Post by RSPost by Bruce MorgenPost by RSPost by Bruce MorgenThe two are similarly
but not identically
modded -- both have a
47k(iirc) series input
resistor added, but only
the troublesome one has
DC on the first hole
filament. That mod
entailed some twisted
pair to bring 6.3VAC
from the PT to a bridge
rectifier circuit
Is it ground-referenced?
No, straight series resistor
a la Leo, not a voltage
divider or some such. Both
amps have it.
Not sure I follow, Bruce. I was referring to whether the filament
supply was ground ref'd or floating. Just trying to narrow down the
differences between the two amps.
The DC to the first hole
socket is floating. The
filament winding of the
PT is ground-referenced
via a pair of 100 ohm 1
watt resistors.
Post by RSPost by Bruce MorgenPost by RSYeah, capacitance has a more pronounced effect there, as it's working
against pickup impedance rather than simple resistance. LC vs RC.
Well, R is still a factor
-- as a matter high coil
resistance via very fine
wire does wonders for the
LCR numbers, it makes a
conventional pot & cap
tone control much more
effective while limiting
the effect of cable
capacitance. That's one
of the virtues of Bill's
new Microcoil pickups --
high R, low L, highly
effective tone control
with a normal 22 nF cap,
little or no worry about
cable length.
I haven't seen the Lawrence Microcoils yet. I'll check them out.
How would -increased- coil resistance limit the effect of cable
capacitance?
Bill showed me the LCR
calculations, but I don't
remember them, sorry --
but I can say that I can
switch from my three foot
practice cable to a
fifteen footer with no
discernible loss of highs
on my Microcoil-equipped
guitar; the same change
with my other guitars
definitely incurs an
audible loss in sparkle
that I have to compensate
for with the amp's
"presence" control.
Here's what they look
like:
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I never finished this
write-up, but you might
find it interesting:
_____
(Note: Below I will try to convey some of my limited understanding of the MicroCoils -- I am
neither a pickup designer nor a physicist, so all the useful and accurate information comes from
Bill while any errors in my explanation are strictly my own.)
Like many revolutionary products, the MicroCoils are deceptively simple in principle -- they are
single-coil guitar pickups, superficially similar to such venerable designs as Gibson's famous
P-90. They feature screw-adjustable, unhardened steel pole pieces charged by a bar magnet, just
like an old P-90. Now I like a good P-90 as much as the next guy and they're featured in many
famous recordings, but their flaws are also very well known. For example, they pick up an enormous
amount of AC hum (a trait that turned Gibson toward humbucking pickups in the 1950s), they tend to
be one-trick ponies tonally (especially in solid-body guitars), and as they age they become
notoriously vulnerable to frustrating microphonic squealing at rock and roll stage volumes.
So, you may well ask, if MicroCoils are like P-90s, what makes them revolutionary? As a player and
listener I can boil that down to one word -- performance -- but that doesn't explain Bill's
achievement. Part of what separates the MicroCoils from all previous single-coil designs is
materials -- neodymium bar magnets were not available circa 1950, and neither was a coil winding
technology capable of handling the extremely fine magnet wire necessary to build a MicroCoil. Being
able to wind with such fine wire at high speed on oblong coil forms without introducing
tone-sucking flaws is another major part of what makes the MicroCoils possible, because using fine
wire allows more turns of wire for any given winding space.
The "Micro" in MicroCoil refers to the very narrow winding space Bill specified. Limiting the
available winding space has a couple of key advantages. First of all, it limits hum to a level
that's so low that many at first don't believe they're listening to a true single-coil pickup. More
importantly, at least as I see it, the narrower the winding space the more phase coherent the
signal coming from the pickup. Most electric guitarists are at least somewhat familiar with phasing
issues -- the famous "quack" tone of a Fender Stratocaster comes from the partial phase
cancellation that occurs when the outputs of two nearly adjacent pickups are combined, as they are
in switch positions 2 and 4 of a Strat.
