Power Amplifiers

PrimaLuna DiaLogue Seven Tube Monoblock Power Amplifier

ARTICLE INDEX

The Design

The DiaLogue Seven is a push-pull tube amplifier (monoblock - one channel), using KT88's in the output stage. The input and buffer stages are handled by two 12AX7's and 12AU7's respectively. The 12AX7 and 12AU7 are native dual-triodes, high and medium voltage gains respectively, while the KT88's are pentodes (actually, called a beam pentode). In a triode, there is a cathode, grid, and anode. The cathode is heated, which forms a cloud of electrons, and the positively charged anode (it has a "plate voltage" applied to it) attracts the electrons from the cathode. The grid, which sits between the cathode and anode, is a screen in form, so that the electrons can pass through. The music signal is applied to the grid, and its varying voltage allows more or less electrons to flow between the cathode and anode. The music signal voltage applied to the grid is very small, and the plate voltage is very large. The voltage and current flowing from the plate to the output transformer is, therefore, higher than the voltage applied to the grid, and voila, the signal has been amplified.

A single-ended tube amplifier uses one or more tubes that all handle the positive and negative portions of the waveform by being operated in Class A. In a push-pull design, such as the DiaLogue Seven, at least two tubes are used, with one tube primarily handling the positive portion of the waveform, while the other tube primarily handles the negative portion. There can be more than one tube for the positive and negative portions.

The DiaLogue Seven has four KT88's. The tubes are biased such that they operate in Class A for a very small portion of the power. However, for most of the power, one pair handles the positive portion of the waveform (and some of the negative), while the second pair handles the negative portion of the waveform (and some of the positive). The amplifier never operates in Class B, which refers to one tube handling only the positive portion of the waveform, and the other tube handling only the negative portion. Operation in Class B produces crossover distortion. In Class AB, both halves of the output stage are operating when the output voltage swings from positive to negative, or negative to positive, until the other half has fully taken over.

Here is a diagram of a pentode. The numbers describe the parts of the tube: the cathode heater (1) has electrical current passing through it, which heats it directly, and then, this heat transfers to the cathode itself (2). By this method, you don't get the heater voltage, which may be AC, introducing hum into the signal path. The control grid (4) is between the cathode (2) and anode (3), as are the screen grid (5) and suppressor grid (6). In the KT88, the suppressor grid is actually shaped like a tunnel, with electrodes on both sides, through which the electrons pass. The diagram shown here is "schematic" rather than illustrative of what the electrodes inside actually look like, or where the connecting pins are located.

pentode-schematic

Here is a screen shot of a 1959 Genalex specification sheet (KT88, Issue 2, March, 1959), showing the schematic of the KT88 that they manufactured (copyright, Genalex). In this case, the pin numbers are indicated, which show how to connect the tube in a circuit.

prima-luna-dialogue-seven-amplifier-kt88-schematic

Many audiophiles love the sound of triode tubes. The DiaLogue Seven allows you to use the amplifier in "triode mode" or "ultra-linear mode", by pushing a button on the included remote control (that is its only function). There is a small LED near the front of the amplifier that is green when in triode mode, and red when in ultra-linear mode. Switching the amplifier to triode mode connects the screen grid to the plate (only in the KT88's). This reduces the maximum power output to 40 watts, compared to the ultra-linear mode, which connects the screen grid of the KT88's to a point 43% in from the end of the output transformer that is connected to the tube's plate. This increases the maximum power output to 70 watts RMS, and also serves as negative feedback, which flattens the frequency response.

The DiaLogue Seven adjusts the bias voltage on the control grid automatically, depending on the demands of the incoming signal. They call their version of this technique Adaptive AutoBias. The adjustment is on the order of a few milliamps, and is used to reduce distortion at high output levels. The process improves the sound quality, because there is now more current flowing that is routed to the output transformer (if this same amount of current were flowing at idle, it would be dissipated as heat, which wastes energy). The Adaptive AutoBias is only applied to the output tubes (the KT88's). It also allows you to use different tubes, such as KT90 or 6550. A "Soft Start" feature slowly raises all of the voltages, including cathode heater and plate voltage, and this extends tube life.

Cross-coupled as well as local negative feedback are employed to reduce distortion, lower the output impedance, and flatten the frequency response.

While the signal path is handled by all tubes, the power supply is solid state.

The rear panel has one RCA input jack and four speaker binding posts, one for ground, and one each for 2, 4, and 8 ohms.

primaluna-dialogue-seven-amplifier-rear-panel

All in all, the build quality of the DiaLogue Seven is superb. The chassis is very heavy at 64 pounds (many multi-channel power amplifiers weigh about this much, and the DiaLogue Seven "only" puts out 70 watts).