PCB Stuffing Tips

tips on PCB stuffing for tube ampsCertain methods of handling components and installing them can make the already “easy” task of PCB assembly even simpler.

With any kit or project, it is best to organize the components. If it is a kit, empty the parts out of the bag, identify them, group them, and make sure everything is present. Look at the kit notes and make sure you have a reasonable understanding of what will be involved. Kit notes are not intended to be circuit tutorials – that is what the books are for – but they will show you how to assemble the given circuit and how to tie it in to other circuits as appropriate.

Some components stay cool during operation, so they can be pushed all the way down to the PCB. That is, their leads are inserted into the holes, and the body of the component made to rest on the PCB. On the solder-side – also called the “foil side” or “copper side” – before installing the components, bend their leads outward or inward slightly. This will keep the part from falling off the board when you flip it over to solder the leads.

Install a few components at a time, starting with parts that mount fully pressed down to the PCB. This includes diodes and low-wattage resistors (1/4W, 1/2W, 600mW).

When soldering each lead, a proper connection should take less than ten seconds. Prior to soldering, add solder to the tip of the iron and then wipe the soldering iron tip on a wet sponge. This removes the solder you just applied and “wets” the tip – primes it with solder so it can more easily heat the solder you wish to apply to the connection. Touch the tip to the lead and trace. Touch the solder to the tip and then move it to the opposite side of the lead. Solder will flow around the lead and cover the pad. This should happen in just a few seconds. As soon as the solder has flowed all the way around the pad, remove the iron.

You can solder several connections in a row with each treatment of the iron tip. Never wipe the iron tip on the sponge prior to placing the iron in the holder. Leave it dirty, and even add some solder to the tip just before placing it in the holder. At the beginning of the next set of connections, wipe the tip on the sponge. Double check that the tip is shiny with solder and no dirt is present on the tip.

Trim the excess leads of the components you’ve just installed. Use flush-cutting snips to cut the leads just at the peak of the solder slope around the lead. There is no benefit in leaving long leads, and in fact, longer leads may cause electrical hazard or failure.

After all of the short components are mounted, move to taller items. If there are semiconductors other than diodes, now is the time to mount them. Push the leads through the holes but leave at least 1/4″ (6mm) above the PCB. The splaying of the leads going through the holes will provide enough friction to hold the parts in place when you flip the board over. Again, solder quickly but effectively. Do not cut across multiple leads of transistors, etc. This can reduce the effective spacing of the leads. Snip each lead individually.

Power resistors must be mounted with space between their body and the PCB. Just because a flame-proof resistor won’t burst into flames, one cannot assume that it won’t get hot enough to burn the PCB. Half-an-inch or 12mm is good here. Hold the resistor body between your thumb and finger. Use your other hand to gently bend the leads perpendicular to the body. They should curve out away from the body rather than bend at a sharp angle. Insert the leads into the holes and push the part down. You must squeeze the leads slightly at the body as you push the leads farther down. This keeps tension on the leads so that the resistor will not fall out when you turn the board over for soldering.

Next, mount taller parts, or heavier and bulkier parts. This includes small transformers, caps, tube sockets, pots, relays, fuse clips, etc. Some components are subject to external forces (tube sockets, fuse holders) while others can impart forces onto the board due to their weight (transformers, large caps). In either case, once the leads are pushed through the PCB, bend the protruding lead over for mechanical retention of the part. Then solder as usual.

Secure the PCB using metal stand-offs and/or metal bolts and nuts. Plastic mounts can be microphonic.

Note that it is important not to bend most component leads right at the body of the component. For devices with short leads that must be formed, use needle-nose pliers to bend the leads as required. Do not bend leads to precisely align with the holes in the board lest the part falls out when you flip it over to solder.

See also PCBs – Colour Considerations and PCBs vs. Hand-Wiring

PCBs vs. Hand Wiring

PCB Facts

Myth: electronic equipment built on printed circuit boards (PCBs) is somehow “inferior” to hand-wired equipment. This belief is perpetuated by some amp builders, hobbyists and amp reviewers who do not have enough of the facts – let alone “all of the facts” – about this subject, rendering their statements too sweeping and frequently incorrect.

As described in our book The Ultimate Tone Vol. 3 (TUT3), you can have good and bad hand-wired assemblies – and you can have good and bad PCB assemblies; there is nothing inherently “better” or “worse” about either process until you get past low-quantity production. At that point, PCBs become far superior.

Yes. PCBs are superior in the end. Here is why:

PCBs provide a stable platform for the small components that comprise a circuit. The card material is usually rigid fiberglass-reinforced epoxy. Copper traces on the board surface create interconnections between the components – an interconnection layout that cannot change with time, vibration, or environmental conditions. So,the circuit will be stable over time, providing consistent performance from initial use to final use, apart from component variation or aging. Also, unit-to-unit consistency is very high.

Hand-wired units can vary in the exact spacing and layout of interconnecting wires and even component mounting. This alters the parasitic elements of the circuit – the ones that will never appear on the schematic, but which are a part of every electrical assembly. Parasitic elements can cause inferior circuit stability, and low unit-to-unit consistency. Most guitar players encounter this at the music store, when they plug into side-by-side identical models, and each sounds different. A good example is old Fenders, which were were fully hand wired, so there is a lot of variability in how things are positioned internally. Old Marshalls used a PCB to support the small components, but extensive wiring to off-board parts introduced instability in the design. Our book TUT3 demonstrates how to correct this.

