The output transistors need to be mounted to your heatsink. Using the heat pads and transistor sockets mount the transistors to the heatsink using #6 3/4" sheet metal screws. The transistors should be mounted in the order shown in the picture above. Now you'll need to solder the leads onto each transistor. The blue wire is 20 gauge and is connected to the collector pin. The white wire (with a yellow stripe) is 20 gauge and is connected to the emitter pin. The purple wire is 22 gauge and is connected to the base pin. These pins are marked with a B, C, or E on the casing. Each wire needs to be about 6 inches in length, but it varies because each transistor is a different distance from the circuit board. The wires for the four transistors near the center of the heatsink won't need to be as long as the wire for the four transistors on the edges of the heatsink.

The transistors leads now need to be connected to their proper locations on the board. Just connect the pins to the correct holes on the circuit board, the labels for these holes can be found in the Circuit Board section. Feel free to take measures to make the wiring neat, but remember that the shorter the wires are the better.

 

Bias Diodes

My implementation of the bias diodes is a deviation from the way Dr. Leach recommends (section marked "Installing Bias Diodes"). You'll have to decide for yourself which one to use, but I think that the way I'll illustrate is both easier and more effective. Instead of mounting the diodes inside the heatsink wall they will be mounted on the surface and thermally coupled to the heatsink with double-sided thermal tape. This process is much simpler because it doesn't involve drilling holes in the heatsink and then gluing the diodes into those holes. I believe my method is probably more effective because most glue doesn't have a very high thermal conductivity. With my method you are coupling the diodes to the wall using a tape that does conduct heat well.

The diode leads will have to be trimmed so that you can solder them closely together. The method of "tie wrapping" involves using a single strand of wire taken from a dissected piece of stranded wire to wrap the leads of the diodes together. This gives you a good mechanical connection between the two leads you intend to solder and makes the whole process much easier. You may wish to use heat shrink tubing over the joints between diodes to protect them from touching the heatsink wall. The thermal tape I used does not conduct electricity, so in theory this should be enough of an insulator, but I added heat shrink just in case. The wire marked A goes to the hole in the circuit board marked 'A' and the wire marked B goes to the hole in the circuit board marked 'B'. I used white 22 gauge wire for my A wire and purple 22 gauge wire for my B wire. Now solder these wires to the circuit board.

 

Mounting Components

Before we can mount the components inside the chassis we need to go ahead and solder some leads to the circuit boards so that they can be connected to the rest of the components. Once the circuit boards are mounted inside the chassis it would be difficult (or impossible) to solder new wires to them. The lengths that these wires need to be will vary depending on where they are going. Below is a list of where each wire comes from, will go to, color, gauge and approximate length. Remember, it's not a big deal to trim a wire that's too long, but extending a wire that's too short is a pain.

From	To	Color	Gauge	Length
-VCC	Either F4 or F5	White	18	8"
+VCC	Either F2 or F3	Blue	18	8"
Output	Red Binding Post	Blue	18	8"
(2) Central Ground	Central Ground	Purple	22	12"
Signal In	Center Conductor of RCA Jack	White	20	8"
Signal Grd	Outer Conductor of RCA Jack	Purple	22	12"

Once you've finished soldering these wires into place your amp should resemble the picture at the beginning of this section. Now we can move forward to mounting the components into the chassis

You should probably mount the heatsink and circuit boards first since they are the most troublesome. Carefully slide the circuit boards through the hole in the back of the chassis until the heatsink if mounted flush with the back wall of the chassis. Then drop (4) #4-40 1" machine screws through the holes in the heatsink and use the nuts to fasten the heatsink to the chassis. Now you can mount the circuit boards into place on their standoffs using the same machine screws you used to hold down Q16 and Q17 in the Circuit Board section. To mount the power supply simple place it into the chassis, then insert the mounting bolt packages with it through the hole in the bottom of the chassis and through the middle of the transformer. Tighten the mounting bolt to the mounting place with the included nut. The binding posts and RCA jacks can simply be tightened to the chassis. I used a little glue on the flaps of my AC receptacle before I pushed it into place. Your amp should now look somewhat like the picture above (crazy wiring and all).

