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The unit is powered directly from the 120 volt AC line, with no power transformers. Filaments are wired in series, with the total adding up to 117 volts (35 + 35 + 35 + 12). The 35W4 forms a half-wave rectifier, which is filtered by a three-stage RC network. The B+ for the output stage plates and screens are taken from the second capacitor, and the B+ for the preamp and phase inverter from the third capacitor in the filter.
The input signal to the amplifier is applied directly to the volume control pot, from whence it passes through a variable high-pass filter (the "Treble" control). When the wiper is set to minimum, response is approximately flat (though actual frequency response will depend somewhat on volume control setting). When it's turned to maximum, higher frequencies are favored, with the lower 3 dB corner at around 1500 Hz. and the higher pole (plateau) around 4000 Hz. For testing purposes, the volume control was set to maximum and the treble control to minimum, to minimise the effect of this control.
The output of the volume/treble control circuit is applied directly to the grid of the first section of a 12AX7 twin triode. A partially bypassed cathode resistor supplies grid bias, while providing a modest amount of local negative feedback to help linearise the stage's response. A 100k resistor provides the plate load for the preamp stage. The cathode also has a 10k resistor to the "Bass" control connected to it; we'll discuss the function of these components a little later, as they are part of the global feedback network.
The input signal to the amplifier is applied directly to the volume control pot, from whence it passes through a variable high-pass filter (the "Treble" control). When the wiper is set to minimum, response is approximately flat (though actual frequency response will depend somewhat on volume control setting). When it's turned to maximum, higher frequencies are favored, with the lower 3 dB corner at around 1500 Hz. and the higher pole (plateau) around 4000 Hz. For testing purposes, the volume control was set to maximum and the treble control to minimum, to minimise the effect of this control.
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The output of the volume/treble control circuit is applied directly to the grid of the first section of a 12AX7 twin triode. A partially bypassed cathode resistor supplies grid bias, while providing a modest amount of local negative feedback to help linearise the stage's response. A 100k resistor provides the plate load for the preamp stage. The cathode also has a 10k resistor to the "Bass" control connected to it; we'll discuss the function of these components a little later, as they are part of the global feedback network.
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