Abstract:
An amplifier for electric guitar including a Cut control for attenuating high frequencies whose operation is expanded via a switch to alternately function as a Master Volume control.

Description:
FIELD OF THE INVENTION 
     This invention relates to audio amplifiers such as for use with an electric guitar. 
     BACKGROUND OF THE INVENTION 
     Guitar players are often fickle with competing requirements for their amplifiers. On the one hand they want simplicity while on the other hand they need versatility. They revere vintage amplifiers for their “classic purity,” but then lament the lack of modem features. They demand a certain type and model of amplifier in order to duplicate influential recordings but are then frustrated by the typical limitations of these early designs, the most common being the “one-good-sound-at-one-volume” syndrome. And that&#39;s usually too loud! 
     The fundamental limitation of these vintage amplifiers is that tone is inseparably related to volume. Many of the vintage amplifiers popularized in the 1960s by famous bands such as the Beatles produce preferred tones only when set at or near maximum volume. While such settings are acceptable for many live venues, they are too loud to be useful in a typical home or small club setting. When adjusted to lower volume, the tone suffers as the overdrive, which alters the tone, evaporates completely. 
     One such vintage amplifier is the Vox AC-30 and its sibling the AC-15. These amplifiers are revered for their unique performance and simple appearance. As testimony to their lasting impact, a modernized version has been introduced by a firm resurrecting the Vox brand name. Furthermore, several copies and derivatives of the original design that have been produced by other manufacturers. But all of these suffer from either being versatile but complex or being faithful to the original but limited in functionality. 
     Even though the Vox AC-15 and AC-30 amplifiers are rated at a mere 15 or 30 watts respectively, they are surprisingly loud. Moreover, their iconic tones really don&#39;t materialize until they are set near the top of the output range via the one simple Volume control 
     This limitation has been dealt with successfully in the past with other circuits, typically by the inclusion of separate controls to regulate preamp output amplitude independent from preamp gain. In many amplifiers one or more extra stages of amplification are added as well. U.S. Pat. No. 4,211,893 (“the &#39;893 patent”) of Randall C. Smith presented one such solution by allowing Gain and Loudness to be preset independently and providing two different foot-switchable sounds—a clean one for Rhythm and an overdriven one for Lead. This solution is adequate for many players but necessitates four separate controls, it is a clear departure from the simplicity of an AC type amplifier with its single Volume control. Furthermore, this design may be inadequate for some players and musical styles because the Rhythm mode of the &#39;893 design may have insufficient gain while the Lead mode has too much. 
     Despite the bare-bones simplicity of the Vox circuit and control panel, the range of gain available from the single Volume control is impressive. Clean sounds at lower settings give way to semi-broken-up sounds to moderately overdriven sounds when the single Volume control is turned up to its maximum. While this range of gain is adequate for many popular styles of music, the most desired sounds only occur at very loud levels, severely limiting the Vox&#39;s usefulness especially in smaller venues. 
     Compounding the problem is that a typical Master attenuator control, coming at the end of the preamp, will not work with the Vox AC type amplifier circuitry. Their original preamp circuit is far too deficient in gain, comprised of only a single triode gain stage ahead of the phase splitter and power tubes. Later Vox AC models added Bass and Treble controls plus a second triode to recover amplitude lost through those tone controls. Even so, there is still not nearly enough gain to obtain usable overdrive characteristics from the preamp alone. 
     The Vox AC amplifiers compensated for this lack of preamp gain by employing a long-tail driver and push-pull output stage and operating them “open loop,” with no negative feedback. The cathode coupled, long-tail phase splitter uses two separate triodes to furnish push-pull drive signals and contributes substantial gain in the process. What&#39;s more, it is this “wide open” output section that is clearly fundamental to the unique character and success of the Vox&#39;s musical performance. 
