Point to Point Versus PCB Amps - What's the difference?

Copyright 2000 R.G. Keen. All rights reserved.
No permission for local copies or serving from web sites other than http://www.geofex.com/.

[Point
to Point] [Stripboard]
[Pros
and Cons] [Capacitances]
[PCB
Right] [PCB
Wrong]
[Good Stripboard] [Bad
Stripboard] [Myths]

Ever hear this one?

"Get a point-to-point wired amp - those PCB amps are no good. They just don't
have the tone they did in the old days."

The deeper you dig into this issue, the more repair techs you'll find that
say that PCB amps are no good. They'll give explanations involving things like
tiny traces run too close together, and the capacitance leaking signal from
place to place and then make the leap that this somehow spoils the tone of the
amp. Is this bit of folk wisdom right?

Yes and no. The short version of the answer is this:
There is no technical
reason that PCB amps *can't* be every bit as good as point-to-point wired amps -
or stripboard wired amps, which is what the grizzled techs usually refer to.
However, there are some serious reliability (not tone!) shortcomings in most of
the PCB based amps that have been put on the market; this is the origin of this
folk myth.

At the bottom of both sides of this question is self-interest. On the side of
the amp makers, most of the time the PCB conversion is done as part of a cost
saving measure; the same motive to change to PCB's also drives changing to
cheaper practices and parts all around. This most often gives worse tone.
They're doing less than the best they could, for money.

On the amp tech side,  PCB amps that have been done poorly have frequent
breakdowns. They are not as easy to repair as hand-soldered terminal wiring.
They require some delicacy in fixing, and they can be a true pain to get the
boards in and out. So to the techs, they break more often, and are fiddlier to
repair. The techs usually can't charge more for PCB amps, so - no surprise -
they don't like them much.

So - PCB based amps, from what we've seen so far, have poorer reliability and
techs don't like to repair them; but that is because they were poorly designed
from a reliability standpoint in the first place. The tone is not necessarily
poorer. Here are some comparisons

PROS and CONS

Advantages

Disadvantages

True Point to Point

Approaches "ideal" in electronic terms; lowest parasitic
coupling and crosstalk if done well.

Sturdy.

Withstands repair surgery well.

Very expensive to design well

Very expensive to produce in today's market

Usually heavy.

Stripboard

Less expensive to make than PTP.

Parasitics and crosstalk OK for most applications.

Parasitics and crosstalk are variable depending on lead dress.

Withstands repair surgery well.

Higher parasitics than PTP.

Variable parasitics.

Moderately expensive to produce in today's market

PCB

Inexpensive to downright cheap to produce

Absolutely fixed, known parasitics - whether good or bad

Extremely reproducible.

Needs special planning and design to avoid mechanical
problems.

Repairs must be done delicately if it needs
repaired.

Most PCB amps that have been produced *have* been poorer than tube amps, for
reasons having nothing to do with the PCB's. Like plastic bobbins in
transformers, this does nothing to change the tone. The *other* poor practices
that go with a cost-cutting attitude that were introduced at the same time may,
but PCB's are unfairly indicted.

If you're interested, here's some more information about the differences in
point to point, stripboard, and PCB construction practices that DO make a
difference.

Point to Point (PTP) - What is it and what's good
about it?

This is one version of real "point to point" construction.

The view is from underneath the chassis surface, looking at the bottom of a 9
pin miniature tube socket and two terminal strips that the circuit is built on.
Good point to point layout makes the actual component bodies bridge most of the
distance from the socket lugs to any other connections. Really, really good PTP
construction will have the components arranged in almost a star pattern, all of
them leading radially away from the socket.

What's important there is that (a) all the component leads are pretty close
to being as short as they can be (b) the component to component wiring lengths
are as short as possible (c) neither the components nor the wires criss-cross
one another (d) the lengths of paralleled wires are about as small as they can
be.

The reason that all these are important is that there is an unavoidable bit
of capacitance between any two electrical conductors that are at different
voltages. You can think of this as a little "ghost" capacitor between every
combination of two points on the schematic - ugly! These ghost capacitors are
often referred to as parasitic capacitors.

This capacitance is proportional to the facing areas of the two conductors,
and inversely proportional to the distance between them. For maximum coupling
between two conductors. they need to be as close together as possible, and have
the maximum area exposed to one another.

