A full-dumping tremolo guitar is provided with a recess extending only part way through the body of the guitar, in which recess are located the springs for balancing the pivotable bridge against the string tension. The screws for adjusting the spring tension are accessible from the front of the guitar when the bridge is pivoted upwardly, in the direction to loosen the strings. The bridge and its supporting structure are readily removable from their connection to the springs in the recess, for adjustment or replacement, and the top bridge itself is readily removable from its supporting structure for replacement purposes. The construction is such that relatively long springs can be used with a relatively short recess. The smooth, undisturbed surface of the back of the guitar contributes to the appearance and tonal quality of the guitar. The base pick-up preferably overlies a portion of the same recess which contains the springs.

FIELD OF THE INVENTION 
This invention relates to stringed instruments which use a bridge pivotable 
during playing to vary the pitch of all strings which traverse the bridge, 
for example to produce a tremolo effect, and which provide for complete 
relief of the string tension to achieve a full-dumping operation. 
BACKGROUND OF THE INVENTION 
Stringed instruments such as guitars are known in which the bridge is 
mounted so that by manual operation of a "trem bar", the bridge can be 
pivoted about an axis transverse to the strings during playing, thereby to 
vary the tension on all strings simultaneously; the result is a controlled 
variation of the pitch of all strings, whereby distinctive sound effects, 
such as a wavering pitch known as tremolo can be produced, as an example. 
In a full-dumping guitar the tension of the strings can be completely 
relieved by pivoting the bridge, which is known as full-dumping. 
One way in which this has been accomplished has been to mount a pivotable 
bridge in an opening extending completely through the guitar, along with 
adjustable springs mounted in the rear of the recess and accessible from 
the rear, which springs urge the bridge in the direction to tighten the 
springs; typically the pivoting torque exerted on the bridge by the 
springs just balances the oppositely-directed torque exerted by the 
strings, so that the bridge "floats" in a balanced equilibrium position 
when no disturbing external force is applied, from which position it can 
be pivoted in either direction by a lever or "trem bar" extending outward 
from the front face of the instrument, thereby to produce the desired 
manually-controlled variation in pitch of all strings. In order to 
maintain the desired balance, i.e. the appropriate spring tension to 
counteract the string tension, the guitar is provided with a rear cover 
which is removed so adjustment of spring tensions can be made from the 
rear side of the instrument. 
In addition, the bridge structure is typically of one piece with its 
underlying support, and therefore causes difficulties in changing the 
bridge itself, making it difficult for example to substitute a wooden 
string-engaging bridge for a metal or plastic one, or vice versa. 
It is therefore desirable, among other things, to provide a stringed 
instrument of the tremolo type in which adjustment of the springs and 
replacement of the bridge are easier, manufacture of the entire instrument 
is easier and less expensive, and the tonal quality of the instrument is 
not compromised by requiring use of a covered opening in the back of the 
guitar. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a tremolo stringed instrument is 
provided having a recess in its front face extending only part-way through 
the instrument, and over which the bridge is pivotably mounted; in the 
same recess there is mounted the spring system which tends to pivot the 
bridge structure in the direction to tighten the strings. This spring 
system comprises spring-tension adjusting means which are accessible for 
adjustment from the front of the instrument when the tremolo bar is 
pressed to rotate the bridge structure in the direction to loosen the 
strings. In the preferred embodiment, normal tuning can be performed at 
the head end of the guitar, and optionally, fine tuning can optimally be 
provided at the body end of the guitar. The bridge is mounted so it can be 
easily removed and replaced by any of various other bridge types and 
materials. The numerous other advantages of the new guitar are set forth 
in detail later herein.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring now to the preferred embodiment of the invention shown in the 
figures by way of example only, the guitar 10 comprises a main body 12 
having a front face 14, and a smooth continuous and uninterrupted rear 
face 16; a finger board 18, with string-tuning means such as 20 at its 
distal end, which in this example may be of the locking machine head type. 
