Abstract:
A vacuum tube power amplifier with a user-operable switch and a single output transformer which selectively provides push-pull or single ended configurations for driving a loudspeaker.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to the use of vacuum tubes in audio power amplifiers, particularly for use in conjunction with an electric guitar.  
       BACKGROUND AND BRIEF DESCRIPTION OF THE PRIOR ART  
       [0002]     Vacuum tube audio amplifiers generally utilize push-pull output configurations as the most economical method of producing ample power. The benefits of push-pull are well known in the art and require no repetition herein. It is sufficient to say that commercial amplifiers offering power levels even as low as 15 watts will use a push-pull pair of output tubes, though one larger tube in a single-ended configuration could also provide similar power. It can be safely asserted that virtually all commercial audio amplifiers providing more than 20 watts are of the push-pull variety.  
         [0003]     In contrast, single-ended vacuum tube output sections have previously been used mostly in 5 to 8 watt so-called “beginner” guitar amplifiers, which offer a stark economy of overall design. Nevertheless, such low powered units occasionally find favor in recording studios or small apartment-sized venues. In such circumstances, their lower power can become a virtue, providing desirable “power amplifier clip” at low volume levels. Many guitarists claim distortion generated in the power section is more legitimate (due to its historical roots), and is more musically pleasing compared to preamplifier overdrive saturation.  
         [0004]     Without discussion of the relative merits of the two, power amplifier distortion remains dependent on the power capability provided and therefore suffers from a lack of versatility regarding loudness. The desired distortion characteristics and the output loudness of an amplifier are so intimately related as to be inseparable and overcoming that limitation is what initially spurred the creation of cascading preamplifier gain structures with separate gain and level attenuation, such as shown in the Smith U.S. Pat. No. 4,211,893, the contents of which are incorporated herein by reference.  
         [0005]     The guitar amplifier design as demonstrated by Smith U.S. Pat. No. 4,211,893 ushered in the “modern era” of guitar amplification and provided the live musician with the ability to set a high-gain sound rich in distortion characteristics independent of its volume level and the ability to switch alternately and selectively to an undistorted clean sound. In order to achieve this goal, distortion is created within the preamplifier section via overdrive saturation, and though it may mimic power amplifier clip, the two are not the same.  
         [0006]     As playing styles and circumstances have evolved over the years, a greater emphasis is now placed on recording, particularly home recording, and on live performance venues of small to medium size. In the prior art, an amplifier capable of producing genuine power amplifier distortion at a volume suitable for home recording or practicing would not be usable in a live performance and vice versa.  
       SUMMARY OF THE INVENTION  
       [0007]     In accordance with the present invention, the limitations of the prior art are overcome by offering musically desirable power section distortion at a plurality of power and loudness settings. The same amplifier can be reconfigured at the flip of a switch to provide ample power for “gigging” as live performances in its push-pull mode, or have substantially reduced power to provide rich, single-ended power clip for recording or practicing at modest volume levels.  
         [0008]     Moreover, in accordance with the present invention, there is provided in a single amplifier both of the differing harmonic tonal characteristics of push-pull and single-ended which may be selected by the musician. Unlike single-ended audio amplifier configurations, push-pull circuits inherently cancel second order distortion harmonics. A sound rich in second harmonic, such as produced by a good single-ended unit, can be characterized as warm and velvety, and as the loudness and distortion are increased, the single-ended distortion stays “glued’ to the note, giving it notable “fatness” and touch sensitive dynamic nuance. These are desirable characteristics not only for immediate player satisfaction, but, in addition, their subtleties are able to record well.  
         [0009]     In contrast, push-pull power sounds much louder due to its relative absence of second harmonic. With its emphasis on third and higher order harmonics predominating, push-pull is sonically better able to cut through the mix of a live band. Furthermore, a pair of tubes in a push-pull configuration will easily provide a three fold increase in undistorted output wattage compared to a single tube of the same type utilizing the same power supply.  
         [0010]     Unfortunately, there are serious technical obstacles that, until the present invention, have prevented successful switching between push-pull and single-ended construction. In a single-ended output, the iron core of the output transformer must include a “gap” to prevent flux saturation by the DC current. Such a gap is an integral structural element of the iron laminations and is achieved by either sawing a small gap in the core or by separating the E and I laminations with an insulator. In a push-pull transformer, such a gap is neither required nor desired. Because DC enters the transformer primary at the center tap and flows equally through both halves, DC core flux is symmetrical, balanced and self-neutralizing. Unfortunately, accommodating single-ended operation by including a gap in the core of a push-pull transformer greatly reduces its magnetic flux capability and causes low frequency roll-off and a general loss of efficiency.  
         [0011]     After studying this difference at length, the present invention overcomes this structural transformer limitation while simultaneously solving the problems of providing appropriate drive, signal and balance conditions for both single-ended and push-pull operation. [How?] 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a schematic diagram of a simplified first embodiment of a circuit illustrating the principles of switching between push-pull and single-ended operation within the same amplifier in accordance with the present invention.  