For two wires, the closest they can get is with their insulation touching.
For the maximum area, they'd have long parallel runs; for minimum area, they'd
cross at right angles. In true PTP, the components and wires are as far from one
another as it's practical to get, and where crossovers are needed, they can be
at almost right angles. Also, the wiring between amplifier sections can be
minimized by laying out the tube sockets in a manner that the signal flows
directly from one tube circuit to the next with no long runs of parallel wires.
Also, the terminals themselves space the wires a goodly distance away from the
presumably grounded metallic chassis.

The worst case for cross coupling in wiring of any kind is where the wires
run parallel and with their insulation touching. I spend part of a Saturday
morning digging through my collection of electronics and physics texts to come
up with some rules of thumb for how big the capacitive coupling is. Of course,
this varies all over the map depending on the layout and wire sizes. However,
for two 22 gauge hookup wires with kind of nominal insulation thicknesses, the
capacitance between the two wires is about 0.381pF per inch. Ten inches of
parallel run, the capacitance is 3.81pF. With the wires crossing, there's only
about 0.025" of wire "parallel", so the capacitance is on the order of
0.008pF.

Oddly enough, the capacitance of two PCB traces under similar conditions is
actually less. For two 0.025" wide traces spaced a conservative 0.075" apart,
the capacitance is 0.317pF per inch. Finer traces even closer together do have a
higher capacitance per unit length, but much less than you'd expect - it goes up
much less than linearly with smaller, finer traces. We have the curious result
that wires and PCB traces have very similar parasitic capacitances, and PCB
traces have a slight edge over wires. That kind of means that it's how you use
them, not which you use, huh?

Note that a poor PTP layout can be a nightmare of criss-crossing parts
and wires that can loose a herd of electronic demons. What makes a PTP wired
circuit good is (a) careful planning and (b) very skilled labor doing the
wiring neatly and carefully. For reason (b), true PTP is very expensive, and has
been even as far back as the late 50's.

True PTP construction is good for frequencies up to UHF (several hundred
MHz!!) if properly done. This is the reason all that old tube radio equipment
was laid out that way. Good PTP layout has wires that almost never come close
to one another, and when they do, the cross instead of running parallel. That's
good, but it's a long way from audio to UHF, so maybe there's something else
that can fill the bill.

Stripboard Construction

The kind of construction in amps that is most often referred to as "point to
point" is not really point to point at all. Most amps use some kind of
insulating board with either eyelets (notably Fender) or turrets set into the
board, and the components strung in neat little rows on the board.

All the connections to tube sockets are made with wires from the stripboard
to the tube sockets and controls. Note that unlike true PTP, the wiring lengths
from the components to the tube sockets are no longer as short as they were.
Also, the wires are often stranded wire, and free to move around, where the
component leads in PTP were solid, and pretty much held in place by being short
and stiff. To get easier wiring, we have sacrificed the very short
interconnections that are a characteristic of good PTP wiring.

The freer, stranded wires can now move around relative to one another. This
means that they have variable capacitances among them! The
coupling of signal from place to place by accident (if it was on purpose, we'd
use a wire or capacitor) is now a variable that depends on how you push the
wires around after they're installed. The placement of wires in this kind of
construction is referred to as lead dress. Not too surprisingly,
poor lead dress can cause the wrong signal to get picked up and amplified. This
can cause hum if the wires get too close or too parallel to those green/black
filament heater wires, or oscillation if an output signal wire gets too near a
sensitive input.

Good stripboard construction can approach the performance of true PTP.
However, stripboard construction abandons the electronic elegance of true PTP
for easier, faster manufacturing by less-skilled (and cheaper!) workers. With
stripboard, assuming a minimum of good workmanship on putting the parts on the
stripboard, the differences in quality will depend primarily on the wires that
lead away from the stripboard to the outboard controls and other components.

This is an illustration of a good style
of stripboard layout. The input and power transformers are at opposite ends of
the chassis. The tube sockets, stripboard, and controls are laid out so that the
wires from the stripboard to the sockets and controls are as short and direct as
possible - no long runs of parallel wires.

Doing this is not all that easy. It takes a good deal of foresight and skill
at the processes of wiring and building things in this style to get the wires
as short and direct as possible.