Even PCB assemblies can have parasitic capacitances between parts, but these will be more predictable and easier to fix or to accommodate than with hand wiring.

Hand wiring may sometimes beconsidered superior because the solder joints are physically larger. It is also said that because a mechanical joint is made first, the solder itself is less critical, so reliability is higher. The truth is that with either hand wiring or with PCBs, one can make mechanical connections prior to soldering that are fully functional electrical connections. In both cases, this will make servicing or later modifying very difficult. But… poor soldering will compromise reliability for either assembly type. And… larger solder connections are of benefit and easily achieved with either method.

Rogues Gallery

Vintage amps had PCBs and were also hand wired. Results were highly variable, depending on the execution. Early Ampegs were hand wired and are extremely difficult to service. Later Ampegs had PCBs but still a fair bit of wiring, and were relatively easy to service. They were very reliable.

Early Fenders were hand wired; later ones used PCBs. Reliability did not change significantly.

Marshall used a combination of PCB and hand wiring. Their amps failed due to poor component choice and poor wiring techniques. The card and its components was the most reliable part of the circuit.

Vox‘s hand-wired amps have a horrible service record and are extremely difficult to work on. A bad combination.

Peavey amps were always PCB construction, but later models ignore the plight of the service tech.

set a standard for new service nightmares with their PCB amps. Their choice to mount the card then attach wiring from all four sides is unbelievably ill-conceived.

Hiwatts were hand-wired and used good parts, so reliability was high. However, they are very difficult to work on.

Matchless/Badcat/Star amps are all hand-wired and very difficult to work on. Reliability problems due to overheating are the result of poor circuit value choices.

And today…

So, our own little niche of electronics gives us good evidence that hand wiring is distinctly not superior – at least in how it has been executed so far. Modern boutique amps have not been around long enough to sway the data. Rather, the large manufacturers using PCBs have demonstrated that PCB construction results in “mostly” reliable products. Not surprisingly, the detractions are related to interconnections – that is, wiring – and primarily, the use of “insulation displacement” (IDC) connectors where the wire is pushed through a knife-edge to make the connection. No wire stripping. No crimping. No soldering. Modern PCB amps of various brands have a neck-and-neck failure rate for poor solder connections and IDC failure.

But in the broader realm of consumer electronics, PCB construction is king. PCBs allow uniform assembly, leading to automated construction in some cases, and uniform quality. In the 1970s, the industry standard for acceptable equipment failure was 2.5%. Five out of every two-hundred units of a given type would fail in the field. The failure rate is much lower today, less than one-fifth of the old standard – and much of this improvement has to do with PCB construction.

The notion that the presence of a PCB means the unit is not hand wired is incomplete; it doesn’t take into account how the parts got on the PCB, how the PCB was soldered, or how the PCB was tied in to other non-card-mounted devices. Items such cell-phones, VCRs, DVD players computers may be assembled without human hands, but there are no guitar amps built that way – yet.

London Power has long since switched all of its amplifier production over to PCB assembly. These boards are hand soldered and hand assembled, just as most other low-production-quantity (custom and boutique) amp products are. Our PCBs themselves are of very high quality, with twice the industry-standard copper thickness, solder-masks on both sides of the board, plated-through holes for increased connection size, silk-screening and proper spacing for voltage and current for each circuit section. The fabrication of these PCBs is highly automated with precision to one-thousandth of an inch. This would be difficult to achieve – if possible at all – doing things by hand.

Servicing Considerations

Servicing a printed-circuit board amplifier does not have to be difficult. However, when designers look at a layout on a computer monitor, it is easy to forget about both the finished product and the tech who might have to service it. This is why you see so many amps with one big PCB inside, supporting all the pots and jacks along the front and rear edges. Servicing for something like this is a nuisance: every knob and every nut has to come off every control and jack just to release the PCB. Although this provides the lowest-cost assembly for the manufacturer – which contributes to the affordability – it also contributes to the extra cost to repair such units.

London Power remembers what it is like to service amplifiers. We try to avoid large PCBs (although in low-profile chassis their use may be unavoidable). Our PCB assemblies are more complex and thus cost more to assemble. This added up-front cost and effort makes servicing much simpler and less expensive. These products can last for generations – something will inevitably break or wear out. It is the fate of all things made by human hands, so why not make that inevitable condition easier to deal with? Routine maintenance, such as spraying potentiometers with lube, is accommodated and does not require removal of the PCB. If an actual control fails and needs replacement, only one or two controls need be released to remove the failed component.

Given that human-made parts eventually fail, servicing a properly designed PCB assembly is no more trouble than servicing a hand-wired amp. In many cases, the hand-wired amp can be more difficult to make look right. Wires might have to be unsoldered from a pot or jack, and this can be difficult to redress and make to look how it did before. The icon of “beautiful” for hand-wired amps is Hiwatt. But – their string-tied wire looms cannot be retied properly without special skill or experience. So, a minor alteration casts a pall over the entire assembly.

See also PCBs – Colour Considerations and Tips on PCB Stuffing