Antenna

Fig. 2. Radio frequency amplifier, schematic.

until I started looking through the manual. If the drive level on the grid of the final amplifiers drops below 10 mA, the keying relays won't pull in. The relays, once keyed, will stay energized, though, even if the drive falls to around 5 mA. But if you drop power on the input, you won't get the rf amplifier going again until you can generate 10 mA of drive. It seems to require about 8W of transceiver power to get 10 mA of grid drive.

A power supply overload control also disables the plate and screen supply to the amplifier when the plate current reaches an excessive value (420 mA). While this does provide protection against momentary overloads, it does amount to a pain in the neck when you're trying to get the thing tuned up. Your best bet is to use the tune position of the set to get everything in resonance, then switch over to high power. When you do this, it will take only a bit of tweaking to redip.

Figure 2 shows the schematic diagram of the amplifier portion of the system (AM-494/GR). The unit comes equipped with a pair of 4CX150A tetrodes, the same lineup Johnson used in its Thunderbolt kilowatt unit. These are extremely rugged tubes and are capable of comfortably running a full thousand watts input by themselves. The big secret is keeping the bottles cool — which is no problem with this amplifier because an efficient air-blowing system is built right in. Construction of the tube sockets is such that the cooling air circulates from the tube bases up past the ceramic envelopes to

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The capacitors shown by broken lines in Fig. 2 consist of the inherent capacitance from the tube plates to the bolt and nut on a feed through insulator; these small capacitance values serve to partially neutralize the grid-to-plate capacitance of the tubes. Since there is nothing critical about the value of the capacitance, adjustment should not be required. If for some reason it becomes necessary to change the neutralization capacitance, washers of various diameters could be used; but the process would not be fun at all, and the hit-and-miss aspects of it should go a long way toward dissuading prospective tinker-ers.

To get the amplifier going, connect a clean rf signal of 8—20W into the input (CW or FM only - this is NOT a linear amplifier). Turn the power switch of the amplifier on, but do not apply plate voltage. Put the TEST switch in the GRID position, and adjust the GRID TUNE control on the amplifier until the

TEST meter indicates at least 10 mA of drive. If you can't get an indication of 10 mA, forget about switching on the high voltage; if you do get at least 10 mA of drive, put the PLATE VOLTS switch on the amplifier in the LOW position and switch on the high voltage. From here on, the tuning is exactly the same as your own transmitter: peak the coupling, dip the plate as long as you have a nice smooth swing on the PLATE CURRENT meter. If the dip looks a little sluggish, back off on the coupling and dip at a lower setting on the meter. That's all there is to it.

With a capacitor input supply, the final should dip at around 300 mA. If you are running 1.6 kV, this is a power input of 480W. Assuming an efficiency of around 70%, which is reasonable with class C amplifiers, your power output should be a little more than 325W. And if you've opted to replace the rectifiers with solidstate diodes, you'll get a signal out that will guarantee you at least a watt out for every dollar you've invested. . . .K6MVH"

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