Abstract:
The disclosure is directed to power supplies for audio amplifiers that focus on the importance of certain power supply characteristics in forming the overall qualities of a tube amplifier. The power supplies may use a power transformer with two dissimilar and selectable secondary windings to supply the voltages for a single vacuum tube power amplifier (B+ voltages). The power supplies may also apply different rectifier characteristics to complement the use of multiple differing secondary windings. The power supplies may also use different capacitor bank options to filter ripple voltage. The power supplies may also use of coordinated grid-bias voltage adjustment that coincides with secondary winding selection. The power supplies may also use coordinated preamp bias voltage adjustment that coincides with secondary winding selection.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention is directed to systems, processes and apparatus for selecting from between multiple power supplies for use with audio amplifiers. More particularly, the invention relates to the field of electronic musical instrument amplifiers that enable selection from between multiple power supplies to achieve differing sonic effects. Accordingly, the general objects of the invention are to provide novel systems, methods and apparatus of such character. 
   2. Description of the Related Art 
   The development of audio amplifiers from the early days of electronics has tracked the development in electronics components in many ways. For example, audio amplifiers, which originally used vacuum tubes as amplifying and rectifying elements, widely migrated to semiconductor elements soon after such elements became available. Nonetheless, the use of vacuum tubes persists in several niche applications such as hi-fi stereos, instrument amplification, recording pre-amplifiers, etc. due to a continuing and widespread belief that vacuum tubes produce superior results in such applications. 
   In the field of musical instrument amplifiers, certain “vintage” instrument amplifiers are much sought after for their ability to provide the sonic qualities of various well known and appreciated musical styles. Unfortunately, such vintage amplifiers typically have deteriorated with age and have become unreliable to the extent that use of such vintage devices is unacceptable. Even in those instances where the performance is satisfactory when measured against the original specifications for the equipment, musicians frequently find that such equipment is extremely limited by today&#39;s standards of versatility. There has, therefore, been a need for equipment capable of producing a range of tones of quality vintage equipment while at the same time providing the versatility and reliability more commonly associated with modern equipment. In order to meet this long standing and accelerating demand, vacuum tube amplification continues to progress around the periphery of a handful of basic designs in an effort to provide more economical, reliable, versatile and user-friendly amplifiers that offer musicians a choice of familiar sonic qualities. 
   Among the areas that have been the subject of such development has been preamp switching design, amplifier switching design, digital amplifier modeling technology, and power tube substitution schemes for vacuum tube guitar amplifiers. However, the sonic contribution of power supply design has only recently come under scrutiny among musicians and amplifier designers. In particular, the sonic contributions of such factors as, preamp bias voltage, power amp bias voltage, type of power amp biasing, the effects of various component types and sizes are being explored more closely. 
   This renewed interest in power supply design has yielded some efforts to provide more versatile power supplies to achieve varying musical effects. One such effort is described in U.S. Pat. No. 5,168,438, entitled Selectable Dual Rectifier Power Supply For Musical Amplifier, which describes a simple switching method for selecting from between either a solid-state rectifier or a vacuum thermionic rectifier within a single power supply. This approach offers a reasonably economical way of approximating the desirable characteristics of tube-based rectifiers with the desirable characteristics of solid-state rectifiers in a single amplifier. Unfortunately, this design trades certain subtle sonic qualities in favor of economy and simplicity in its approximation of both styles of power supplies. For example, since this design uses the same secondary power transformer coil and the same ripple-voltage filter regardless of the type of rectification selected only a limited range of changes are possible. 
