Valve amplifiers

Abstract

A valve amplifier (105, figure 1) having an output transformer (110, figure 1) and a capacitor 210 connected in parallel with secondary windings 205 of the transformer. A method of connecting a capaci

Description

Field The present disclosure relates to valve amplifiers. The valve amplifiers may be for musical instruments, such as electric guitars. Background Most valve amplifiers comprise an output transformer. The output transformer converts a high-voltage, low-current signal to a low-voltage, high-current signal to drive one or more loudspeakers. Different output transformer designs have different sonic characteristics. Summary Accordingto first embodiments, there is provided a valve amplifier comprising: an output transformer; and a capacitor connected in parallel with secondary windings of the output transformer. According to second embodiments, there is provided a method comprising connecting a capacitor in parallel with secondary windings of an output transformer of a valve amplifier. Brief Description of the Drawings Various embodiments will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a block diagram of an example of a system; and Figure 2 shows an electrical schematic of part of the example system shown in Figure 1. Detailed Description Referring to Figure 1, there is shown an example of a system 100. The system 100 comprises a valve amplifier 105. The term “valve amplifier” is used herein to mean an amplifier with at least one thermionic valve (also known as a “valve” or “tube”). In examples described herein, the valve amplifier 105 is an audio amplifier in that the valve amplifier 105 amplifies an audio signal. The valve amplifier 105 may be for a musical instrument. An example of such a musical instrument is an electric guitar. In this example, the valve amplifier 105 comprises an output transformer 110. As explained above, the output transformer 110 converts a high-voltage, low-current signal to a low-voltage, high-current signal. Some valve amplifiers do not comprise output transformers. The valve amplifier 105 comprises other components not shown in Figure 1. Examples of such other components include, but are not limited to, a power supply, a preamplifier, and a power amplifier. In this example, the valve amplifier 105 is an amplifier “head”. For example, the amplifier head may be a guitar amplifier head. However, the valve amplifier 105 can have a different form in other examples. In this example, the valve amplifier 105 is connected to a loudspeaker cabinet 115. The loudspeaker cabinet 115 may be referred to as a “speaker cabinet”, a “cabinet”, or a “cab”. The loudspeaker cabinet 115 comprises one or more loudspeakers. In this example, the valve amplifier 105 is connected to the loudspeaker cabinet 115 via a cable 120. In accordance with examples that will now be described, at least one capacitor is added in parallel with secondary windings of the output transformer 110 of the valve amplifier 105. In such examples, one or more capacitors are permanently connected in parallel with the output transformer 110 and one or more additional capacitors are selectively connected in parallel with the output transformer 110. For example, a single capacitor may be permanently connected in parallel with the output transformer 110 and one or more additional capacitors may be selectively connected in parallel with the output transformer 110. This arrangement enables different sonic characteristics to be achieved compared to there being no capacitor(s) connected in parallel with the output transformer 110. The capacitor(s) may cut high frequencies. Such high frequencies may be in the 10-20 kilohertz (kHz) band. This provides high frequency roll-off. Sonically, this helps make the sound from the loudspeaker cabinet 115 be more pleasing and less fatiguing, thereby also reducing hearing fatigue. Further, this arrangement protects output valves of the valve amplifier 105 from damage. When a power amplifier is clipping, the impedance of the loudspeaker cabinet 115 produces voltage spikes which are reflected on the primary windings of the output transformer 110, for example ten-fold. This can damage the output valves as result of current voltage arcing between valve pins. Such back electromotive force (EMF) is one of the main causes of damage to such output valves. The capacitor(s) can therefore help reduce, or even prevent, output valve damage. Existing output transformers could be redesigned to filter out high-end content without using the capacitor-based arrangement described above. However, examples described herein enable existing output transformers to be used, by using the capacitor(s) in conjunction with the power amplifier and loudspeaker impedance, to create a low-pass filter (LPF). Different capacitor values may be used for different desired output responses. For example, during manufacture, a capacitor having a particular capacitance can be selected from a set of capacitors. Such selection may be based on the loudspeaker cabinet 115 with which the valve amplifier 105 is expected to be used. In accordance with some examples described herein, multiple capacitors may be included in the valve amplifier 105 at manufacture and the user may be able to select between one or more of the capacitors during use. In particular, the desired output response can change with different loudspeaker cabinets 115. Thus, the ability to switch capacitor values after manufacture allows different output responses for different loudspeaker cabinets 115. In addition, the user may select the capacitor(s) to be used based on personal taste, a type of grill cloth used on the loudspeaker cabinet 115, whether effects pedals are used, and / or any other criterion. Grill cloth may already filter out some high frequencies, and effects pedals often already cut off frequencies above 20 kHz. Referring to Figure 2, there is shown an example of a circuit 200. The circuit 200 may be comprised in the system shown in Figure 1. In particular, the circuit 200 comprises secondary windings 205 of the output transformer 110 of the valve amplifier 105 and comprises a capacitor 210 connected in parallel with the secondary windings 205. The capacitor 210 is referred to herein as the “first capacitor” 210. The first capacitor 210 may be “connected” in parallel with the secondary windings 205 in the sense that the first capacitor 210 is permanently connected in parallel with the secondary windings 205, or at least is intended to remain permanently connected in parallel with the secondary