In the the valveamp schematic below you see that the amplifier is built with only 5 vacuum tubes! The circuit is a classic push-pull amplifier using an output transformer with a center tap. This topology is widely used for tube amplifiers, because it is impossible to make a complementary P-type tube: you cannot make holes in vacuum as you do in semiconductor materials.
The circuit works as follows:
The first ECC81 triode amplifies the input signal, the second ECC81 triode acts as an 180 degree phase shifter The amplified audio signal is presented on C3 and (180 degrees shifted) on C2.
The output stage is built with 2 x 2 EL84 penthodes in parallel. The phase shifter drives the high side 180 degrees out of phase from the low side: when the low side pulls, the high side pushes and the other way around.
Because both high and low side of the output stage draw current form a seperate primary winding, the push pull output current will cause an AC only flux in the magnetic core of the output transformer which will result in an AC signal on the secondary winding which drives the speakers.
The quiescent current of the output stage will also pass trough the transformer. This current has a substantial 50/60Hz component because
the power supply of is not stabilised. However, if accurately biased, this current will not result in a secondary signal: because the bias currents are in phase, the low side bias current will cancel the high side bias current.
In this circuit the current sources ensure well defined bias currents for the output stage, minimising hum and cross over problems.
Most designs use cathode series resistors and an adjustable Negative Grid Voltage to set the bias currents of the output stage.
In this way, the bias current is at the mercy of the valve characteristics which are different per valve, but also change over time!
The current sources (the schematic will follow next) bias all four EL84 pentodes on 20mA DC, regardless of tube characteristics
The AC audio signals pass through the parallel capacitors of the current sources(about 100uF). The output transformer is a toroid type from Amplimo: type 3A524.
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