There is also a built-in service problem with this style. Notice that the
stripboard is "tied down" by the wires coming off both sides of the stripboard
to tube sockets on one side and controls on the other. To lift the stripboard
itself requires unsoldering all the wires from one or the other side.
Fortunately, most of the parts are easy to get to and replace from the top side
if the board has NO components or wires running under the board. Sadly, this is
not always true. Poor layout is not limited to PCB's.

Stripboard has a dark side, too. Consider the
following picture.

In an attempt to be neat and tidy, the wires have been gathered into
neat, tidy bundles where the wires from some mechanical area are gathered into a
paralleled bunch, often tied into the parallel bundles by lacing ties (beeswax
impregnated cotton is the old standard!) or nylon cable ties. The advantage here
is that you can drive nails into a dummy amp baseboard at the corners of the
wiring, to pre-cut and lace in place all the wires before ever putting them in
an amp. What's wrong with this picture?

That ugly word - parallel. This is how you maximize coupling, remember.
Putting the wrong components side by side can cause performance problems in
high impedance circuits. Putting the wrong wire in the wrong bundle can make
this construction oscillate uncontrollably, or can introduce treble loss.
Conversely, not every signal wire coupling makes a difference to every other
wire, so there's usually some bundling arrangement that can make this
arrangement work OK. Sometimes a signal wire must be shielded to run in a bundle
without unwanted coupling.

Which brings up an interesting point - shielding. Shielding a signal wire is
most commonly taken to mean running a coaxial wire with the signal on the
central conductor and the shield connected to ground. The shield does in fact
keep any other signals out side the shield from touching the central conductor
by shunting those signals to ground. However, it does it by completely
enclosing the signal wire in an enveloping parallel (ack! there's that word
again!) sheath. As you might expect from the earlier comments on parallel wires,
a wire with a shield completely surrounding it has a lot of capacitance to the
shield. Indeed, coaxial cable often has specifications on the capacitance per
length of cable. Good cables can be down in the range of 10-15pF per foot. Bad
cables can be significantly more. Put a couple of feet of bad shielding cable on
a grid signal, and you're losing treble. Better than oscillating, but not
perfect by any means.

Wiring capacitances - how much matters?

It's easy to calculate how much is too much for audio purposes. There are two
important cases - a capacitor in parallel with a resistor and a capacitor in
series with a resistor. We don't consider inductors and capacitors
because

there's at most one real inductor in essentially all amps - and that's in
the power supply; lots of tiny parasitic ones, though

parasistic wiring capacitances are so small that they are swamped by
parallel real capacitors, and series real capacitors are so much larger that
the real capacitor is effectively a short circuit.

Fortunately, we can calculate the frequency where the capacitor and resistor
interact the most very easily. This is the frequency where the capacitor's
impedance equals the resistance, or F = 1/ (2*pi*RC). To a first approximation
we can consider every frequency below this to be unaffected by the resistor,
and above it to be unaffected by the resistor.

Guitar amps are a special case of audio amplifiers. Hifi amps have to handle
20kHz, and some purists would insist that 40kHz is a minimum. However, the best
wound guitar pickups pretty much cut off above 7kHz and our speakers don't put
out much above 4kHz. Even for fuzz-box generated hash, there's not much
happening above 10kHz. To make some sense of the huge numbers of
variables, we'll have to make some assumptions. The capacitances we calculate
may be wrong by a factor of two either way, but then calculating capacitances
between free-swinging wires is prone to some error anyway. We'll at least be
close enough to know where to dig deeper if we need to.

For a resistor, the "significant capacitance" is C = 1/ (2*pi*Rf), or at
10kHz, C= (15.9/R) uF. If the resistor in question is a 1M resistor, the
capacitance will be C = 15.9pF. So a resistor between 8 and 32pF makes a
"significant difference" to a 1M resistance. We can see then that the higher the
impedances involved, the smaller the capacitance that makes a difference to
tone. Likewise, the lower the resistance, the bigger the capacitance must be to
make a difference.

One "resistance" we don't see listed on schematics also has a big effect -
the equivalent plate resistance. Since we already know that the higher the
resistance, the smaller the capacitance that has an effect, we'll consider two
high-end examples - the 12AX7 plate and a small-signal pentode plate resistance.
The 12AX7 has a plate resistance of about 60K under the conditions it's most
often used in preamps. This resistance is effectively in parallel with the plate
resistor, so the parallel combination makes for a resistance of about 37.5K in
the very common instance of a 100K plate resistor.