   Another effort of this general type is disclosed in U.S. Pat. No. 5,091,700 and entitled Amplifier With Mains Voltage Reduction. This patent describes the use of a switch to adjust the mains voltage such that it is possible to reduce the voltages in an amplifier for a different musical effect. This switch adjusts the mains input voltage by selecting a primary winding tap on the primary side of a power transformer to thereby change all the voltages in the amplifier. While this design does appreciate that some transformer characteristics are important in creating the overall amplifier characteristics, its simplistic approach is incapable of changing a limited set of voltages while leaving others unaffected (or changed in the opposite direction). Thus, once again, this design offers economy and simplicity at the expense of some of the finer elements of the several vintage amplifiers it tries to approximate. For example, this design also uses the same secondary power transformer coil and the same ripple-voltage filter regardless of the type of rectification selected. 
   Yet another effort to offer a more versatile amplifier power supply for a vacuum tube amplifier is embodied in an amplifier known as the FENDER® PROSONIC®. This amplifier uses a switching system slightly more complicated than those noted above to offer a number a rectification options. Once again, however, this design also uses the same power transformer and the same ripple-voltage filter regardless of the rectification circuit selected. It, thus, also fails to appreciate the important role of transformer design in the overall amplifier characteristics. 
   All of the designs noted above suffer from the deficiency that they all sacrifice some degree of sound quality in an effort to provide approximations of the sonic qualities of multiple amplifiers. Thus, after many years, no amplifier design has been able to faithfully emulate the essential characteristics of several different vintage amplifiers in one simple, economical, reliable and user-friendly amplifier. 
   There is, accordingly, a need in the art for novel methods, systems and apparatus that faithfully emulate the desired characteristics of several vintage amplifiers in one simple, economical, reliable and user-friendly package. Such methods and apparatus should go further than prior efforts in capturing various subtle qualities of various amplifier designs without presenting users with undue additional complexity, weight size and/or expense. 
   SUMMARY OF THE INVENTION 
   The present invention satisfies the above-stated needs and overcomes the above-stated and other deficiencies of the related art by providing methods, systems and apparatus offering improved power supply options for vacuum tube audio amplifiers. The power supplies of the present invention offer better emulation of disparate vintage amplifier tonality while also maintaining a high degree of economy, reliability and simplicity. 
   One aspect of the present invention is directed to an improved power supply for vacuum tube audio amplifiers that focuses on the importance of certain power transformer characteristics in forming the overall qualities of a tube amplifier. In this regard, the invention uses a power transformer with two dissimilar and selectable secondary windings to supply the plate voltages for a single vacuum tube power amplifier (B+ voltages). Power supplies in accordance with the invention may, or may not, also apply different rectifier characteristics to complement the use of multiple differing secondary windings and, thus, further augment the range of power supply characteristics provided. Power supplies in accordance with the invention may or may not also use different capacitor bank options to filter ripple voltage in conjunction with one or both of the inventive features noted above. Yet another optional aspect of the present invention is directed to the use of coordinated power amplifier grid-bias voltage adjustment that coincides with the secondary winding selection noted above. Still another important optional feature of the present invention concerns the use of coordinated preamp plate voltage adjustment that coincides with the secondary winding selection noted above. 
   In a related form, the invention is directed to methods of selecting various power supply options for supplying vacuum tube audio amplifiers with varying characteristics. Methods in accordance with the invention may include coordinated selection of differing secondary transformer windings, differing rectifying characteristics, differing ripple-voltage filter elements, and/or differing pre-amp plate voltages. 
   Naturally, the above-described methods of the invention are particularly well adapted for use with the above-described apparatus of the invention. Similarly, the apparatus of the invention are well suited to perform the inventive methods described above. 
