Table of Contents
Getting Started
- Resistors
- Tube Sockets
- Semiconductors
- Capacitors
- Final Assembly
- Checkout
- Setting the Bias
- Enclosures
Getting Started
This section will get you started assembling your circuit board. I have been assembling electronic devices both from kits and from scratch for over 40 years. I have designed one of a kind electronics devices, and electronic devices that have been put into mass production. In the production cases I have assisted in setting up the assembly lines, and tweaked the designs to improve manufacturability. I attempt to share some of these experiences here. If you are an experienced builder, please read this section and feel free to adopt any of these concepts that may help you. If you are relatively inexperienced, please follow these concepts, unless you have a better idea, or a unique circumstance. If this is your first electronic project, it would be wise to follow these concepts as closely as possible, and seek the assistance of an experienced person if you don't understand anything.
It is important to take your time and don't try to rush the assembly. It takes me about two hours to populate (put all of the parts on, and solder) a board, with all of the parts laid out in advance. I have built about 10 of these boards, and I know where all of the parts go. One of my associates who had never built a board before took about 8 hours to populate his board. It worked the first time.
These concepts can be used to help you assemble other kits, which may have a different order of assembly, but this board should be assembled in the following order:
- Resistors
- Tube sockets
- Semiconductors and their heat sinks
- Capacitors
There is a final board assembly page, where the board begins to take on its individual personality. Here a few jumpers are installed to set the filament voltage, and enable the internal, or external power supply. You will also install the volume control, if desired. This order should be followed to assure that the small parts (resistors) get put on the board first. You may not be able to easily insert them after the large parts have been inserted.
After your board has been populated, set it down, and take a break. Come back later with fresh eyes and go over your work. I have found errors in my own work this way. Don't skip this step. Look for poor solder connections, solder bridges, wrong parts installed in the right holes or the right parts installed in the wrong holes. I would check the installation of each electrolytic capacitor to make sure that they are facing in the right direction. After you are convinced that all is well, proceed to the preparation for checkout page.
Tools
You will need some tools to assemble your board. You will obviously need a soldering iron and some solder. Make absolutely sure that your solder is rosin core or flux core solder made for electronics use. Acid core solder will destroy the parts and the board. Acid core solder is the type typically used by plumbers to solder pipes.
You will also need a set of wire cutters (also called diagonal cutters or dikes). A pair of needle nosed pliers are also needed. The following other tools may be useful, wire strippers, tweezers, screwdrivers, hemostats (also called seizers) and some desoldering braid (Solder Wick). It is likely that these tools will be needed later in the final assembly and checkout. These are covered below. You will also need one or more digital voltmeters (also called a multimeter) to check out and adjust the amplifier. The preferred checkout procedure for this board uses three meters. This will be covered later.
The Soldering Iron
In order to solder parts onto the board you are going to need a soldering iron. It doesn't need to be a fancy microprocessor controlled digital wonder iron, although there is nothing wrong with that. At work I use a Metcal digital wonder iron, and it does practically solder by itself, but it costs over $1000. For the rest of us who have to buy our own equipment, an adjustable temperature iron is nice but not mandatory. You need an iron with a heating element between 40 and 100 watts, capable of holding the tip temperature around 600F or 325C. If you have to purchase an iron, I would suggest an adjustable temperature model, similar to the one shown below.
I have used this one for over 10 years and it still works great. I got it from Jameco, it is a Xytronic model 168-3CK. It is still shown on their web site at $89.95. They also list a 40 watt Weller for $48.95 and a 45 watt Xytronic for $59.95, but I have not used either one of them. It would be wise to purchase some spare tips in different sizes.
Your iron should have replaceable tips. For this board the best tip will have a 1/8" point. The typical pencil point used on modern surface mount boards is too small to heat up the large pads used on this board.
Make sure that the irons cord does not hang too far over the edge of the bench. If it gets caught on the arm of your chair, the iron could be pulled out of its holder when you move your chair. Trust me, it is really NO FUN to drop a hot soldering iron in your lap.
The Small Tools
These are some of the tools you will need to work with electronic devices. These can be purchased at Radio Shack, Harbor Freight, Jameco.com, JDR.com and other stores.
I like to keep them in a small plastic tray to keep the work area neat. This is helpful during the testing phase of the project where loose conductive items could be dangerous.
Your Work Space
Populating your board (soldering the components onto it) is easiest if you have sufficient unobstructed work space to set up the tools, soldering iron, the parts, and the board. Since this board has an extensive set of instructions that are computer based, it helps to have a computer nearby. I will spread out the parts that I am going to use next such that they can be easily seen and identified.
In this photo I have placed all of the resistors in a manner that they can be easily identified. I keep all of the other parts in another tray so they will not get lost. If you have been collecting other electrical parts, keep them separated from the ones used for the board to avoid any confusion. The parts not being used immediately are in the tray to the right.
Some means of supporting the board is needed to allow for easy soldering. There are several gadgets out there designed for this purpose, but I prefer the low tech approach, two large books that are the same size. They must be thicker than the tallest part on the board.
The books are set on the work surface, spaced slightly less than the board is wide. The board can be set on top of them such the components can hang freely between the books. This way the parts can be inserted in the board, and the board flipped over. The parts can be soldered and then allowed to cool without being disturbed. The technique will be explained further in the next section.