     Another unique aspect of the Vox was the inclusion of a so-called “Cut control” to partially compensate for the initial lack of tone controls and to allow some taming of the brash sounding wide-open output section. This consisted of a capacitor and variable resistor functioning as a high pass filter between the input grids of the push-pull output tubes. Advancing the Cut control clockwise increasingly attenuates high frequencies by decreasing the resistance between the opposing grids such that the high frequencies were selectively cancelled through the capacitor. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention satisfy the long felt need for providing optionally selectable modern versatility while maintaining performance and appearance. Using the invention of the current disclosure, a Master Volume control may be added to a vintage amplifier, such as the Vox AC-30 and its sibling the AC-15, while preserving the performance and classic appearance of the vintage amplifier. 
     In one example of an embodiment the amplifier presently disclosed, the same basic controls of the simple Vox panel(s) may be retained. For the early versions these are only two: the Volume control and the Cut control. For later so-called “Top Boost” models, these were complimented by the addition of Treble and Bass controls. Whether retrofitting original Vox amplifiers or creating newly-built recreations as “tributes,” the Cut control, in embodiments of the present invention, is modified by adding a switch to optionally alter its function. With this switch in a first state, the potentiometer of the Cut control&#39;s circuitry is reconfigured to perform the function of a Master Volume control, enabling the amplifier to produce its iconic overdriven sounds, largely independent of playing loudness. With the switch in a second state, full traditional performance is restored: the Master Volume control circuitry is decoupled from the amplifier circuit. By adding the switch means and altering the traditional Cut control circuitry in accordance with the present invention, a dual-function Cut/Master control is created. 
     This dual-function Cut/Master control successfully enables a vintage amplifier to deliver its essential tonal character at any volume, thus overcoming the principle limitation or objection to this type of amplifier. The importance of the ability to decouple the Master Volume control circuitry from the amplifier cannot be overstated to the legion of “purists” who often regard the inclusion of any Master as an illegitimate sacrilege certain to defile vintage tone even when turned up fully. 
     Further, the dual-function control presently disclosed, especially with an integrated pull-switch, has considerable value for modifying existing Vox and similar amplifiers because the retrofit could be accomplished without the need to alter the control panel with additional holes, pots or switches. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified schematic diagram of a prior art Vox AC circuit; 
         FIG. 2  is a schematic diagram of an example of one embodiment of the present invention; 
         FIG. 3  is a schematic diagram of an example of a second embodiment of the present invention; and 
         FIG. 4  is a schematic diagram of an example of a third embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In order to overcome the lack of preamp gain and provide a successful Master Volume function, in accordance with the present invention a variable resistor is coupled directly between the opposing grids of the push-pull output tubes in a vacuum tube amplifier thereby preserving the gain and overdrive capabilities inherent in long-tailed driver amplifiers. There are a number of ways to accomplish this result and create the dual-function Cut/Master control of the present invention. 
     In embodiments of the present invention, a switch is used to alternate the function of the dual-function Cut/Master control. The switch has two positions, the Cut position and the Master position. In the Cut position, a variable resistor is coupled in series to a capacitor and both elements are coupled between the opposing grids of the push-pull output tubes. In the Master position, a variable resistor is coupled directly between the opposing grids of the push-pull output tubes. 
     The preferred mechanical configuration for alternating between the two functions of the dual-function Cut/Master control is a pull-switch integrated with the dial that controls the potentiometer or variable resistors contained in the circuitry of the dual-function Cut/Master control of the present invention. The dial would be labeled something like “Cut/Pull Master.” When the user pulls the integrated pull-switch, the functionality of the dial changes from adjusting the Cut control to adjusting the Master Volume control. A switch separated from the dial could also be used, but the amplifier would not retain the simple and traditional visual appeal that is so important to many players. 
