring
  Test-point diagram
  Final chassis picture

  PP-18 kit purchase page

  PP-18 construction guide
  1 - Kit contents
  2 - Block diagram
  3 - Reading your schematic
  4 - Chassis assembly
  5 - Turret board assembly
  6 - Turret board wiring
  7 - Chassis wiring guide I
  8 - Chassis wiring guide II
  9 - Chassis wiring guide III
  10 - Initial testing
  11 - Final test

  Main diagrams:
  Schematic
  Input wiring
  Control wiring
  Turret board layout
  Turret board wiring
  Test-point diagram
  Final chassis picture

  PP-18 kit purchase page