   Numerous other advantages and features of the present invention will become apparent to those of ordinary skill in the art from the following detailed description of the preferred embodiments, from the claims and from the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred embodiments of the present invention will be described below with reference to the accompanying drawings where like numerals represent like steps and/or structures and wherein: 
       FIG. 1  is selectable power supply for use with a vacuum tube audio amplifier in accordance with one preferred embodiment of the present invention; 
       FIG. 1   a  is a logic table illustrating the preferred coordinated switching of the selectable power supply of embodiment of  FIG. 1 ; 
       FIG. 2  illustrates a selectable power supply for use with a vacuum tube audio amplifier in accordance with another preferred embodiment of the present invention; and 
       FIG. 2   a  is a logic table illustrating the preferred coordinated switching of the selectable power supply of the embodiment of  FIG. 2 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   With joint reference to  FIGS. 1 and 1   a , an amplifier power supply  10  according to one preferred embodiment of the present invention is shown. The power supply  10  is capable of operation in, preferably, two modes: (1) referred to herein as the “punch” mode which is characterized by a relative voltage insensitivity to different current-demand conditions dictated by the connected audio amplifier; and (2) referred to herein as the “sag” mode which is characterized by a relative voltage sensitivity to different current-demand conditions dictated by the connected audio amplifier. These modes are user-selected to achieve differing sonic effects and are changed by a number of coordinated switches. The states of these switches for the embodiment of  FIG. 1  are summarized in  FIG. 1   a  and discussed in detail below. 
   As shown in  FIG. 1 , the inventive power supply may include a conventional connection  28  for receiving AC power, connected through a conventional fuse  26  and a power on/off switch S 1  coupled to a primary winding  14  of a power transformer  12 . The power transformer  12  preferably includes a plurality of secondary windings, including a 6.3 volt heater winding  24 , a first high voltage secondary  22  coil with a center tap WB, a second high voltage secondary  20  coil with a center tap WA, a power amplifier negative bias winding  16  and a five volt filament winding  18 . As shown, the center tap terminals WB and WA of the first and second secondary windings  22  and  20  may be selectively connected to ground via toggle switch S 2 A. When one of the center taps is coupled to ground in this way, the corresponding winding will pass AC power voltage to the corresponding rectifiers ( 30 / 32  or  34 / 36  as the case may be) and, thus, the winding has been selected. 
   It will be appreciated, that switch S 2 A may also have a third position in which it is not connected with either of WA or WB. If this third position (not shown) is provided, switch S 2 A serves as both a winding selector and a power supply stand-by switch. Additionally, a single-pole single-throw switch S 2 B selectively permits activation of additional capacitors by permitting selective connection with ground. It will also appreciated that the switch S 2 B is coordinated with the switch S 2 A such that the capacitor  44  is only electrically activated when the first secondary winding  22  is activated. In this way, capacitor  44  may be selectively disabled so that it does not interfere with use of thermionic vacuum element  38 . These various features are described in greater detail below. 
   When the switch S 1  is closed (thereby delivering power to the transformer  12 ) and the user selects the “punch” mode, switch S 2 B is closed and switch S 2 A is connected with terminal WB and, thus, high voltage AC from the winding  22  is applied to solid state rectifiers  30  and  32 . High voltage winding  22  is specifically designed with a volt-amp rating that far exceeds the expected demand of the connected audio amplifier so that the voltage provided therefrom is relatively insensitive to the changing demands of the connected audio amplifier. Filter capacitor bank  40  reduces the ripple voltage from the rectified DC in a conventional manner and, thus, presents at point A a DC voltage adapted to supply vacuum thermionic elements of an audio amplifier. Similarly, capacitor bank  40  is specifically designed with a capacitance that exceeds the expected needs of the connected audio amplifier due to the added capacitance from capacitor  44  so that the voltage provided therefrom is relatively insensitive to the changing demands the amp. As shown, capacitor bank  40  may comprise a voltage divider formed of a pair of resisters  43  and  47  to which capacitors  42  and  46  are connected. The voltage presented at point A may be further filtered in a conventional way by the use of an inductor L and capacitor  60  to present another DC voltage at point B. Further dropping resistors  70 ,  72  and  74  may apply voltage to power supply points C, D and E, to which other stages of an amplifier circuit (not shown) may be connected. Further decoupling and filtering may be provided by capacitors  63 ,  64  and  66  in a conventional manner as shown. 