       FIG. 1  shows the audio circuitry of the traditional Vox AC amplifier  60 . A guitar input terminal  1  is coupled to the grid  2  of the preamplifier vacuum tube V 1 . This is a basic resistance coupled amplifier with a cathode bias resistor  3  and a bypass capacitor  4 . An amplified signal is developed across a plate resistor  5  and AC coupled to the Volume control  7  via the blocking capacitor  6 . The Volume control  7  includes a wiper element  8  that is coupled via a resistor  9  to the grid  10  of vacuum tube V 2  which is the first of a long-tail vacuum tube pair V 2 -V 3  comprising the phase splitter/driver amplifiers. The resistors  11 ,  12 ,  13  and  14  comprise the biasing network of the pair V 2 -V 3  in a typical arrangement that provides signal coupling from a cathode  15  to a cathode  16  to provide amplified opposing phase signals developed across the resistors  19  and  20  from the plates  17  and  18 , respectively. A pair of coupling capacitors  21  and  22  deliver AC signals to the grids  23  and  24  of the push-pull output tubes V 4  and V 5 , which are cathode biased via a resistor  25  and a bypass capacitor  26 . The screen grids  27  and  28  are fed through the resistors  29  and  30  from a power supply point B. The power supply point A furnishes high-voltage to the plates of V 4  and V 5  through a primary winding  31  of the output transformer. C and D indicate other high-voltage points of the power supply, which are not shown in this simplified drawing. The secondary winding  32  drives loudspeaker  33 . 
     The resistors  34  and  35  provide grid leak for the grids  23  and  24  respectively. A variable high-pass filter of the capacitor  36  and the variable resistor  37  comprise the Cut control, which functions by selectively canceling out high frequencies at the grids  23  and  24  of the output tubes V 4  and V 5 . 
       FIG. 1  is provided as an example of circuitry that may surround the examples shown in  FIGS. 2 ,  3  and  4 .  FIGS. 2 ,  3  and  4  each show examples of embodiments of the present invention.  FIGS. 2 ,  3  and  4  may be used in conjunction with the Vox AC circuitry shown in  FIG. 1 , or with other push-pull amplifier circuitry. 
       FIG. 2  is a schematic diagram of an example of an embodiment of the present invention illustrating the circuitry of a power amplification section of an amplifier that includes the dual-function Cut/Master control. In this example, the Cut control  37  of  FIG. 1  is replaced with potentiometer  37 A. As shown, the potentiometer  37 A has its wiper element  40  coupled directly to the grid  24  of the output tube V 5  while one end  41  of its fixed element is coupled to terminal  42  of a Double Pole Double Throw (DPDT) switch whose poles are shown at  43 A and  43 B. The other end  46  of the fixed element of potentiometer  37 A is coupled to terminal  47  of the DPDT switch. 
     In  FIG. 2 , the DPDT switch is shown in the Cut position. In this position, terminal  41  of potentiometer  37 A is coupled to capacitor  36  and the amplifier functions as a traditional push-pull amplifier with a Cut control, for example, the amplifier shown in  FIG. 1 . 
     In  FIG. 2 , when the DPDT switch is in the Master position (as indicated by the dashed lines) the  43 B switch segment couples the terminal  46  of the potentiometer  37 A to the grid  23  of the output tube V 4 . This provides a variable attenuation path directly between the grids  23  and  24  of the output tubes V 4  and V 5  to make potentiometer  37 A function as a Master Volume control. Additionally, a capacitor  36  may be coupled through a pole  43 A and a switch terminal  44  to couple with a resistor  45  to provide a fixed substitute for the formerly adjustable Cut function. 
     The circuitry surrounding  FIG. 2  may be the circuitry shown in  FIG. 1 . The output tubes V 4  and V 5  may be coupled to power supply B and coupled to the output device as shown in  FIG. 1 . Capacitors  21  and  22  may be coupled to the preamplification circuitry shown in  FIG. 1 , or other preamplification circuitry. 
       FIG. 3  is a schematic diagram of an example of another embodiment of the present invention illustrating the circuitry of a power amplification section of an amplifier that includes the dual-function Cut/Master control. In this embodiment, a second variable resistance element  37 B is used to form the Master Volume control circuitry. Variable resistance elements  37  and  37 B may be ganged to a shared actuator shaft so that one dial may adjust both elements. 
