A number of low voltage vacuum tube circuits include using supply voltages well below the manufacturer's recommended voltages applied to the plate or screen grid. Some of the tube circuits operate at near zero plate and or screen grid voltages. Other low voltage circuits have forward biasing on one or more grids that are normally biased at a non positive voltage or a grid that is normally connected a cathode. Substantially lower supply voltages allow for example, the filament supply to also supply voltage to the plate and or grid for providing an output signal at a grid and or a plate.

An audio device for amplifying an audio signal enables a user to instantly switch between two or more active amplifying components, thereby allowing the user to evaluate the effect of a particular active amplifying component in the audio device. In one example, the audio device is an audio vacuum tube preamplifier, and has a circuit configuration that facilitates A/B comparison between two or more preamp tubes by hot switching between preamp tubes in the audio signal path. The input to the two or more tubes is switched between each tube by the listener on demand without pause or interruption to the music or program.

An analog amplification vacuum tube of the present invention suppresses influences of filament vibration on amplification characteristics. The analog amplification vacuum tube of the present invention is provided with a filament, an anode, a grid and a vibration-proof part. The filament is tensioned linearly and emits thermal electrons. The anode is disposed parallel to the filament. The grid is disposed between the filament and the anode so as to face the anode. The vibration-proof part includes a thin film usable in a vacuum environment and the thin film comes into contact with part of the filament.

An analog amplification vacuum tube of the present invention suppresses influences of filament vibration on amplification characteristics. The analog amplification vacuum tube of the present invention is provided with a filament, an anode, a grid and a vibration-proof part. The filament is tensioned linearly and emits thermal electrons. The anode is disposed parallel to the filament. The grid is disposed between the filament and the anode so as to face the anode. The vibration-proof part includes a thin film usable in a vacuum environment and the thin film comes into contact with part of the filament.

An analog amplification vacuum tube of the present invention suppresses influences of filament vibration on amplification characteristics. The analog amplification vacuum tube of the present invention is provided with a filament, an anode, a grid and a vibration-proof part. The filament is tensioned linearly and emits thermalelectrons. The anode is disposed parallel to the filament. The grid is disposed between the filament and the anode so as to face the anode. The vibration-proof part includes a thin film usable in a vacuum environment and the thin film comes into contact with part of the filament.

An analog amplification vacuum tube of the present invention suppresses influences of filament vibration on amplification characteristics. The analog amplification vacuum tube of the present invention is provided with a filament, an anode, a grid and a vibration-proof part. The filament is tensioned linearly and emits thermalelectrons. The anode is disposed parallel to the filament. The grid is disposed between the filament and the anode so as to face the anode. The vibration-proof part includes a thin film usable in a vacuum environment and the thin film comes into contact with part of the filament.

An amplifier circuit including a first stage, second stage and output stage. The first stage includes a cathode biased triode. The amplifier circuit includes a second stage having two long tailed pair triodes, each of the second stage tubes output signals inverted from each other wherein the long-tailed pair triodes are direct current coupled to the output tube's grids, Each cathode biased triode section is direct current coupled to one of the two long tailed pair triodes. The output stage includes two output tubes are in a Circlotron configuration with each output tube's cathode connected to the loudspeaker terminals. Each Bluetooth receiver's ground is connected to the ground of the first stage and the output of each Bluetooth is connected to the grid of the first stage.

An amplifier device includes a back-EMF voltage shorting circuit between a power supply terminal located on a primary side of an output transformer and a constant potential terminal grounded. The back-EMF voltage shorting circuit is provided with a capacitor and a diode in parallel. When a back-EMF voltage input into a circuit of an amplifier device is a positive potential, a back-EMF current flows from the power supply terminal toward the constant potential terminal, and the back-EMF current is short-circuited by the capacitor at this time. On the other hand, when the input back-EMF voltage is a negative potential, the back-EMF current flows from the constant potential terminal toward the power supply terminal, and is short-circuited by the diode at this time.

An amplifier device includes a back-EMF voltage shorting circuit between a power supply terminal located on a primary side of an output transformer and a constant potential terminal grounded. The back-EMF voltage shorting circuit is provided with a capacitor and a diode in parallel. When a back-EMF voltage input into a circuit of an amplifier device is a positive potential, a back-EMF current flows from the power supply terminal toward the constant potential terminal, and the back-EMF current is short-circuited by the capacitor at this time. On the other hand, when the input back-EMF voltage is a negative potential, the back-EMF current flows from the constant potential terminal toward the power supply terminal, and is short-circuited by the diode at this time.

An audio device for amplifying an audio signal enables a user to instantly switch between two or more active amplifying components, thereby allowing the user to evaluate the effect of a particular active amplifying component in the audio device. In one example, the audio device is an audio vacuum tube preamplifier, and has a circuit configuration that facilitates A/B comparison between two or more preamp tubes by hot switching between preamp tubes in the audio signal path. The input to the two or more tubes is switched between each tube by the listener on demand without pause or interruption to the music or program.