   With continuing attention on  FIG. 1 , it is noted that a user may, alternatively, select a “sag” mode. In this alternative mode, the switch S 2 B deactivates capacitor  44 , and the switch S 2 A connects with terminal WA to thereby apply high voltage AC from the winding  20  to solid state rectifiers  34  and  36 . Additionally, filter  48  of relatively small capacitance reduces the ripple voltage from rectifiers  34  and  36 . Rectified DC is, thus, applied to vacuum thermionic element  38 . High voltage winding  20  is specifically designed with a lower volt-amp rating than that of winding  22 . In particular, the volt-amp rating of winding  20  is intentionally selected to cause a substantial voltage drop when the current demand of the connected audio amplifier increases substantially. In this way, the voltage provided therefrom is relatively sensitive to the changing demands of the connected amplifier and an audible change in the character of the amplifier results. 
   Since the secondary windings  20 ,  22  may have different voltage and current ratings, winding  20  may have less voltage at quiescence than winding  22 , and more loss of voltage (sag) as the current demands of the amplifier increase. The rate of sag, or how fast the voltage drops and recovers under operation, is directly related to the filtering of filter bank  40 . The use of lower filtering with winding  20  makes the sag more noticeable, while the higher filtering used with winding  22  makes the supply “stiffer”. As the B+voltage in the amplifier lowers when using winding  20  the preamp voltage is also lowered. Thus, the use of switch S 2 D and resister  76  may be used to readjust the voltage as discussed herein. 
   By way of non-limiting example only, the VA rating for winding  22  may be 136, and the VA rating for winding  20  may be 83.75 or 38.4% less than winding  22 . Thus, the second winding may have a VA rating at least 25% less than the first winding which may yield a noticeable change in performance. Further, by way of non-limiting example only, the connected audio power amplifier may use EL34 power tubes and the quiescent voltage for EL34&#39;s in punch mode may be 475 vdc. Under full load that voltage may drop (sag) to 438 vdc (37 volts). In sag mode the quiescent voltage may be 440 vdc and with a full load the voltage may drop (sag) to 376 (64 vdc). Thus, the quiescent voltage difference between punch and sag may be about 35 vdc and under full load the difference may be about 62 vdc. The use of a solid-state rectifier with winding  20  may give a quiescent voltage of 461 vdc, and that may sag to about 421 vdc under a full load. 
   A filament of the element  38  is heated by the secondary winding  18 . DC voltage from rectifiers  34 / 36  is connected to element  38 . B+ from the cathode of the thermionic element  38  is connected to the point A. Thermionic vacuum element  38  may be one of many known types such as a GZ34/5AR4, 5Y3, 5U4, etc. (all commonly used as diodes) and primarily serves to provide additional voltage sensitivity to the current flowing therethrough. Those of ordinary skill will appreciate that only one of rectifiers  34 / 36  or element  38  needs to be provided for the power supply to function in the sag mode. Nonetheless, it has been discovered that the internal resistance of element  38  provides desirable sonic characteristics and that the additional use of rectifiers provides an additional measure of safety to the connected audio amplifier against damage if element  38  fails. 
   A filter capacitor bank  40  reduces the ripple voltage from the rectified DC in a conventional manner and, thus, presents at point A a DC plate voltage adapted for use with vacuum thermionic elements of an audio amplifier. When switch S 2 B is opened, capacitor bank  40  presents a lower capacitance so that the voltage filtering provided thereby is relatively sensitive to such changing demands. Capacitor bank  40  may comprise a voltage divider formed of a pair of resisters  43  and  47  to which capacitors  42  and  46  are connected. As noted above, the switch S 2 B is coordinated with the switch S 2 A such that the capacitor  44  is electrically deactivated when the second secondary winding  20  is activated to prevent capacitor  44  from interfering with the use of element  38 . In this way, the invention ensures that a relatively large capacitance can be used with winding  22 , while ensuring that element  38  is not damaged by the presence of too much capacitance (preferably no greater than about 60 uF) and that the desired sensitivity provided by the use of winding  20  and element  38  is not compromised by the presence of too much capacitance. The total capacitance of bank  40  will depend on various known factors such as the number and type of power tubes used in the connected audio amplifier. 