         [0013]      FIG. 2  is a schematic diagram of an expanded second embodiment of a circuit in accordance with the present invention which, by adding an additional pair of output tubes shows an amplifier with three distinctly different power levels in addition to providing the ability to switch between single-ended and push-pull operation.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]     Referring to  FIG. 1 , opposing phase push-pull AC signals are provided for the signal grids  1 ,  2  of vacuum tube power pentodes V 1  and V 2  respectively. The AC signals are coupled through blocking capacitors  3 ,  4  and grid stop resistors  5 ,  6 . Grid leak resistors  7 ,  8  provide DC reference to ground  30  for the grids  1 ,  2 . Anodes  9 ,  10  of pentodes V 1  and V 2  are fed DC high voltage through respective halves of the output of transformer primary winding  11 ,  12 . DC voltage from the power supply (not shown) enters the output transformer primary winding  11 ,  12  through center tap  31 . Screen grids  13 ,  14  are coupled to the high voltage DC source at  32  via current limiting resistors  15  and  16 . Cathodes  17 ,  18  are coupled to ground  30  through respective cathode bias resistors  19 ,  20 . Switches A, B, C and D, E, F operate in conjunction with each other and may be a double pole double throw (DPDT) relay device. When configured as shown by the arrows, connection D to E effectively grounds the grid  2  of tube V 2 , thereby shunting any drive signal and preventing the drive signal form appearing on the grid  2 . It should be understood that, in an amplifier with a fixed DC bias arrangement, a capacitor may be used for AC signal shunting, thereby preserving the DC bias to the grid  2 . Simultaneous with shunting drive signal away from grid  2 , switch A, B, C also in the position indicated by the arrow, decouples the cathodes  17  and  18  from one another. This prevents the power tube V 2  from being cathode driven by tube V 1  in what would be a grounded-grid amplifier and prevents tube V 2  from continuing to operate as half of a push-pull arrangement. Therefore, when the pair of switches A, B, C plus D, E, F is configured as shown by the arrows, the circuit of  FIG. 1  will operate in single ended configuration. The two cathodes  17 ,  18  of the power tubes V 1  and V 2  are then independently biased through resistors  19  and  20  such that amplification occurs in tube V 1  only while tube V 2  operates as a DC current sink. No AC component or amplification is allowed, with DC flux currents in the two halves of the output transformer primary winding  11 ,  12  remaining substantially equal and balanced. [Is this what permits use of a transformer without a gap?] 
         [0015]     When the switches A, B, C, plus D, E, F are configured as shown by the dashed lines, balanced push-pull operation results. By removing the shunt D-E, drive signal is allowed to couple to the grid  2  of tube V 2 . By connecting A to C, the cathodes  17 ,  18  are coupled together and both are biased simultaneously through the parallel network of resistors  19 ,  20 . By coupling together the cathodes  17 ,  18 , improved balance is achieved between the separate power tubes V 1  and V 2  while operating push-pull.  
         [0016]     In the second embodiment of  FIG. 2 , there are shown all the same elements operating in identical fashion as describe in  FIG. 1 , however an additional pair of power tubes V 3  and V 4  is also provided. An additional switch G, H, I is utilized such that this second pair of power tubes V 3 , V 4  may be switched on in parallel with the first pair, V 1 , V 2 , to offer a third level of output power. As is conventional in a push-pull parallel amplifier, drive signal is coupled to grids  41 ,  42  through grid stop resistors  43  and  44  to prevent oscillation. Screen grids  45 ,  46  are fed from high voltage DC supply  32  through resistors  47 ,  48  to stabilize the circuit and limit screen dissipation under conditions of heavy clip. Cathodes  49 ,  50  are connected together and biased to ground  30  through resistor  51  when the switch G, H, I is configured as shown by the arrow.  
         [0017]     An added enhancement to the preferred embodiment amplifier of  FIG. 2  would be its operation in conjunction with a power supply such as described in Ser. No. 10/376,338, the contents of which are incorporated herein by reference. In that disclosure, the power supply automatically switches to provide optimum power delivery to an amplifier with selectable output devices as various devices are added.  
         [0018]     In the case of the amplifier of  FIG. 2  employing two of the popular 6BQ5/EL-84 power pentodes, for example, and working together with a version of such an improved power supply, the following would occur: In the lowest power position, the amplifier would be configured for single-ended operation and fed a somewhat reduced DC high voltage by the power supply, utilizing its vacuum tube rectifier and/or a voltage dropping resistor. This reduced plate voltage enables the single tube to be biased for heavier current draw such that operation is centered around the mid-point of its linear region and pure Class A operation ensues. Power produced at the onset of clip is a very rich sounding 4 watts, perfect for recording or soft practicing. Switching the amplifier to operate push-pull on a single pair of output tubes, while still utilizing the slightly lower DC voltage as provided by the tube rectifier and/or resistor, will produce about 12 watts of punchy, pure Class A push-pull power. Finally, at its highest output configuration, the second pair of push-pull power pentodes comes on line in parallel with the first pair while the B+ operating voltage is simultaneously increased by the above described power supply automatically switching to silicon diode rectifiers.  
         [0019]     Referring again to  FIG. 2 , it will be seen that when the switch G, H, I turns on the second pair of push-pull tubes V 3 , V 4  (as shown by the arrow), it simultaneously provides for resistor  60  to come into series with the parallel network of bias resistors  19 ,  20  such that the first pair of output tubes V 1 , V 2  is re-biased to accommodate the higher B+ voltage now supplied. Total output before clip then can exceed 30 watts to provide an amplifier capable of playing mid-size venues.  
         [0020]     Though the invention has been described with reference to the specific preferred embodiments thereof, many variations and modifications will immediately become apparent to those skilled in the art. For example, a load resistor could be substituted in place of the tube V 2  to provide DC flux balance through the transformer primary during single-ended operation. However, the tube as shown would still be required for push-pull power. A further possible refinement could be the inclusion of a tapped secondary winding in the output transformer combined with a switch to provide more correct impedance matching under varying conditions. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modification.