windings 205. The first capacitor 210 may be permanently connected by being soldered. At least one additional capacitor is connectable in parallel with the secondary windings 205 of the output transformer 110. The least one additional capacitor may be “connectable” in parallel with the secondary windings 205 in the sense that the least one additional capacitor is non-permanently connected in parallel with the secondary windings 205, or at least is intended to be selectively connected in parallel with the secondary windings 205 or disconnected from the secondary windings 205. In this example, at least two additional capacitors are connectable in parallel with the secondary windings 205, namely a first additional capacitor 215 and a second additional capacitor 220. Thus, and as can be seen from Figure 2, in this example, the first additional capacitor 215 is connectable in parallel with the first capacitor 210 and the second additional capacitor 220 is also connectable in parallel with the first capacitor 210. In this example, the capacitance of the first additional capacitor 215 and the capacitance of the second additional capacitor 220 are different from the capacitance of the first capacitor 210. In this example, the capacitance of the first additional capacitor 215 is different from the capacitance of the second additional capacitor 220. Thus, in this example, the capacitances of the first capacitor 210, the first additional capacitor 215, and the second additional capacitor 220 are all different from each other. Using different capacitances may enable different sonic characteristics to be achieved. In some examples, the capacitance of the first additional capacitor 215 is at least 1.5 times the capacitance of the first capacitor 210, and the capacitance of the second additional capacitor 220 is at least 1.5 times the capacitance of the first additional capacitor 215. The capacitance of the first additional capacitor 215 may be from ten to twenty times (inclusive) the capacitance of the first capacitor 210. The capacitance of the second additional capacitor 220 may be from two to four times (inclusive) the capacitance of the first additional capacitor 215. In a specific example, the first capacitor 210 has a 100 nanofarad (nF) capacitance, the first additional capacitor 215 has a 1 microfarad (piF) capacitance, and the second additional capacitor 220 has a 2piF or 3pF capacitance. The capacitance of the first capacitor 210 may, however, be selected such that the first capacitor 210 cuts off frequencies just above the highest audible frequency. The main effect of the first capacitor 210 may therefore be spike protection rather than high frequency roll-off. The capacitance selected for the first capacitor 210 may be dependent on the output transformer 100. In some examples, the first capacitor 210 has a capacitance of at least 47 nF, preferably at least 100 nF. A capacitor having a very low capacitance, for example below 47 nF or below 100 nF, may be used for electromagnetic compatibility (EMC) performance, while having no or minimal sonic impact and while not protecting the output transformer 110 from back EMF. Additionally, having relatively large relative capacitance differences enables more pronounced differences in sonic characteristics to be achieved. The capacitances may be selected from a set of standard capacitor values, such as E12 series values. At least one loudspeaker 225 is comprised in the loudspeaker cabinet 115. The at least one loudspeaker 225 is connected in parallel with the secondary windings 205. The at least one loudspeaker 225 is therefore connected in parallel with the first capacitor 210. Additionally, the at least one loudspeaker 225 is connectable in parallel with the either the first or second additional capacitor 215, 220 at any one time. In this example, the first capacitor 210 and the first and second additional capacitors 215,220 are bipolar capacitors. Bipolar capacitors may also be referred to as “unpolarised” or “non-polarised” capacitors. Bipolar capacitors are used in this example since current in the circuit 200 is alternating current (AC) and not direct current (DC). The circuit 200 comprises a switching arrangement 230. The switching arrangement 230 may be in the form of a toggle switch or otherwise. The switching arrangement 230 may be on a rear face of the valve amplifier 105. The switching arrangement 230 may be used less regularly than controls on a primary control panel of the valve amplifier 105. Mounting the switching arrangement 230 on the rear face of the valve amplifier 105 preserves space on the primary control panel for more regularly used controls. When the switching arrangement 230 is in a middle switch position, as is shown in Figure 2, the first capacitor 210 is connected in parallel with the secondary windings 205, and neither the first nor second additional capacitor 215, 220 is connected in parallel with the secondary windings 205. When the switching arrangement 230 is in a bottom switch position, the first capacitor 210 and the first additional capacitor 215 are connected in parallel with the secondary windings 205, and the second additional capacitor 220 is not connected in parallel with the secondary windings 205. In this switch position, the first additional capacitor 215 is connected in parallel with the first capacitor 210 and their capacitances are, in effect, added together such that a capacitance equal to the sum of the capacitance of the first capacitor 210 and the capacitance of the first additional capacitor 215 is connected in parallel with the secondary windings 205. When the switching arrangement 230 is in a top switch position, the first capacitor 210 and the second additional capacitor 220 are connected in parallel with the secondary windings 205, and the first additional capacitor 215 is not connected in parallel with the secondary windings 205. In this switch position, the second additional capacitor 220 is connected in parallel with the first capacitor 210 and their capacitances are, in effect, added together such that a capacitance equal to the sum of the capacitance of the first capacitor 210 and the capacitance of the second additional capacitor 220 is connected in parallel with the secondary windings 205. Thus, in a specific example in which the first capacitor 210 has a 100 nF capacitance, the first additional capacitor 215 has a 1 pF capacitance, and the second additional capacitor 220 has a 3.3pF capacitance, a capacitance of 100 nF, 1.1 pF or 3.4 pF is connected in parallel with the