   With continuing reference to  FIG. 1 , it will be seen that the voltage presented at point A may be further filtered in a conventional way by the use of an inductor L and capacitor  60  to present another DC voltage at point B. Further dropping resistors  70 ,  72  and  74  may apply voltage to power supply points C, D and E, to which other stages of an amplifier circuit (not shown) may be connected. Further decoupling and filtering is provided by capacitors  63 ,  64  and  66  in a conventional manner as shown. 
   In the case of an amplifier circuit using cathode bias throughout, the power supply circuit described above would be complete. Cathode bias in the power amplifier tubes provides the advantage of its natural tendency to compensate for B+ voltage changes which naturally occur from switching between the secondary coils  20  and  22  and their corresponding rectifying circuits. 
   However, a fixed bias supply  80  may be successfully used in such a power amplifier circuit even though the B+ operating voltage will differ significantly depending upon which secondary winding is connected. Such a bias supply preferably includes a current limiting resistor  82 , a solid-state rectifying diode  83  whose cathode end is fed from a tap of the secondary coil  16 , filtered by capacitor  84 , fed through dropping resister  86  to a bias supply output point F where rectified DC is filtered by a capacitor  85 . An adjustable voltage divider may also be provided via potentiometer  87  and resister  88  as shown in  FIG. 1 . Additionally, a single-pole single-throw switch S 2 C selectively permits engagement of the resister  89  as shown. It will be appreciated that the switch S 2 C is coordinated with the switch S 2 A (see  FIG. 1   a ) such that the resister  89  is only electrically engaged when the second secondary winding  22  is activated. In this way, supply  80  automatically rebiases the tubes of the connected audio power amplifier for proper operation regardless of which secondary coil is selected. 
   With joint reference now to  FIGS. 2 and 2   a , an amplifier power supply  10 ′ according to another preferred embodiment of the present invention is shown which uses non-center tapped secondary windings. The power supply  10 ′ is capable of operation in, preferably, two modes: (1) referred to herein as the “punch” mode which is characterized by a relative voltage insensitivity to different current-demand conditions dictated by the connected audio amplifier; and (2) referred to herein as the “sag” mode which is characterized by a relative voltage sensitivity to different current-demand conditions dictated by the connected audio amplifier. These modes are user-selected to achieve differing sonic effects and are changed by a number of coordinated switches. The states of these switches for the embodiment of  FIG. 2  are summarized in  FIG. 2   a  and discussed in detail below. 
   As shown in  FIG. 2 , the inventive power supply may include a conventional connection  28  for receiving AC power, connected through a conventional fuse  26  and a power on/off switch S 1  coupled to a primary winding  14  of a power transformer  12 . The power transformer  12 ′ preferably includes a plurality of secondary windings, including a 6.3 volt heater winding  24 , a first high voltage secondary  22 ′ coil, a second high voltage secondary  20 ′ coil, a power amplifier negative bias winding  16  and a five volt filament winding  18 . As shown, the first and second secondary windings  22 ′ and  20 ′ may be directly connected to rectifiers  30 / 31 / 3233  and  34 / 35 / 36 / 37  respectively. The rectified DC voltage from  34 / 35 / 36 / 37  is then connected to element  38 . 
   The preferred embodiment of  FIG. 2  further includes an toggle switch S 2 A′ that may be used to select either of windings  20 ′ or  22 ′ and their corresponding rectifiers by connecting with either point RA or point RB as shown. Further, capacitors  58  and/or  59  may also be connected as shown to reduce the tendency for the release a momentary “pop” or “surge” upon selection as is known in the art. It will also be appreciated, that switch S 2 A′ may also have a third position in which it is not connected with either of RA or RB. If this third position (not shown) is provided, switch S 2 A′ serves as both a winding selector and a power supply stand-by switch. 