We can quickly see then that the wire coming from a 12AX7 plate may
have  a 37.5K source resistance - that is, it can supply
current exactly like a perfect voltage source with a 37.5K resistor in series
with it. This wire couples the signal it carries to every conductor anywhere
near it - and is loaded by every conductor near it through the parasitic
capacitances. If the wire is run right along the chassis, insulation actually
touching, this is just like a capacitor to ground connected to the wire. If
that capacitance is more than 424pF, then we lose treble above 10kHz, and the
amp starts to sound a little dull. We've lost some high-end tone.

If there are 10Vac of signal on that plate wire (not at all unusual),
and it runs near a wire connected to an input tube's grid, Then to couple in a
feedback signal equal to a typical guitar signal of 100mv, we only need a
capacitor of  C=15.9/100M uF  = 0.159pF - YIKES!! A SIXTH OF A
PICOFARAD!!!! We suddenly see how oscillations start - a grid wire too close to
a plate wire and a bit of bad luck on the phasing and the amps starts screaming
at 100kHz.

OK. So now we know what we're playing with. A fraction of a pF can give us
oscillations, but we need something between 8 and 400pF to start cutting treble
off, depending on the circuit.

Here's some detailed considerations on the implications:

The picture shows a somewhat representative tube amplifier voltage gain
circuit. The "real" circuit is shown in black, and the parasitic capacitances
are shown in red. The excess capacitors may come from wires or PCB traces,
doesn't matter which.

Cp1 shunts signal away from the grid when its impedance is less than Rsrc.
Cp3 shunts amplified signal around the plate resistor Rp, and Cp2 leaks
inverted and amplified signal from the plate to the grid. Every single one of
these results in treble loss. Cp2 is the worst because its real value is
"amplified" by the voltage gain of the stage. It can be modeled by another
capacitor in parallel with Cp1 of Cp2*voltage gain.

So how much capacitance matters?

Cp1 and Cp2: for a representative Rsrc of 35K (typical of a 12AX7 plate with
a 100K plate resistor), when the sum of Cp1 and Cp2 times the voltage gain is
Cep=1/(2*pi*35K*20kHz) = 227pF, this "dulling capacitor" is eating half the
signal. If we assume that all of this is from Cp2, and a voltage gain of 30 for
the stage, then the max capacitance we can stand for Cp2 is 7.5pf.  As we
know from the calculations above, either PCB land or #22 wires are both about .3
to 0.4pF/inch, so we can stand a maximum of about 18 inches of absolutely
parallel run wires before we start getting treble loss even at 20kHz. That's a
lot! Even when we correct for our assumption that Cp1 and Cp3 didn't matter,
allowing for a fifth of the capacitance to be in them, we would be safe with
12-15 inches of parallel, side by side signal run. While the laced-and-bundled
stripboard techniques might do this, it's unlikely to be a real problem. Good
stripboard and good PCB construction will not even come close.

On Rp, Cp3 could be C= 1/(2*pi*100K*20kHz) or C=79.5pF before we'd get treble
loss at 20kHz. That's 227 inches of paralleled wire.

For Rk, there can be no effect if Ck is part of the circuit. Cp4 is
completely swamped by Ck.  If there is no Ck, the gain of this stage
will start rising at the frequency F=1/(2*pi*Rk*Cp4), or for ballpark values,
F=1/ (2*pi*1500*7pF) = 15.1MHz!! The tube itself will be cutting off at
that frequency. So wiring capacitance from this source has no effect on the
frequency response of this tube.

Between two stages the picture changes a bit. This illustration shows
two stages, one following the other, but connected with a feedback capacitor
that we'll assume was caused by bad wiring. The signal at the second plate is in
phase with the signal at the first grid, and considerably larger. If each stage
has a gain of 30, the second signal is 30*30= 900 times as big as the input
signal. For a 0.1V signal, we'd see 90Vac of signal on the second plate if it
was not otherwise limited.

The feedback through the parasitic Cfb is limited by the plate resistance of
the second triode, maybe the 37.5K of a 12AX7, and is divided down by the other
capacitances at the grid of the first triode, Cgeq, and by any loading at the
input other than the grid leak resistor. Let's take worst case, the input
open.