secondary windings 205 depending on the switch position being used. In this example, the first capacitor 210 is always connected in parallel with the secondary windings 205 irrespective of the switch position of the switching arrangement 230. In addition, in this example, the circuit 200 is configured such that the first and second additional capacitors 215, 220 cannot both be connected in parallel with the secondary windings 205 at the same time. Thus, the switching arrangement 230 is configured such that, when the switching arrangement 230 is in a first switch position (for example, the bottom position described above), the first additional capacitor 215 is connected in parallel with the secondary windings 205 and the second additional capacitor 220 is not connected in parallel with the secondary windings 205. The switching arrangement 230 is also configured such that, when the switching arrangement 230 is in a second switch position (for example, the top position described above), the second additional capacitor 220 is connected in parallel with the secondary windings 205 and the first additional capacitor 220 is not connected in parallel with the secondary windings 205. The switching arrangement 230 is also configured such that, when the switching arrangement 230 is in a third switch position (for example, the middle position described above), neither the first additional capacitor 215 nor the second additional capacitor 220 is connected in parallel with the secondary windings 205. In the third switch position, the first capacitor 210 is connected in parallel with the secondary windings 205. Where the first capacitor 210 has a nominal capacitance, such as 100 nF, the switching arrangement 230 is effectively off in the third switch position. With reference to Figure 1, the secondary windings 205, the first capacitor 210, the first and second additional capacitors 215, 220, and the switching arrangement 230 may be comprised in the valve amplifier 105, and the loudspeaker 205 may be comprised in the loudspeaker cabinet 115. A method is also provided in which a capacitor is connected in parallel with the secondary windings 205. The capacitor may bethefirst capacitor 210 and the connecting may comprise permanently or temporarily connecting the first capacitor 210 in parallel with the secondary windings 205. The capacitor may be the first or second additional capacitor 215, 220 and the connecting may comprise temporarily connecting the first or second additional capacitor 215, 220 in parallel with the secondary windings 205, for example usingthe switching arrangement 230. In addition, a valve amplifier 105 comprising an output transformer 110, at least one capacitor 210, 215, 220, and a switching arrangement 230 is also provided. The switching arrangement 230 may be operable to connect the at least one capacitor 210, 215, 220 in parallel with the secondary windings 205 and to disconnect one or more of the at least one capacitors 210, 215, 220 from being connected in parallel with the secondary windings 205. In particular, in accordance with examples described above, the first and second additional capacitors 215, 220 may be temporarily connected in parallel with the secondary windings 205. Additionally, although in examples described above, the first capacitor 210 is permanently connected in parallel with the secondary windings 205, in other examples the first capacitor 210 may be temporarily coupled to the secondary windings 205, for example in a similar manner to that for the first and second additional capacitors 215, 220. Further, a valve amplifier 105 comprising an output transformer 110 and at least one capacitor 210, 215, 220 connectable in parallel with the secondary windings 205 is provided. Thus, examples described herein add at least one capacitor in parallel with the output transformer 110 to enable desired sonic characteristics to be achieved using an existing outputtransformer 110 while providing protection against back EMF. Some such examples enable a user to select different sonic characteristics, for example based on a particular loudspeaker cabinet 115 they are using with the valve amplifier 105. In examples described above, the valve amplifier 105 is an amplifier head that is connected to aseparate loudspeaker cabinet 115. In other examples, the valve amplifier 105 is in a combination amplifier in which the valve amplifier 105 and the loudspeaker(s) are in the same enclosure as each other. A combination amplifier may also be known as a “combo amp” or “combo”. Even where the valve amplifier 105 is in a combination amplifier, the valve amplifier 105 may still be useable with a separate loudspeaker cabinet 115. Thus, the user may still select from different output responses where the valve amplifier 105 is in a combination amplifier. In examples described above, a single capacitor 210 is permanently connected to the secondary windings 205. However, zero capacitors or more than one capacitor may be permanently connected to the secondary windings 205 in other examples. In examples described above, two additional capacitors 215, 220 are connectable in parallel with the secondary windings 205. However, in other examples, a single additional capacitor or more than two additional capacitors may be connectable in parallel with the secondary windings 205. In examples described above, no more than one of the additional capacitors 215, 220 is connected in parallel with the secondary windings 205 at any point in time. In other examples, more than one additional capacitor may be connected in parallel with the secondary windings 205 at the same point in time. In such other examples, some or all of the additional capacitors may have the same capacitance as each other. For example, the first capacitor 210 may have a 100 nF capacitance and the first and second additional 5 capacitors 215,220 may both havea 1 pF capacitance. If onlythe first capacitor210were connected in parallel with the secondary windings 205, then the capacitance connected in parallel with the secondary windings 205 would be 100 nF. If the first capacitor210 and one of the first and second additional capacitors 215, 220 were connected in parallel with the secondary windings 205, then the capacitance connected in parallel with the 10 secondary windings 205 would be 1.1 pF. If the first capacitor 210 and both the first and second additional capacitors 215, 220 were connected in parallel with the secondary windings 205, then the capacitance connected in parallel with the secondary windings 205 would be 2.1 pF.