The strings such as 24, in this example six in number, extend from the 
string-tuning means 20 to a bridge 25 which is provided with a string 
retainer such as 28 for each string (see FIG. 1D for details); in this 
example each string is provided with a ball such as 30 (FIG. 3) at its 
end, which holds the string from being pulled through its associated slot 
such as 34 (also FIG. 3) when placed under tension. The bridge 25 is 
mounted on the front of the guitar, and carries adjustable intonators such 
as 38, one for each string, the string in each case extending over and 
against the top of its corresponding intonator. Preferably, fine tuning of 
each string is enabled by a fine-tuning screw such as 40 (FIG. 3) 
screw-threaded into the top of the bridge in a position such that, by 
screwing it downwardly, it presses the underlying string downwardly to 
sharpen the pitch of the string, and vice versa when it is screwed 
upwardly. The intonators 38 are mounted on the bridge by screws such as 46 
(FIG. 6) which pass through slots such as 49 in the bridge which extend 
parallel to the strings, whereby each intonator can be adjusted without 
changing the pitch of the corresponding string by loosening its screw, 
sliding the intonator along the bridge, and then re-tightening the screw. 
Also mounted on the front of the guitar are a bridge pick-up 50 adjacent to 
the bridge and a second pick-up 51 nearer the fingerboard, for responding 
to the vibration of the strings to produce corresponding electrical 
signals for amplification and subsequent application to a speaker system, 
thereby to provide audible dissemination of the music being performed by 
the player. Manual control knobs 52, 53 and 54 (FIG. 1D) are provided to 
adjust volume and tone, and to switch pick-ups. Interconnections between 
the controls and the guitar electrical output are provided by way of 
channels 58, 60, 62, 64 buried within the guitar body. 
The bridge 25 is mounted on the guitar so that by pressing down on the trem 
bar 68, the bridge pivots and tilts upwardly away from the guitar face to 
loosen the strings, and by releasing the trem bar the bridge returned to 
its original position and re-tightens the strings. The degree of pivotal 
motion is such that the string tension can be completely relieved, so as 
to accomplish what is known as a "full dump", rather than just a minor 
variation in tension to produce only a tremolo effect. 
The bridge system of the invention is mounted in the front-opening, 
closed-back recess 70 (FIGS. 2 and 3), which extends only part way through 
the thickness dimension of the guitar main body. Where, as in this 
example, the main body is solid rather than hollow, the recess can be 
formed by routing with a wood router, but it can be made by forming the 
main guitar body of successive bonded lamina of wood, each lamina except 
the rear one being cut out in the shape of the cross section of the 
recess, so that when placed on top of each other and bonded together by a 
suitable adhesive they form the desired recess. 
In the recess 70 there are mounted a plurality of spiral springs 71, in 
this example three of them, mounted at one end on a first crossbar 72 (see 
FIG. 2) by screws such as 75 (FIG. 3), and at the other end on a second 
crossbar 74 by screws and washers such as 77 and 78. Bar 72 is mounted to 
the wood of the guitar body near one end of the recess 70 by a pair of 
adjustable fasteners such as 80, in this case comprising wood screws the 
slotted heads such as 81 of which are spaced from crossbar 72 by 
respective spacer cylinders 84 and 86. The three springs 71 are used to 
oppose the tension which the strings exert on the bridge, as described 
below. 
The bridge 25 is mounted, by five screws 88, on a hard-metal pivot plate 90 
which extends laterally beyond the bridge on both sides, and is provided 
at its forward corners (facing the fingerboard) with a first horizontal 
knife-edge structure 94 and a second horizontal knife-edge structure 96. 
Knife edge structure 94 is substantially semicircular, while knife-edge 96 
is substantially quarter-circular, as an aid in slipping the entire bridge 
and supporting structure into and out of the position shown. These 
knife-edge structures mate with, and pivot in, a corresponding pair of 
pivot anvils 98 and 100 threaded into the wood of the guitar front on 
opposite sides of the bridge. Each pivot anvil has a frusto-biconical 
shape with its axis vertical, and the two knife edges of the pivot plate 
fit part-way around the anvils at their minimum diameters, to establish a 
horizontal pivot axis transverse to the strings for the pivot plate and 
the bridge carried on it. 
Beneath the bridge and the pivot plate and extending at right angles 
therefrom into the recess 70 is an actuator plate 108 (FIG. 4) having two 
downwardly-extending arms 110 and 112 between which a cut-away region 116 
provides a passage for springs 71. Above this passage are two through 
holes 120 and 122 large enough to accommodate the shaft of a screwdriver, 
and as will be seen they are aligned so that when the bridge is tilted up, 
a screwdriver can be inserted through the holes to turn the screws such as 
80, and thereby adjust the longitudinal position of the springs, and hence 
the tension which they exert on the actuator plate 108, as now to be 
described. 