   When the switch S 1  is closed (thereby delivering power to the transformer  12 ) and the user selects the “punch” mode, switch S 2 A′ is connected with terminal RB and, thus, high voltage rectified and filtered originating from the winding  22 ′ is provided at output point A. High voltage winding  22 ′, is specifically designed with a volt-amp rating that exceeds the expected demand of the connected audio amplifier so that the voltage provided therefrom is relatively insensitive to such changing demands. A filter capacitor bank  50 ′ reduces the ripple voltage from the rectified DC in a conventional manner and, thus, presents at point A a DC plate voltage for use with vacuum thermionic elements of an audio amplifier. Similarly, capacitor bank  50 ′ is specifically designed with a capacitance that exceeds the expected needs of the connected audio amplifier so that the voltage provided therefrom is relatively insensitive to the changing demands of the amp. As shown, capacitor bank  50 ′ may comprise a voltage divider formed of a pair of resisters  53  and  57  to which capacitors  52  and  56  are connected. The voltage presented at point A may be further filtered in a conventional way by the use of an inductor L and capacitor  60  to present another bias voltage at point B. Further dropping resistors  70 ,  72  and  74  may apply voltage to power supply points C, D and E, to which other stages of an amplifier circuit (not shown) may be connected. Further decoupling and filtering is provided by the capacitors  63 ,  64  and  66  in a conventional manner as shown. 
   With continuing attention on  FIG. 2 , it is noted that a user may, alternatively, select the “sag” mode. In this alternative mode, high voltage AC from the winding  20 ′ may be continually applied to solid state rectifiers  34 / 35 / 36 / 37 . Additionally, capacitor  48  of relatively small capacitance may be used to reduce some of the ripple voltage from rectifiers  34 / 35 / 36 / 37 . Rectified DC is, thus, applied to vacuum thermionic element  38 . High voltage winding  20 ′ is specifically designed with a lower volt-amp rating than that of winding  22 ′. In particular, the volt-amp rating of winding  20 ′ is intentionally selected to cause a substantial voltage drop when the current demand of the connected audio amplifier increases substantially. In this way, the voltage provided therefrom is relatively sensitive to the changing demands of the connected amplifier and an audible change in the character of the amplifier results. 
   A filament of the element  38  is heated by the secondary winding  18 . DC rectified voltage from elements  34 / 35 / 36 / 37  is connected to element  38 . B+from the cathode of the thermionic element  38  is connected to the point A. Thermionic vacuum element  38  may be one of many known types such as a GZ34/5AR4, 5Y3, 5U4, etc. (all commonly used as diodes) and primarily serves to provide additional voltage sensitivity to the current flowing therethrough. Those of ordinary skill will appreciate that only one of rectifiers  34 / 36  or element  38  needs to be provided for the power supply to function in the sag mode. In both cases rectifiers  35 / 37  will be needed to complete the bridge rectifier. Nonetheless, it has been discovered that the internal resistance of element  38  provides desirable sonic characteristics and that the additional use of rectifiers provides an additional measure of safety to the connected audio amplifier against damage if element  38  fails. 
   A filter capacitor bank  40 ′ reduces the ripple voltage from the rectified DC in a conventional manner and, thus, presents at point RA a DC plate voltage adapted to vacuum thermionic elements of an audio amplifier. By contrast with bank  50 ′, capacitor bank  40 ′ is specifically designed with a lower capacitance so that the voltage filtering provided thereby is relatively sensitive to such changing demands. Capacitor bank  40 ′ may comprise a voltage divider formed of a pair of resisters  43  and  47  to which capacitors  42  and  46  are connected. As shown, the switch S 2 A′ may be connected such that the capacitor bank  50 ′ is electrically decoupled from the connected audio amplifier when the second secondary winding  20 ′ is selected. This prevents bank  50 ′ from interfering with the use of element  38 . In this way, the invention ensures that a relatively large bank  50 ′ can be used with winding  22 ′, while ensuring that element  38  is not damaged by the presence of too much capacitance and that the desired sensitivity provided by the use of winding  20 ′ and element  38  is not compromised by the presence of too much capacitance. Similarly, bank  40  is decoupled from the connected audio amplifier when winding  22 ′ is selected. The total capacitance of bank  40 ′ will depend on various known factors such as the number and type of power tubes used in the connected audio amplifier. 
