Single-ended triode

A SET tube audio amplifier.

A single-ended triode (SET) vacuum tube electronic amplifier 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; the cancellation of distortion allows push-pull amplifiers also to operate in Classes AB or B without excessive net distortion. See power amplifier classes for a detailed discussion.

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.

An older, but quite different, use of the term single-ended was to distinguish tubes in which all pins were connected via the base from those that had a top cap for high-voltage (plate) or interference-susceptible (control grid) electrodes.

Audio power amplifiers

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 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).

Benefits

• 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.

Drawbacks

• Low efficiency. All Class A amplifiers consume power regardless of the signal amplitude.
• Second harmonic distortion at high signal level is a normal feature of single-ended amplifiers.
• Compared to the amplifier's low power the output transformer is heavy and expensive.

Speaker matching

Historically, negative feedback in single ended pentode amplifiers was quite common (for example, the Mullard 3-3 design built around EL84). Today negative feedback is less popular with SET amplifiers, with many having no overall feedback loop. Their frequency response, limited by transformer passband, is then modulated by irregularities in loudspeaker impedance. This, and the very low attainable power levels (3 Watts for 2A3 to 20 Watts for 6C33C), requires careful matching of amplifier to speakers; selection is usually limited to high-efficiency loudspeakers with a sensitivity exceeding 90 dB/Watt.

Audio quality

Single-ended triode (SET) amplifiers are considered a classic design among certain audiophiles and have achieved a cult status because of their excellent midband performance (argued to be the most important part of the audio spectrum in music reproduction)^{[citation needed]}, musicality, and directness. This perceived high sonic quality is mainly attributed to the simplicity and minimalistic approach of the circuits involved and the triode amplifying tubes that are typically used.^{[citation needed]}

The configuration will generally provide higher measured distortion performance compared to high feedback amplifiers.^{[citation needed]} This distortion is predominantly second harmonic which is not unpleasant to the ear (the second harmonic for an A on the musical scale at 440Hz is 880Hz which is, obviously, also an A i.e., the same note just an octave higher).^{[citation needed]} In a push pull amplifier this second harmonic distortion is canceled in the output transformer. Several percent THD is not unusual at full power output, but will be much lower at normal music levels. Most builders and users have concluded that while global feedback reduces distortion across the harmonic range, it also reduces the dramatic dynamics associated with a SET amplifier and the highly efficient speaker needed to enjoy a low power amplifier.

Estimating power output for a Class A1 amplifier from valve specs

In class A, in order to produce a full sine wave, the tube must be exactly half-way on. Therefore the maximum current swing is +/- 50%. No actual amplifier will ever achieve this.

Obtain the following parameters:

• Pa (or P[a+g2] for pentodes) (in watts).
• Rp in triode (in ohms),
• load resistance (Rl) (in ohms) (which may vary depending on the design.)

Compute maximum theoretical power output by P[ot] = Pa / 2.

While the valve can be driven all the way to shut-off fairly easily, the maximum current will be limited by the internal resistance of the valve as the grid reaches the voltage of the cathode and is not impeding electron flow. For this reason, valves that have a low Rp can yield more power in class A1 than other valves with similar Pa ratings.

Next, compute P[o] = P[ot] * (1 - Rp/Rl) to obtain an estimate of the maximum obtainable output power.

Deduct 10% from this figure to account for the relatively heavy distortion encountered as the valve approaches cut-off.

Class A2

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.

See also

• Valve sound
• High-end audio
• European triode festival
• List of vacuum tubes

External links

• www.pmillett.com A public domain depository for various audio designs and projects, including tubes and SET.
• www.enjoythemusic.com Article titled "Why The SET And Efficient Loudspeaker Approach Works".
• The Single-Ended Amplifier: Cary's Dennis Had in Stereophile, September, 1995