Sound reference kt88 tube power amp4/17/2023 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). Many audiophiles love the sound of triode tubes. In this case, the pin numbers are indicated, which show how to connect the tube in a circuit. 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). The diagram shown here is “schematic” rather than illustrative of what the electrodes inside actually look like, or where the connecting pins are located. In the KT88, the suppressor grid is actually shaped like a tunnel, with electrodes on both sides, through which the electrons pass. The control grid (4) is between the cathode (2) and anode (3), as are the screen grid (5) and suppressor grid (6). By this method, you don’t get the heater voltage, which may be AC, introducing hum into the signal path. 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). 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. Operation in Class B produces crossover distortion. 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. 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 tubes are biased such that they operate in Class A for a very small portion of the power. There can be more than one tube for the positive and negative portions. 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. 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.Ī 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. The music signal voltage applied to the grid is very small, and the plate voltage is very large. 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 grid, which sits between the cathode and anode, is a screen in form, so that the electrons can pass through. 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. ![]() In a triode, there is a cathode, grid, and anode. 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). The input and buffer stages are handled by two 12AX7’s and 12AU7’s respectively. The DiaLogue Seven is a push-pull tube amplifier (monoblock – one channel), using KT88’s in the output stage. Input Sensitivity for Full Output: 1.1 Volts.Frequency Response: 10 Hz – 100 kHz, ± 3 dB. ![]()
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