Valve audio amplifier technical specification
The characteristics of valves as gain devices have direct implications for their use as audio amplifiers, notably that power amplifiers need output transformers (OPTs) to translate a high-output-impedance high-voltage low-current signal into a lower-voltage high-current signal needed to drive modern low-impedance loudspeakers (cf.
Two power valves (may be triodes or tetrodes) being differentially driven to form a push–pull output stage, driving a push–pull transformer load.
The CC may be approximated by a resistor dropping a large voltage, or may be generated by an active circuit (either valve, transistor or FET based) The long-tail pair can also be used as a phase splitter.
As an alternate to the long-tail pair, the concertina uses a single triode as a variable resistance within a potential divider formed by Ra and Rk either side of the valve.
In a 1951 engineering paper published by David Hafler and Herbert Keroes, they determined that when the screen tap was set to approximately 43% of anode voltage, an optimized condition within the output stage occurred, which they referred to as ultra-linear.
This design require numerous valves, run hot, and because they attempt to match impedances in a way fundamentally different from a transformer[citation needed], they often have a unique sound quality.
This type of design results in an extremely simple distortion spectrum comprising a monotonically decaying series of harmonics.
See paragraphs further down regarding high-power commercially available SET amplifiers offering up to 40 watts with no difficulty, following the development of output transformers to overcome the above restrictions.
One reason for SETs being (usually) limited to low power is the extreme difficulty (and consequent expense) of making an output transformer that can handle the plate current without saturating, while avoiding excessively large capacitive parasitics.
Push–pull output stages can use triodes for lowest Zout and best linearity, but often use tetrodes or pentodes which give greater gain and power.
The effect of this is that class A amplifiers perform extremely well with music that has a low average level (with negligible distortion) with momentary peaks.
In addition, transformers introduce frequency-dependent phase shifts which limit the overall negative feedback which can be used, to keep within the Nyquist stability criteria at high frequencies and avoid oscillation.
In recent years, however, the development of improved transformer designs and winding techniques greatly reduce these unwanted effects within the desired pass-band, moving them further out to the margins.
Noise is due to device imperfections plus unavoidable temperature-dependent thermal fluctuations (systems are usually assumed to be at room temperature, T = 295 K).
For the EF86 low-noise audio pentode valve, for example, this voltage noise is specified (see e.g., the Valvo, Telefunken or Philips data sheets) as 2 microvolts integrated over a frequency range of approximately 25 Hz to 10 kHz.
It can be reduced by choosing very pure materials for the cathode nickel, and running the valve at an optimized (generally low) anode current.
Unlike solid-state devices, valves are assemblies of mechanical parts whose arrangement determines their functioning, and which cannot be totally rigid.
If a valve is jarred, either by the equipment being moved or by acoustic vibrations from the loudspeakers, or any sound source, it will produce an output signal, as if it were some sort of microphone (the effect is consequently called microphony).
For high-end audio, where cost is not the primary consideration, valve amplifiers have remained popular and indeed during the 1990s made a commercial resurgence.
In response, many modern valve push–pull amplifiers are more powerful than earlier designs, reflecting the need to drive inefficient speakers.
When valve amplifiers were the norm, user-adjustable "tone controls" (a simple two-band non-graphic equaliser) and electronic filters were used to allow the listener to change frequency response according to taste and room acoustics; this has become uncommon.
However, there is some small demand for valve preamps and filter circuits for studio microphone amplifiers, equalising preamplifiers for vinyl discs, and exceptionally for active crossovers.
Since the 1990s a niche market has developed again in the west for low-power commercial SET amplifies (up to 7 watts), notably using the 300B valve in recent years, which has become fashionable and expensive.
Mainstream modern loudspeakers give good sound quality in a compact size, but are much less power-efficient than older designs and require powerful amplifiers to drive them.
This construction style is satisfactory due to ease of construction, adapted to the number of physically large and chassis mounted components (valve sockets, large supply capacitors, transformers), the need to twist heater wiring to minimise hum, and as a side effect benefiting from the fact that "flying" wiring minimises capacitive effects.
Another picture shows exactly the same circuit constructed using Russian military production Teflon capacitors and non-inductive planar film resistors, of the same values.
The main problem with these designs is constructing output transformers able to sustain the plate current and resultant flux density without core saturation over the full audio-frequency spectrum.
Some high-power commercial amplifiers use arrays of standard valves (e.g. EL34, KT88) in the parallel push–pull (PPP) configuration (e.g. Jadis, Audio Research, McIntosh, Ampeg SVT).
The output transformer (OPT) is a major component in all mainstream valve power amplifiers, accounting for significant cost, size, and weight.
A common myth is that a short-circuit in an output valve may result in the loudspeaker being connected directly across the power supply and destroyed.