 

Final Wiring

Now it's just a matter of tying up all the loose ends. I'd start by running all of those wires you soldered on the circuit boards to their respective locations. Regarding the power supply rail fuses, I'm using F2 for the positive rail of the circuit board on the left of the picture and F4 for the negative rail of the circuit board on the left (assuming the fuses are numbered F2 - F5, left to right). Take a look at your power switch at this point, notice that it has 4 pins. The two larger pins (unlabeled) are to be connected to transformer and the AC receptacle (it doesn't matter which pins you choose of these two). The other two (labeled + and -) are to power the LED. Once all those wires are connected you'll need to connect the blue wire that extends the brown/gray pair from the transformer to one side of the power switch and a new 18 gauge blue wire from the other side of the power switch to one of the AC pins on the AC receptacle. Solder the white wire that extends the blue/purple pair from the transformer to the other AC pin on the AC receptacle and the green wire to the AC Safety Ground pin of the AC receptacle. Now solder a new blue 18 gauge wire to either of the positive fuse terminal on the capacitor side of the fuses (not on the side where the circuit boards are connect). Then connect this wire to the 3.0K ohm 3W resistor and connect the resistor to the + pin of the power switch. Now solder a 22 gauge purple wire from the - pin of the power switch to central ground. Below is a picture of what my central ground looked like with all the wires attached. I used a lot of tie wraps in the construction of this.

Now you'll need to attach R50 and C25 to the binding posts. Soldering these together should be much like solder the 4 bias diodes together. Attach a 22 gauge white wire to the remaining resistor lead and a 22 gauge purple wire to the remaining capacitor lead. Now solder the white lead to the red binding post and the purple lead to the black binding post. You may have to heat the binding posts for a long time before the solder will adhere to them (turning up the heat on your soldering iron will shorten this).

 

Setting The Bias Current and Final Tests

The first thing to do before setting the bias current is to have a look at P1 on both of your boards. Ensure that it is set at the position for maximum resistance by turning it fully counter-clockwise (that is when the arrow is pointing at 1). Now place F4 into place and ensure that no other fuses are in place. Set your multi-meter to measure DC current (usually denoted by a line with three dots under it) up to 200mA (usually denoted by a 200m). Place the black lead on the capacitor side of the fuse holder for F2 and the red lead on the circuit board side of that lead. Connect the AC power and turn on the power switch. From this point forward to do touch ANY components with your bare hands or with non-insulated metal objects. Pending no explosions, you should read a very small amount of current on your multi-meter (approx. 10mA), even if you are not follow the next step. Take a small flat head screwdriver (preferable plastic) and give P1 about 1/4 of a turn. You should definitely be seeing current on your meter now (probably 20mA), if you aren't, check the fuse on your multi-meter's amp-meter. Once you know that your multi-meter is correctly measuring current, wait about 10 minutes for the amp to warm up, during this time you will notice the current begin to drift upward. If at any point the current reaches 100mA turn it down slightly. After 10 minutes have passed adjust P1 until your multi-meter reads 100mA. P1 is not linear so you may have to turn quite a bit before the current begins to move upward quickly. Once the current is staying stable at 100mA by itself you can unplug the amp and discharge the filter capacitors as described in the Power Supply section under "Testing". Then remove F4 and put F5 into place. Repeat this process to bias your other amp.

After both channels are biased and the power supply has been discharged you can put F2 - F5 into place. Now connect the power and turn on the power switch. Set your multi-meter to read DC voltage under 20V, then place the red lead on the casing of Q12 and Q13 on one of the amps, it should read 3.4V, if it does not then that channel is not biased correctly. Repeat for the other channel. Once you've confirmed that your bias current is set correctly on both channels you should power the amp down again.

At this point your amp is ready for real world testing and use.