Arms 110 and 112 of the actuator plate 108 are provided with threaded 
through holes 130 and 132 (FIG. 4) in which threaded stubs 134 and 136 are 
adjustably held, and from which they extend outwardly. Stubs 134 and 136 
are seated in corresponding loosely-fitting openings in the crossbar 74 so 
that when the assembly of bridge 25, pivot plate 90 and actuator plate 108 
is pivoted upwardly, the springs 71 are stretched and hence exert a 
restraining spring force. Trem bar 68 has a horizontal portion 138 and a 
vertical end portion 140 threaded into the top of actuator plate 108, so 
that by manual up-and-down motion of the trem bar the bridge and bridge 
support structure can be pivoted on the knife edges 94 and 96. 
As the strings are being initially tuned, the bridge structure will tend to 
be pivoted somewhat upwardly by the tension of the strings; to adjust the 
springs so that the bridge structure "floats" in an equilibrium between 
string tension and spring tension, the trem bar is pressed to raise the 
bridge structure sufficiently to expose the heads of the screws 80 through 
the holes 120 and 122 in the actuator plate, whereby a screwdriver can be 
inserted through the holes to adjust the screws and thereby set the spring 
tension to balance it against the string pressure. If in some cases it is 
only desired to dump the strings at certain times, the springs can be 
adjusted normally to hold the bridge structure in one extreme position, as 
against a stop, in which it maintains full string tension, the "trem bar" 
then constituting in effect a dump bar which can only be operated in one 
direction from its rest position. 
Various of the advantages of the new guitar can now more readily be 
appreciated; 
(1) The springs which counteract the string tension can be more readily 
adjusted then in guitars in which they can only be accessed through a 
removable plate on the back of the guitar; one need merely operate the 
trem bar to lift the bridge structure above the front face of the guitar 
so that a screwdriver shaft can be inserted through the holes 120 and 122 
in the actuator plate to adjust the screws 80. 
(2) The sound and appearance of the instrument are improved due to its 
intact rear side, as compared with a guitar in which there is a covered 
hole in the back of the guitar. 
(3) The full-dump tremolo bridge structure of the invention can be readily 
installed on nearly any standard type of electric guitar body. 
(4) The bridge section of the bridge system is easily removable by removing 
five screws, so the system can readily be optionally used with a different 
type of bridge. e.g. one of wood, aluminum, steel, etc. for different 
types of effects; also,by using an electrically insulating bridge 
material, the structure of the invention can be used with instruments such 
as the touch-guitar which require electrical insulation of each string 
from the other. 
(5) The guitar of the invention is especially easy and economical to 
manufacture. 
(6) The intonators such as 38 are easily adjustable in position without 
detuning the guitar. 
(7) The cut-away portion of the actuator plate in the bridge system permits 
the springs to extend through the actuator bar, and thus uses a shorter 
recess in the guitar main body than when the springs are required to be 
entirely on one side of the actuator plate. 
(8) Strings are easily removable by merely tilting the bridge structure 
forwardly by means of the trem bar, and then popping the ball ends of the 
strings out of their retainers. 
(9) Fine tuning can be provided, optionally, at the head end of the guitar 
as described. 
(10) Locking nuts are not required for the strings at the distal end of the 
fingerboard. 
(11) The threaded stubs 134 and 136 can be adjusted by screwing them in 
either direction in the holes 130 and 132, as a further means of adjusting 
the spring tension. 
(12) The feature of connecting the actuator plate to the cross-bar 74 by 
means of the threaded stubs 134 and 136 is important in that it permits 
the easy removal of the entire bridge assembly of actuator plate 108, 
pivot plate 90 and top bridge 25 as a unit, for example, to accomplish the 
adjustment of the threaded stubs as described in paragraph (11) above, or 
to remove the top bridge 25 for repair or replacement. 
While the invention has been described with particular reference to 
specific embodiments in the interest of definiteness, it may be embodied 
in a variety of diverse forms without departing from the invention as 
defined by the claims.