weight by the fact that it finds its way into the most modern equipment. The grid tank coil is similarly tuned by an aluminum disc.
___Proper excitation of the 5-kw. linear amplifier is obtained by varying the coupling to the modulated

amplifier plate inductance and adjusting the grid-shunt resistor until the amplifier carrier output is 5-kw. With carrier power output of this value, a maximum undistorted power output of 20 kw. is possible on the modulation peaks. The excitation adjustment is both critical and important. Distortionless performance on the part of the linear amplifier is impossible unless it is correct. A detailed explanation of the method of adjusting the excitation of the linear amplifier of an amateur 'phone transmitter is given in the April, 1929, issue of QST.

___A 50,000-watt radio frequency linear power amplifier is most decidedly an innovation in the art of radio telephony and has been made a practical actuality as a result of the perfection of the 100-kw. tube. The designation of the rating of the amplifier might well be 200 kw., for its power output on the modulation peaks has this value. Two UV-862, 100-kw. water-cooled tubes in a push- pull circuit do the work.
___Of all the interesting features of this amplifier, the UV-862 is undoubtedly the most interesting. To one accustomed to lesser tubes its ratings are almost staggering. Filament current is 207 amperes at a filament voltage of 33. Almost 14 kw. of power is required for heating the filaments of

the two tubes; enough power to operate a goodly number of ham transmitters. The plate voltage is 18,000 and seems quite reasonable. But the plate current is around 5 amperes per tube; 10,000 milliamperes plate current for a single push-pull amplifier almost numbs the amateur operation. Grid bias for Class B operation is 320 volts, a comparatively low value since the tubes have a high amplification factor. Two kilowatts of actual grid excitation are required for the full 50-kw. power output of the amplifier. When the mind has become accustomed to such values, the good old 75-watter seems to shrink to the proportions of a 199.
___The heat generated by the filament and plate power consumed is considerable to say the least, and it is obvious that an effective means for rapid cooling of the tube elements must be provided. The heat dissipated by the filament alone would be sufficient to wreck a tube in short order if no means, other than air, were available for cooling. Such catastrophe is made remote by the effective water-cooling system incorporated as an auxiliary to the transmitter and will be described in detail later.
___The circuit arrangement of the 50-kw. amplifier is the same as that of the preceding stage. Its tuning and excitation adjustments are identical with those of the 5-kw. amplifier. Its components differ considerably from those of the preceding stage,

however, and details for their construction may be of interest.
___The plate inductance is made up of flat wound copper strip on notched glass bars fastened to a form of wood ribs and is quite similar in construction to the edge-wise wound inductance of the preceding stage. Proponents of both flat and edgewise