Rf W

Fig. 1. Linear amplifier schematic*

The grounded grid rf amplifier described here covers a continuous frequency range of 3,0 MHz through 38 MHz in five steps. The circuit is straight forward, built around a pair of 4E27 tubes in parallel, with a pi-network output, If you prefer, 813 tubes may be substituted for the 4E27s without any electrical change in the circuitry. Mechanically you will have to change the plate caps for the 813's, the rest of the hardware remains the same.

The amplifier is built on a 12 x 12x3 inch aluminum chassis. The total height of 8 inches is achieved by recessing the tubes 1 inch below the chassis. Drill plenty of V* inch vent holes in the chassis around the tube bases for proper heat dissipation, To conserve additional room, the coaxial con-

nectors in the rear are recessed 112 inch into the chassis.

t jti unit are available from surplus houses, wholesale or retail outlets, junk boxes or friends.

The amplifier schematic is shown in Fig. 1. Capacitors (CI, C2, C3, C4) are Centralab 850 type transmitting ceramics. After assembling the unit with the vacuum tubes in place, use a grid dip meter to tap the final tank. Set the plate tuning capacitor (C5) as follows when tapping tank coils Li and L2:

C5 setting 90% of full mesh 50% of full mesh 20% of full mesh 20% of full mesh 15% of full mesh

Turns in our circuit 18 turns on L2 8 turns on L2 ■ turn on L2 4 turns on LI 1 turn on LI

The pi-network output capacitor (C6) is a three gang capacitor out of an old broadcast band radio, The three sections of capacitor (C6) are tied together in parallel. Three additional capacitors (C7, C87 C9), transmitting micas rated at 5000 volts test —2500 volts working, may be necessary for loading on the lower bands. The output

I mH 600 mA

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