Single-ended triode
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In a single-ended triode (SET) vacuum tube electronic amplifier the entire audio signal waveform is amplified by the power triode, in contrast to push-pull amplifiers where two output tubes work together in phase opposition. Small-signal audio and radio frequency amplifiers are single-ended by default, with the exception of balanced amplifier stages. Thus, in practice, the term single-ended triode amplifier applies only to 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.
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[edit] Audio power amplifiers
A typical circuit will have a driver that provides voltage gain, coupled to a triode (like 2A3 and 300B), or even pentodes like EL34 or the KT88 in triode configuration, connected to the loudspeaker through an audio transformer in a common cathode arrangement. Audio power triode is biased to Class A operation by applying negative bias voltage to its input grid (see diagram), or by raising cathode potential with biassing 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 gap in transformer core to prevent core saturation by DC current; gapping decreases primary inductance and limits bass response. In practice, these limitations are handled by increasing transformer size.
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.
[edit] Speaker matching
Historically, negative feedback in SET amplifiers was quite common (notably, the Mullard 3-3 design built around EL84). Today nfb is less popular with SET amplifiers, with many having no global 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.
[edit] Audio quality
Single-ended triode (SET) amplifiers are considered a classic design and have achieved a cult status among certain audiophiles, because of their excellent midband performance (argued to be the most important part of the audio spectrum in music reproduction) and directness. This high sonic quality is mainly attributed to the simplicity and minimalistic approach of the circuits involved.
Critics of this design point to its electrical inefficiencies in that it consumes the same amount of electricity with no signal as it does when driven to full power. This is true because SET amps are necessarily configured to operate in class A. Push pull designs may be operated in class AB, which idle at lower power levels, at the cost of some distortion as they are driven out of class A into class B operation. Also, there are higher levels of distortion which are canceled out in push-pull designs.[citation needed]
[edit] Estimating power output for a Class A1 amplifier from valve specs
From the available data sheets, obtain the following parametrs, Pa (or P[a+g2] for pentodes) (in watts). Rp in triode (in ohms), and the desired load resistance (Rl) (in ohms) which may vary depending on the design.
Compute maximum theoretical power output by P[ot] = Pa / 2.
Reasoning: 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.
Next, compute P[o] = P[ot] * (1 - Rp/Rl) to obtain an estimate of the maximum obtainable output power.
reasoning: 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.
Finally, knock another 10% off to account for the relatively heavy distortion encountered as the valve approaches cut-off.
Class A2 amplifiers can overcome a high Rp by driving the grid positive with respect to the plate. Because this makes the grid a secondary anode, it too will draw current from the cathode while accelerating the remaining electrons towards the plate. Because these grid currents can place extreme demands on the driving circuitry, sometimes requiring as much as 8 watts for larger tubes such as the 211, Class A2 operation is not recommended for Hi-Fi reproduction. A hybrid design called the powerdrive circuit [1] can be used to overcome this problem and achieve as much as 45% * Pa total output power.
The grid goes positive with respect to the cathode, to draw grid currant in class AB2, not the plate.
[edit] See also
[edit] References
[edit] 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".