Unless we've done something special to make Cgeq smaller, it's probably on
the order of 2-3pF. Let's see what Cfb makes the loop gain one, so this thing
oscillates. We'd have to have a feedback loss of 900 to make this true, so the
impedance of the parallel combination of Rgrid and Cgeq have to equal 1/900th
of Cfb's impedance. At frequencies below F=1/(2*pi*1M*3pf) = 53kHz, the
impedance is dominated by Rg, and decreases above that. Phase shift also
accumulates above that, so let's take the resistance as the limiting resistance.
To get oscillation, the impedance of Cfb has to be no more than 900 times Rg, so
Cfb=1/(2*pi*900M*10kHz) = 0.018pF. ACK!! 18 FEMTO-FARADS???? HOW DOES THIS THING
EVER WORK AT ALL??

It works at all because that picture does not show the whole story, of
course; it's too simplistic. There are other parasitics that are not shown in
this picture that actually raise the needed coupling well above the 18 fF
level. The phase shifts and leakages we've blithely ignored, as well as the
parasitics from the first tube itself enter into it as well. However, the math
complexity goes right through the roof, so it gets to being a poor illustration,
lost in the complexity.

It makes a great morality play for keeping your input and output wires well
separated though, doesn't it?

This is actually one of the places that PCB construction would be a big help.
The Cfb path would likely be much nearer other signal traces and especially
ground traces, which would eat a lot of the feedback signal.

Parasitic capacitance effects on the tone stack

--- coming ---

How to do PCB construction wrong

This is easy. Just use the technology in ways that ignore the mix of strong
points and failings that every technology has. Some examples of WRONG
techniques:

Using a PCB designer that is not experienced with analog circuits, or
worse yet, an automated layout program that is intended primarily for laying
out digital circuits. This gets you the tiniest possible PCB - with all the
worst aspects of the technology for analog circuits.

Using too-thin PCB materials, possibly combined with using too few
mechanical supports. This lets your PCB flex and either crack the thin
traces or break solder joints to heavy parts.

Mounting input/output jacks and other connectors on the PCB - this lets
the regular mechanical stress of insertion/removal flex the PCB

Mounting preamp tube sockets on the PCB with no reinforcement so tube
insertion/removal stresses and can crack board lands and solder
joints.

Mounting controls on the PCB, or worse yet, mounting the PCB to the
chassis through controls soldered to the PCB - there is always some mis-fit
between the PCB and the chassis. Using controls to mount the PCB to the
chassis builds in a mechanical stress. Using both controls and other
mechanical mountings lets both the mountings stress the PCB.

Mounting power tubes to PCB's - ACK!! They're big, they're heavy, they get
removed and reinserted, they're hot; about the only thing worse would be
...

Mounting transformers to PCB's without special
reinforcement.

Unfortunately, PCB technologies are most cost efficient when everything is
connected to the PCB, which encompasses all the things you can do wrong. So you
get a clear conflict between cost efficiency (that is, the manufacturer making
money) and long term reliability.

How to do PCB construction right

Don't let the boards flex

Use thick enough board material - 1/8" instead of the normal 1/16"
should be enough

Stiffen and reinforce the board if it's large - say, over 5" in any
dimension; use lots of mounting points to the chassis and reinforcement
wherever stress is unavoidable.

Mount any modestly large/heavy components to the board with mechanical
restraints that take the stresses off the solder joints. Component leads
for anything bigger or heavier than about a 5W power resistor or a 1" by 2"
capacitor must not be relied on to hold the component physically.

Mount mechanical components which will be moved, stressed, adjusted, etc.
off the board and connect with flexible leads; this includes pots, jacks,
switches, and tube sockets. The flexible leads don't transmit the stress to
the board.

Don't let the boards get too hot

Glass-epoxy board materials softens and begins to char at temperatures
easily generated by power resistors and power tubes. Mount these items off
the board or provide special thermal shielding.

Don't use solder as a mounting glue

Wires coming into the PCB should terminate in either plated-through holes,
eyeletted holes, or stress-relieved connectors. Otherwise, any wire
movement stresses the solder holding the wire in and the bond of the wire
pad to the base PCB material. This can crack the solder joint, or it
can crack the copper foil itself.

Repetitive stress on solder joints eventually causes the joint to fail -
this happens with switch and pot mountings, as well as places where heated
parts cause thermal expansion stress.

Don't treat high impedance analog signals the same way you do low-impedance
digital signals.