My work area is shown here. Not quite visible in this photo is the shelf above the transformers. All of the test equipment is on this shelf, and the shelf above it. The scope probe is visible in the upper left. This keeps the equipment out of the way until needed. The PC visible in the photo is also used for amp testing via the high end sound card and specialized software. I have the board overlay on the PC screen since the silkscreen on the actual PC board can be hard to read in some places. I also have a checklist on paper to keep track of each part as it is installed.
Working With the PC Board
Working with a PC board requires some soldering techniques somewhat different than point to point wiring. First, your soldering iron should be set at 600 to 650 degrees F. If it is too cold, the soldering time will be too long, and the solder may not be evenly melted. If it is too hot, the board could be burnt or delaminated.
To insert a component gently bend its leads at right angles to the body of the part. The bends should be smooth, bent by hand. I don't recommend it, but you can bend the leads with needle nosed pliers. if you do this, the leads must be formed in such a way that there is no physical stress on the part once it is installed in the board. The part must have enough slack in the leads so that no stress is applied to the part as it (and the board) expands as it gets hot.
The leads should then be inserted into the appropriate holes in the board. The part is pushed down toward the board, but not flush to it. There should be a space between the part and the board. For most parts the spacing should be about 1/4 inch. There are two reasons for this. Many parts generate heat. The space allows for air all around the part, assisting in heat transfer. Tube amplifiers get hot, this heat causes thermal expansion. Parts drawn tightly to the board don't have room to move as they expand. This turned out to be the reason for failure of many components on the circuit boards in the first generation PC board products, especially early TV sets ( I worked in a TV repair shop in the 60's).
After the part is inserted into the PC board, the leads should be slightly bent outward to prevent the part from falling out when the board is flipped over for soldering. Do not bend the leads more than needed to keep it from falling out. Doing this will make it difficult to remove the part if it becomes necessary.
Each lead of the component is then soldered in place. The pads on this board are oversize where possible to aid in soldering.
The excess part leads should then be clipped off using diagonal cutters. Save a few of the thinner resistor leads that you clip off. You will need them later in final board assembly.
The completed solder joint should look like this.
When soldered correctly enough solder should flow through the board to the top side to cover the pads on the top. Too much solder will pile up and flow down the lead to the part. Not enough solder is far better than too much. The board has plated through holes. Solder on the front side of the board is not necessary. It does help physically stabilize the part. If the part is soldered OK on the back, and no solder flows through to the front, the solder joint is probably OK.
If you look carefully at the parts shown here you can see that there is actually see that there is more solder on the front side than on the back. This is an indication that enough heat was applied to the joint, and that gravity did its job. If there was not enough heat applied there may be far more solder on the back side since heat (and melted solder) never made it through to the front side. I would not recommend trying to solder the front side of the board. If the back of the board looks OK, leave the joint alone. If the back of the board does not look shiny reheat the joint and apply a little more solder. The reason that I am explaining this is for you to get practice on the resistors (which are not terribly critical) before attempting the tube sockets and semiconductors, which are far more difficult.
This is what your board should look like after a few resistors have been installed.
There are two parts lists included on the CD. The parts purchase list shows each unique part and the quantity of each that are required per board. Also listed are suppliers for each part. The assembly parts list itemizes each part in alphabetical order, there is a check box next to each part. It is recommended that this list be printed on paper. Check off each part as it is installed. This makes it far easier to keep track of your progress. It is especially valuable if you can't finish the whole board in a single session.
My components don't exactly match the holes in the board. What gives?
I have built several amplifiers using this board, from a 1/2 watt per channel headphone amp to a 200 watt (single channel) prototype amp. I have made a few optimizations that improve the performance of the amplifier, especially when it is used at higher power levels than it was originally designed for. Since these optimizations were done after the board was laid out, some of the components that are on the current parts list do not exactly match the holes in the board. If you source your own components your choices may be of a different size than the original part. In that case, some extra steps are required.
For instance the parts list in the photo above shows R6 as a 10K 3 watt resistor. We found that on boards operated above 400 volts, a 3 watt resistor was marginal. We now specify a 6.5 watt resistor in this location. The 6.5 watt resistor is larger than the original part (but smaller than the 5 watt white ceramic ones). The 6.5 watt part will fit in the board, but it is slightly longer than the hole spacing. You can sharply bend the leads at right angles to the part and force it into the board, but this will lead to early failure. It is better to carefully bend the resistor leads in the manner shown below using needle nosed pliers. The board shown in the photos above reveals the proper mounting of this part. The parts list shown above is the old one. The parts list on this disc is up to date.
Some of the newer metal film resistors are smaller than the original parts. In some cases 1/4 watt metal film resistors are specified (or supplied) where the original was 1/2 watt. In this case the part is simply inserted normally. Center it in the original pads. It may look funny, but these parts offer lower noise than the original part. See the resistors page for details and photos.
When I designed the board I left space for large power supply capacitors C4 and C5. In the early prototypes I provided two sets of holes allowing for different size capacitors. This turned out to cause more problems than it solves, since some of the larger capacitors touched the extra pads causing shorts. The value of C4 can be changed to adjust the B+ voltage. For most applications the value of C4 will result in a part physically smaller than space on the board. Often this will be a capacitor with wire leads instead of stiff pins. To use a leaded capacitor in the C4 or C5 position simply insert the leads through the holes, push the cap down toward the board leaving at least 1/4 of an inch between the cap and the board. You can pre - form the leads with needle nosed pliers if you like. The idea is to avoid any physical stress on the part. Further details and photos are on the capacitors page.
The resistors are the first components to be installed on the board. When you are ready to begin assembling your board proceed to the resistors page.