A FLEXIBLE tone-control system is an important part of any amplifier. Most high-quality amplifiers use more or less complex networks of L, C and R. They usually also require one or more supplementary tubes to make up for the loss of gain in the compensating network.
This can be avoided by splitting the input stage into three independently controlled channels. In this amplifier, two channels cover only a part of the audio frequency spectrum (bass and treble). The third channel covers the whole spectrum.
Fig. 1501. Top view of the three-channel amplifier.
Fig. 1502. Three-channel amplifier. Each power output tube can be metered
and then adjusted by means of the 600-ohm, wire-wound potentiometers
used as cathode resistors. Although not shown, primary of power transformer
should be fused.
There are six separate tone controls. (The photo, Fig. 1501, shows only two, P4 and P6, bass and treble controls. The other four, S1, S2, P2, and P3, were added after the photo was taken.) The circuit is shown in Fig. 1502.
As the amplifier was designed for living room use, 8 watts was judged sufficient output. A pair of 6V6's in class AB1 deliver this at the plate voltage chosen. To lower the output impedance, negative feedback is used from the voice-coil winding of the output transformer over three stages to the 6J5 cathode circuit. The amount of feedback can be changed by varying the 1.2K resistor connected from the under grounded side of the voice coil to the cathode circuit of the 6J5. A 2K, wire-wound potentiometer can be used as a feedback control. If the feedback causes oscillation, transpose the voice coil leads.
The input of the amplifier is designed for a crystal pickup. R1 is selected to limit signal voltage at point X to 0.5 volt on peaks. From here the signal goes through volume control P 1 and then divides into three parts, going to a 6S J7 and each half of a 6SL7. The output of the 6SJ7 has a low-pass filter in its plate circuit. The crossover frequency of this filter can be adjusted by Si. The amount of bass boost is regulated by P2. The over-all gain of the bass boosting channel is controlled by the potentiometer P4 in the grid circuit of the 6J5.
One-half of the 6SL7 functions as the uncompensated stage which amplifies all frequencies. P5 controls the gain of this channel.
The other half of the 6SL7 is used as the treble channel. This channel has a variable high-pass filter in its plate circuit. S2 selects the frequency at which the channel's response begins to fall. Potentiometer P3 regulates the amount of fall. The over-all gain of the treble channel is controlled by potentiometer P6.
Fig. 1503. The under-chassis wiring. The output transformer is at the
right.
The output of the three separate channels is combined at the grid of the 6J5 amplifier. The 6SN7 which follows the 6J5 functions as the phase inverter using a cathode-coupled circuit. In this circuit, only one of the triode grids is driven. The other triode's grid is at ground potential as far as a.c. is concerned. The signal is fed through the cathode which is connected to the cathode of the first half of the 6SN7. Since both cathodes are well above ground, they vary at an audio rate when a signal is fed to the first triode grid. Because of the grounded grid, the second triode's plate has the same phase as the cathode, and differs by 180° from the plate of the first triode.
The 6V6 push-pull output stage is connected in conventional fashion. However, each tube has a separate cathode bias resistor (a 600 ohm, 4-watt potentiometer). A closed-circuit jack in each cathode permits metering cathode current of each 6V6. Adjust the bias to give a cathode current of 35 ma for each tube.
To get the maximum quality from this amplifier, use a high-grade output transformer. The capacitors and resistors on each side of the inverter and push-pull circuits should be accurately matched for best results. Grounds must be grouped by stages, and all groups connected to the chassis at one point. The filament winding on the power transformer was center-tapped and all filament leads were shielded. An under-chassis view is shown in Fig. 1503.