Be aware of the parasitic capacitances that exist in wiring, and route
sensitive lines in the shortest, most direct path possible, NOT paralleling
other low impedance lines that could couple noise, hum, or feedback signals
into it.

Keep parallel runs of non-related signal lines to a minimum

Use interspersed ground lines to "decouple" parallel traces where
needed.

Like good PTP practice, lay out the circuits so that trace/wire lengths
are minimized, and the signal flow is from small to large. Keep outputs well
separated from inputs.

PTP Wire Amp Myths You'll Hear

PCB's use traces to connect components that add a small degree of
capacitance between the parts which changes the tone/sound
It is
correct that PCB's use thin copper traces to connect parts and that there is
capacitance between all the traces. However, the implication that point to point
wiring does NOT have such parasitic elements is
completely false. Point to point wiring uses "wires" to connect components;
these "wires" also add a small degree of capacitance
between the parts. In fact, for some instances, hookup wire can have MORE
wire-to-wire capacitance per unit length than PCB traces. There are no wires
that *don't* have self inductance, capacitance,
resistance and all the other ills that every electronic part is heir to. Which
has more? It depends... you have to know the mechanical
specification of exactly where every part is in relation to every wire and the
chassis to compute the capacitance for either traces or
wires. A well designed PCB with attention paid to where the traces go may have
bigger or smaller capacitances to other traces and
parts.

As to this necessarily degrading tone - almost always no.
The capacitances may or may not have any audible effect, and if there is an
effect on the tone, it may or may not be positive, or
may or may not be swamped out by the actual parts instead of the
wires/traces. The only way to tell is to figure out what the capacitances
actually are and see if it makes a difference - what's the capacitance of a
signal wire lying against the chassis for 6"? Is it more or less than the
capacitance of a PCB trace for 6"? Hmmm... can't tell without
measuring and seeing what this capacitance couples together in the
amp.

Companies used to use carbon comp resistors which yield a
richer, more complex tone.
Ah, yes. The old "They don't make them like
they used to". They really don't make them the same any more - thank God.
Resistors used to be 20% tolerance, drifted another 5-10%, and had a "hole in
the middle" where all the ones that did by accident get within 5% and 10% of the
nominal value had been selected out. A 20% resistor was guaranteed
to not be any closer that 11% to the nominal value because all the 10% and
better ones had been selected out by the manufacturer. Old carbon comp processes
had build in distortions, rectification effects, and so on. Process control is
much better than it used to be. The differences in carbon comp and modern film
resistors are real, but that whole thing is a topic for another article. Suffice
it to say that just because it's carbon comp, it's not tone magic. Carbon comps
were the cheapest available resistors in the "golden age", and were used for
that reason. Leo Fender was a businessman, not a guitarist.

The quality of the parts has been reduced /
Companies used to use better quality caps.
Another one. This one
is pure nonsense. Modern capacitors are more stable, more consistent, have
closer tolerances, lower drift with time and soldering, and so on.

Ah, but the old paper capacitors were tone magic/mojo
magnets/whatever.
Nonsense. Old paper capacitors had higher ESR and ESL
than some modern capacitors, lower than others. It's entirely possible to get
*better* modern caps and then add resistors to "ugly them down" to what the
older caps did. You do have to know what you're doing, though.

Modern parts that cost next to nothing when bought bulk quantities are
loaded by computers.
Leo Fender would have jumped on that!! He'd
have loved it. As it was, he used the cheapest parts and labor that would
produce his amps reliably enough to sell well. Leo was a businessman, not a
guitarist.

Cheaper parts and pcb's lead to an amp or effect
that sounds stiff, lifeless or one dimensional compared to the earlier
ones
Huh? Why? What parts affect the tone? This is a form of creeping
superstition. Most audio research indicates that the terms "stiff", "lifeless",
"tiring", and so on are associated with low levels of intermodulation
distortion. This is a circuits thing, not a wiring thing, no matter how it's
done.

Handwired, point to point will always sound better than any stock PCB
amp.
Lets face it, in the 50's and early 60's amps were wired
point-to-point, because it was cheap...the skilled labor required to solder
wires to the turrets was cheaper than buying PCB making equipment, which was
still experimental at the time. But there's no real evidence that you can't do
as well or better with a PCB. And the PCB's will come out the same way, every
time. No little lead dress problems making some of your amplifiers oscillate
when the rest of them are OK.