     In  FIG. 3 , the switch  50  is shown in the Cut position. In this position, the first variable resistance element  37  is coupled to the capacitor  36  and the amplifier functions as a traditional push-pull amplifier with a Cut control, for example, the amplifier shown in  FIG. 1 . When the switch  50  is in the Master position, as indicated by the dashed line, the second variable resistance element  37 B is coupled between the two grids  23  and  24  of the output tubes V 4  and V 5 . In the Master position, variable resistance element  37 B functions as a Master Volume control. In this embodiment, individual resistance tapers may be used in  37  and  37 B which would offer customized performance of the Cut control and Master Volume control. 
     A conventional Master Volume control and a traditional Cut control have opposite functional rotations. The Master Volume control increases the loudness when the control is turned clockwise by increasing the resistance of the variable resistor in the Master&#39;s circuitry. A traditional Cut control functions in the opposite direction, decreasing the resistance of the variable resistor when turned clockwise. For this reason, in some embodiments of the present invention, the functional rotation of variable resistance element  37  may be configured to be the opposite of the functional rotation of variable resistance element  37 B as shown in  FIG. 3 . Using opposite functional rotations will make the amplifier operate in a manner that conforms to the user&#39;s expectations. 
     In the embodiment shown in  FIG. 3 , a second switch element  50 A may be included to provide a preset level of the Cut function through the resistor  45  when the amplifier is in the Master position as shown in  FIG. 3 . As described with respect to  FIG. 2 , the circuitry surrounding  FIG. 3  may be the circuitry shown in  FIG. 1 . 
       FIG. 4  is a schematic diagram of an example of another embodiment of the present invention illustrating the circuitry of a power amplification section of an amplifier that includes the dual-function Cut/Master control. In  FIG. 4 , a switch  51  is used to shunt across the capacitor  36 . The potentiometer  37  is configured as a variable resistor. When switch  51  is open, the amplifier is in the Cut position. In this position variable resistor  37  is coupled to the capacitor  36  and the amplifier functions as a traditional push-pull amplifier with a Cut control, for example, the amplifier shown in  FIG. 1 . When the switch  51  is closed, the amplifier is in the Master position and variable resistor  37  is coupled directly between the two grids  23  and  24  of the output tubes V 4  and V 5 . As described with respect to  FIG. 2 , the circuitry surrounding  FIG. 4  may be the circuitry shown in  FIG. 1 . 
     In  FIG. 4 , the functional rotation of the variable resistor is the same for both the Master Volume control and the Cut control. This may be confusing to users. For example, one way to overcome the confusion would be to would be to re-label the Cut control, calling it “Bright” or “Presence” instead. Then the user&#39;s expectation would be that clockwise rotation would increase high frequencies rather than roll them off, as the Cut control traditionally does. Labeling a tone control with another name does not remove it from the scope of the present invention. 
     In each of the embodiments a variable resistance element such as a variable attenuator or variable resistor, is used. The variable resistance element may comprise a potentiometer or a potentiometer type variable resistor, for example. In some embodiments, the player may adjust the control using a manually-operated dial, but other types of adjusters could be used to adjust the control. 
     Embodiments of the present invention may be applied to amplifiers other than Vox AC amplifiers. For example, embodiments of the present invention may be used with preamplifier sections different from the preamplifier shown in  FIG. 1 . Embodiments of the present invention may also be coupled to output configurations different from the configuration shown in  FIG. 1 . 
       FIG. 1  shows a typical Vox AC type amplifier, presented as a combo amplifier with a loudspeaker present in the unit. The present invention could be applied to a combo amplifier or to a head unit with an output device capable of being connected to one or more speakers. 
     The embodiments disclosed above are intended to be illustrative only. One of ordinary skill in the art will recognize that changes may be made to the embodiments described herein without departing from the scope of the invention, which is defined by the claims, below.