PrimaLuna is a Dutch-based company that released its first products, the ProLogue One and ProLogue Two integrated tube amplifiers in 2003. Then, in 2006, they released the ProLogue Three preamplifier, and ProLogue Five, Six, and Seven power amplifiers. Late in that same year, the PrimaLuna DiaLogue series hit the market, which had the ProLogue’s basic features, but with upgraded parts and additional capabilities. Here, we review the DiaLogue Seven, which is a monoblock power amplifier, capable of delivering 40 watts RMS in triode mode, and 70 watts RMS in ultra-linear mode (see The Design section for an explanation of what this means). The bottom line is that the PrimaLuna DiaLogue Seven monoblock is more than capable of delivering a clean, detailed, and lush sound (ah, those tubes are responsible for that).
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.
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.
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.
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.
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. 
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).
PrimaLuna’s products and sound were unfamiliar to me before this review. Now, they are familiar, and I like what I heard. If you are a tube-ophile, you also will like what you hear. If you are a purist, you will likely prefer the triode mode as I did, but there certainly was nothing wrong about the sound in ultra-linear mode. It was just “different”. This is not an inexpensive product by any means, but you get what you pay for, and in this case, you are paying for some great music listening.
Source: www.primaluna-usa.com
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