What's not so well known is that more subtle -- but definitely audible -- partial phase
cancellations also occur within a pickup's coil because part of the signal is coming from turns
close to the string and part of it comes from turns considerably further away. For true phase
coherence, every turn of the pickup's coil would have to be equidistant from the string, a
practical impossibility. What Bill did was to minimize phase cancellation to the extent that's
practical by specifying that very narrow MicroCoil winding space. This is what allows the
MicroCoils' exquisite tonal purity, which I'm sure Bill could explain in proper technical terms but
to my ear comes across as a sense of intimacy with the string that I've never heard from any other
guitar pickup -- it's like a MicroCoil is a freshly washed pane of clear glass, while traditional
single-coil designs are more like a piece of waxed paper or even cheesecloth, with the pickup's
coil dimensions impacting clarity in a way that cannot be readily corrected elsewhere in the signal
chain. In contrast, the more phase coherent, transparent clarity of a MicroCoil can be readily
altered to achieve all manner of "waxed paper" or "cheesecloth" effects -- but the player is never
stuck with a sound he may only want once in a while. One of Bill's design watchwords has always
been "versatility," and their transparency helps make MicroCoils amazingly versatile.
Efficiency
Another advantage of the MicroCoils' narrow winding space is that more turns closer to the strings
make for improved efficiency -- more signal for any given inductance. An additional factor in the
MicroCoils' impressive efficiency is the meticulous design of the magnetic circuit. Total magnetism
-- which experts call the "energy product" -- has two components. Just as electricity has voltage
and current (amperes), magnetism has magnetic attraction (gauss) and demagnetization force
(oersted). While magnetic attraction is what causes string-strangling problems like "Stratitis" and
loss of sustain, it turns out it's the demagnetization force that's more relevant to how much
signal is generated. One type of magnet that favors the desirable demagnetization force over the
often-troublesome magnetic attraction is the modern neodymium variety. Unfortunately, such magnets
are far too strong -- they have a very large total energy product -- to charge pole pieces
directly. Bill's MicroCoil magnetic circuit includes a steel moderator bar between the neodymium
bar magnet and the pole pieces -- this maintains a favorable ratio of demagnetization force to
magnetic attraction while reducing the latter to a level that avoids loss of sustain or
"Stratitis."
Wonderful "side effects"
Bill's design goal of versatility implies that the instrument's controls be highly effective in
altering the sound -- for high inductance designs like the Bill's famous L-500XL humbucker, the
best way to meet that goal is to employ an inductor (passive coil) in a tone control circuit that
optionally also includes a resistor along with the usual capacitor in a circuit Bill calls a
"Q-Filter." Without getting into the actual math, the very fine wire used in the MicroCoils
increases the electrical resistance (Er) and thus alters the LCR
(inductance/capacitance/resistance) interaction that determines (among other things) how the tone
control operates. This in turn allows a conventional tone circuit (comprising a potentiometer wired
as a variable resistor in series with a capacitor) to operate much more effectively than it does
with conventional single-coils and eliminates the need for an inductor as in the "Q-Filter." This
means that the MicroCoils can be perfect, "drop-in" replacement pickups -- at most, you might find
you like a different capacitor value, but the tone circuit can remain just like it came from the
factory. I found the typical 22 nanofarad (.022 mFd) "chicklet" capacitor in my guitar to be pretty
much perfect.
Another wonderful "side effect" of installing MicroCoils is that the same altered LCR interaction
that permits a conventional tone circuit also minimizes the effect of cable capacitance. Excessive
cable capacitance has a major impact on the performance of guitars with high inductance pickups and
even many lower inductance models wound with a typically thicker magnet wire -- to avoid severe
losses in the cable, low capacitance cable (25 picofarad/foot or lower) is a must and even then
cable length needs to considered. With the MicroCoils, this issue becomes pretty much a non-problem
-- you can use virtually any cable for a short (ten feet or less) run, or choose a low-capacitance
cable (I use Bill's cable and solderless plugs) that's considerably longer with no ill effects -- I
just measured the cable I've been using regularly and it's nearly 15 feet long!
_____
Like I said, it's unfinished -- I just hope you find it helpful!
