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WF-55 Chassis wiring - part 3


There are just a few connections left to make. Start with the last ground connection from the turret board, the black wire that runs from the turret at the bottom end of C6. Trim, connect and solder it to the ground lug that's next to the rubber grommet (right).


Output transformer - secondary

Start by soldering the wires to the output transformer's secondary winding - the lower row of four 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.
      For speaker output, the first decision you need to take is which impedance you want to use. The original 5F1 Champ had only a 4-ohm speaker output, but your WF-55 can be tweaked to work with 4-ohm, 8-ohm or 16-ohm speakers
* For 4-ohm speaker: First, solder an orange wire to the 0 lug (the lug at the right of the row of four). Then solder a yellow wire to the 4-ohm lug. Both wires must be long enough to travel through the rubber grommet, and then run over to the speaker jack socket. Twist the two wires together and thread them through the grommet. Then jump to the next section on the output transformer - primary.
* For 8-ohm speaker: First, solder an orange wire to the 0 lug (the lug at the right of the row of four). Then solder a yellow wire to the 4-ohm lug (this is needed for the amplifier's negative feedback). Then solder a green wire to the 8-ohm lug. All three wires must be long enough to travel through the rubber grommet, and then run over to the speaker jack socket. Twist the wires together and thread them through the grommet. Then jump to the next section on the output transformer - primary.
* For 16-ohm speaker: First, solder an orange wire to the 0 lug (the lug at the right of the row of four). Then solder a yellow wire to the 4-ohm lug (this is needed for the amplifier's negative feedback). Then solder a grey wire to the 16-ohm lug. All three wires must be long enough to travel through the rubber grommet, and then run over to the speaker jack socket. Twist the wires together and thread them through the grommet. Then jump to the next section on the output transformer - primary.


If you opt for an 8-ohm speaker output, you'll solder wires to the bottom row of lugs as shown above. If you opt for 4-ohm, there'll be no green wire. If you opt for 16-ohm, there'll be a grey wire attached to the left lug and no green wire


With the wires soldered to the output transformer, remount it on the chassis (if necessary), twist the wires together and poke them through the rubber grommet

Output transformer - primary

You now have two sets of wires passing through the rubber grommet. Take the red and brown pair that come from the inside of the chassis and twist them together and run them over to the top row of solder lugs on the output transformer.
      It's important to remember that when the amp is running, these lugs 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 (see Insulating sleeves, right). Note: using the insulating sleeve and heatshrink can be fiddly, but it's a necessary safety precaution.
      The wires connect as follows: brown wire to the left lug; red wire to the right lug. Ignore the central lug in this row. Once the wiring and insulating is done, the fully wired-up transformer will look like the one below (depending on the speaker impedance you chose).



Insulating sleeves
With the wires trimmed to length, first slide them through the insulating sleeve (making sure you have it the right way round). Then slide an 8mm piece of heatshrink over each wire. When you solder the wire to the lug, don't make a fat ball of solder. Keep the joint small, so that the insulating sleeve will be able to slide over it (below).


With the wires soldered, pull the heatshrink back up the wire to cover the 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 you can slide the insulating sleeve up to cover the row of solder lugs.

Speaker socket wiring

There are just a few 'loose ends' left. First, complete the speaker output and negative feedback wiring. Inside the chassis, take the twisted bundle of wires from the output transformer's secondary and run them to the back of the chassis and then along to the speaker socket. First, trim, connect and solder the orange wire to lug 3 of the socket.
      Now connect the wire for your chosen speaker output (yellow for 4-ohms, green for 8-ohms, grey for 16-ohms) to lug 1 on the speaker socket. Don't solder it just yet, because what you do next depends on the speaker output you've chosen:
* If you chose 8- or 16-ohms, you can solder lug 1 now. This leaves you with the yellow wire left over: solder it to the turret at the end of R7 that's closest to the mains socket (shown right). Jump to the next section.
* However, if you chose 4-ohms, you have no wires left to make this negative feedback connection. So just make a short yellow connecting wire and solder one end to the turret at the end of R7 that's closest to the mains socket, and connect and solder the other end to lug 1 of the speaker socket.


Wire up the speaker socket using the orange wire for ground, and the yellow/green/grey wire (depending on the speaker impedance you have chosen)


Negative feedback for the WF-55 is taken from the 4-ohm speaker output (yellow wire) and injected back into the preamp circuit via the 22k resistor, R7

High voltage supply

The final connections for your WF-55 are the high voltage supply wires. Take some brown and white wire and solder them to the turrets at the junctions of the rectifier diodes: one to D1+D3 and the other to D2+D4 (either way around is fine).
      Twist these two wires together and run them around the back of the chassis and over to the power transformer, to the row of three lugs marked '275-190-0'. Solder the brown wire to the 275 lug and the white wire to the 0 lug.
      This completes the wiring. In the next section, you will test your chassis wiring and soldering work before powering up your amplifier.


The final connection is the high-voltage supply to the rectifier diodes


Once again, run the twisted brown and white wires around the back of the chassis - keeping these wires away from the signal-carrying part of the amplifier circuit helps to minimise hum

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