   With continuing reference to  FIG. 2 , it will be seen that the voltage presented at point A may be further filtered in a conventional way by the use of an inductor L and capacitor  60  to present another DC voltage at point B. Further dropping resistors  70 ,  72  and  74  may apply voltage to power supply points C, D and E, to which other stages of an amplifier circuit (not shown) may be connected. Further decoupling and filtering is provided by capacitors  63 ,  64  and  66  in a conventional manner as shown. 
   In the case of an amplifier circuit using cathode bias throughout, the power supply circuit described above would be complete. Cathode bias in the power amplifier tubes provides the advantage of its natural tendency to compensate for B+ voltage changes which naturally occur from switching between the secondary coils  20  and  22  and their corresponding rectifying circuits. 
   However, a fixed bias supply  80  may be successfully used in such a power amplifier circuit even though the B+ operating voltage will differ significantly depending upon which secondary coil and/or rectifier circuit is selected. Such a bias supply preferably includes a current limiting resistor  82 , a solid-state rectifying diode  83  whose cathode end is fed from a tap of the secondary coil  16 , filtered by capacitor  84 , fed through dropping resister  86  to a bias supply output point F where rectified DC is filtered by a capacitor  85 . An adjustable voltage divider may also be provided via potentiometer  87  and resister  88  as shown in  FIG. 2 . Additionally, a single-pole single-throw switch S 2 C selectively permits engagement of the resister  89  as shown. It will be appreciated that the switch S 2 C is coordinated with the switch S 2 A (see  FIG. 2   a ) such that the resister  89  is only electrically engaged when the second secondary winding  22  is activated. In this way, supply  80  automatically rebiases the tubes of the connected audio power amplifier for proper operation regardless of which secondary coil is selected. 
   In the preferred embodiments described herein switch S 1  is preferably a single-pole/single-throw mechanical switch, but may be any one of the many well-known equivalents. Further, switch S 1  may also be a single-pole/double-throw switch to allow it to be used as a power supply standby switch. Further, the preferred embodiments described herein may use one quadruple-pole/single-throw switch to mechanically serve as switches S 2 A, S 2 B, S 2 C and S 2 D. Those of skill in the art will know how to use other mechanical switches to effectuate all or fewer than all of the above-noted features in one or more similar mechanical switches. Further, some or all of the switching functions noted above may be executed with some combination of mechanical, solid state, optical, electrical, MIDI or other switches such as opto-couplers, LDR&#39;s, and relays. Moreover, many of the well known stand-by switches (such as double-pole/double-throw switches between the transformer secondary windings and the rectifier elements) may be used in combination with the invention. Finally, it will be appreciated that both secondary windings, their respective rectifiers and their respective capacitor banks may be operated simultaneously with the use of a logical AND switch and various component value changes. 
   As discussed throughout, the solid-state diodes may be made of silicon, germanium or any of the other known materials as desired. Further, those of ordinary skill will know how to apply the principles of the invention described herein with various rectification circuits such as either half-wave rectifiers and full-wave rectifiers. 
   While the present invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to encompass the various modifications and equivalent arrangements included within the spirit and scope of the appended claims. With respect to the above description, for example, it is to be realized that the optimum dimensional relationships for the parts of the invention, including variations in size, materials, shape, form, function and manner of operation, assembly and use, are deemed readily apparent to one skilled in the art, and all equivalent relationships to those illustrated in the drawings and described in the specification are intended to be encompassed by the appended claims. Therefore, the foregoing is considered to be an illustrative, not exhaustive, description of the principles of the present invention.