Claims

1. A valve amplifier comprising:an output transformer; anda capacitor connected in parallel with secondary windings of the output transformer.

2. A valve amplifier according to claim 1, wherein the valve amplifier comprises at least one additional capacitor connectable in parallel with the secondary windings.

3. A valve amplifier according to claim 2, wherein the at least one additional capacitor comprises a first additional capacitor connectable in parallel with the secondary windings and a second additional capacitor connectable in parallel with the secondary windings.

4. A valve amplifier according to claim 3, wherein the valve amplifier is configured such that the first and second additional capacitors cannot both be connected in parallel with the secondary windings at the same time.

5. A valve amplifier according to any of claim 3 or 4, wherein a capacitance of the first additional capacitor and a capacitance of the second additional capacitor are different from a capacitance of the capacitor.

6. A valve amplifier according to claim 5, wherein the capacitance of the first additional capacitor is different from the capacitance of the second additional capacitor.

7. A valve amplifier according to claim 5 or 6, wherein the capacitance of the first additional capacitor is at least 1.5 times the capacitance of the capacitor, and wherein the capacitance of the second additional capacitor is at least 1.5 times the capacitance of the first additional capacitor.

8. A valve amplifier according to any of claims 5 to 7, wherein the capacitance of the first additional capacitor is from ten to twenty times the capacitance of the capacitor.

9. A valve amplifier according to any of claims 5 to 8, wherein the capacitance of the second additional capacitor is from two to four times the capacitance of the first additional capacitor.

10. A valve amplifier according to any of claims 3 to 9, wherein the valve amplifier comprises a switching arrangement, and wherein:when the switching arrangement is in a first switch position, the first additional capacitor is connected in parallel with the secondary windings and the second additional capacitor is not connected in parallel with the secondary windings,when the switching arrangement is in a second switch position, the second additional capacitor is connected in parallel with the secondary windings and the first additional capacitor is not connected in parallel with the secondary windings, andwhen the switching arrangement is in a third switch position, neither the first additional capacitor nor the second additional capacitor is connected in parallel with the secondary windings.

11. A valve amplifier according to claim 10, wherein the switching arrangement is on a rear face of the valve amplifier.

12. A valve amplifier according to any of claims 1 to 11, wherein the capacitor has at least a 47 nanofarad capacitance.

13. A valve amplifier according to claim 12, wherein the capacitor has at least a 100 nanofarad capacitance.

14. A valve amplifier according to any of claims 1 to 13, wherein the valve amplifier is for a musical instrument.

15. A system comprising a valve amplifier according to any of claims 1 to 14 and aloudspeaker cabinet.

16. A method comprising connecting a capacitor in parallel with secondary windings 5 of an output transformer of a valve amplifier.

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