___The output of the second screen-grid amplifier is capacitively coupled to the control-grid of the UX-860, 75-watt screen-grid buffer-amplifier which is mounted in the compartment above the crystal-oscillator-amplifier units. Excitation is sufficient to cause this tube to draw 250 watts from the plate supply at a plate voltage of 3000. It is nearly biased to cut-off, bias voltage being

realization of 100% modulation, and has been described previously in QST. The Class-C amplifier is neutralized to prevent self-excited oscillation. Its plate tank is similar to that of the buffer-amplifier; a copper-strip inductance tuned by a Cardwell transmitting condenser.
___Two UV-849 tubes in parallel are used in the modulator. At first consideration the use of two tubes may seem unnecessary, but their desirability becomes apparent when the ratings and characteristics of the UV-849 are consulted. The

obtained from the station grid-bias generator. Screen-grid voltage of 750 volts is obtained from the plate supply through a suitable resistor. The plate supply for the buffer-amplifier, as well as for the UV-849 modulators and modulated amplifier, is obtained from the three-phase series rectifier using six UX-866 tubes mounted on the left end panel. The plate tank inductance is of copper ribbon wound on a hard-wood strip form and is tuned by a 7200-volt Cardwell transmitting condenser.
___The buffer-amplifier must furnish a healthy kick to the grid circuit of the modulated Class-C amplilfier to supply the grid losses and insure grid saturation. The UX-860, capacitively coupled to the grid circuit of the Class-C, UV-849 modulated amplifier, does this without difficulty. Plate input to the UV-849 is 300 watts at 2000 volts with the grid biased well beyond cut-off. Plate current is supplied to the modulated amplifier and two UV-849 modulator tubes through a common modulation choke; plate voltage on the modulator tubes is 3000 and this is dropped to 2000 for the modulated amplifier through a suitable resistor which is by-passed by a large fixed condenser. This arrangement is essential to the

load resistance of the modulator is the plate resistance of the modulated amplifier and the conditions for maximum output are satisfied when the load resistance is twice the plate resistance. The plate resistance of the two modulator tubes in parallel is practically half the plate resistance of the single amplifier. Maximum modulator output, therefore, should be obtainable. Moreover, the use of two tubes in the modulator makes possible complete modulation of the Class-C amplifier output without overloading the modulator tubes. The plate input to each modulator tube is 100 milliamperes at 3000 volts, a total input of 500 watts. Their grids are biased so that they operate on the linear portion of their characteristic. Sufficient grid-swing is possible to vary the modulated amplifier plate potential between zero and twice the operating voltage without noticeable distortion. Non-inductive resistors are connected in the modulator grid leads to prevent oscillation at ultra-high radio frequencies. Negative grid bias voltage is obtained from the bias generator.
___Grid input to the modulator is from a two-stage speech amplifier consisting of two resistance- coupled UV-203-A tubes. This amplifier is at the

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