Бір триодты вакуумдық күшейткіш

Vacuum tube electronic amplifier that uses a single triode to produce an output

A single ended triode (SET) is a vacuum tube electronic amplifier that uses a single triode to produce an output, in contrast to a push pull amplifier which uses a pair of devices with antiphase inputs to generate an output with the wanted signals added and the distortion components subtracted. Single ended amplifiers normally operate in Class A; push pull amplifiers can also operate in Classes AB or B without excessive net distortion, due to cancellation. The term single ended triode amplifier is mainly used for output stages of audio power amplifiers. The phrase directly heated triode single ended triode amplifier (abbreviated to DHT SET) is used when directly heated triodes are used. There are also single ended tetrode, beam tetrode/beam power tube/kinkless tetrode, and pentode amplifiers with the same functionality and similar circuitry; e. g. this Mullard design.

A typical triode audio power amplifier will have a driver that provides voltage gain, coupled to a triode (like 2A3 and 300B) or a pentode or kinkless tetrode such as EL34 or KT88 connected as a triode, connected to the loudspeaker through an audio transformer in a common cathode arrangement. The triode is biased to Class A operation by applying a suitable negative bias voltage to its input control grid (see diagram), or by raising the cathode potential with biasing components. In traditional SET amp, the direct current of output triode (from 30 mA for triode strapped 6V6 to 250 mA for 6C33C) flows continuously through the primary winding of a transformer. This requires inserting a gap in the transformer core to prevent core saturation by DC current; adding a gap decreases primary inductance and limits bass response; the inductance and bass response can be restored by using a larger transformer than if the DC were not present. An alternative schematic, parafeed amplifier, solves bandwidth problem by blocking direct current from output transformer (which does not need to be gapped, thus improving its bass response). Power supply is reconfigured into a constant current source, usually with a massive, high inductance anode choke (gapped inductor), so there is little, if any, gain in cost and weight of magnetic components. A stereo class A single ended design with KT88 kinkless tetrodes which produces 15W of output power per channel, and 5W when triode connected, is the Antique Sound Lab MG SI15DT. By comparison a pair of the same tubes in class AB push pull claim to output about 50W at 1% distortion (higher powers at high distortion are quoted for guitar amplifiers).

Simplicity. An audio amplifier cannot be much simpler than a Class A single ended. Overall low harmonic distortion with small signals. By nature Class A amplifiers do not suffer from crossover distortion. Small roaming of anode bias current does not do anything catastrophic.

Low efficiency. All Class A amplifiers consume full power regardless of the signal amplitude. Second harmonic distortion at high signal level is a normal feature of single ended amplifiers. (Not all consider this a drawback. Some audio enthusiasts find even order harmonics to be pleasing to the ear and more natural sounding.) Compared to the amplifier's low power the output transformer is heavy and expensive.

Historically, negative feedback in single ended pentode amplifiers was quite common (for example, the Mullard 3 3 design built around EL84 SET designs require output transformers which are able to cope with a strong DC component in the signal, which causes them to have worse performance in regard to frequency response, distortion and efficiency (although the latter is not generally a priority for most SET enthusiasts, or audiophiles in general). Furthermore, as SET amplifiers have a relatively high output impedance, it is hard to couple them effectively to a loudspeaker which hasn't been designed especially to be driven by a high output impedance amplifier, as this will cause the amplifier to be much more sensitive to the loudspeaker's impedance characteristics across the spectrum, resulting in coloration. In general, the configuration will usually provide higher measured distortion performance compared to high feedback amplifiers.

Class A2 amplifiers can overcome a high Rp by driving the grid positive with respect to the cathode. Because this makes the grid a secondary anode, it too will draw current from the cathode while accelerating the remaining electrons towards the plate. Grid currents can place extreme demands on the driving circuitry, sometimes requiring as much as 8 watts input for larger tubes such as the 211.