4cx250b Amplifier Schematic

This voltage and current is maintained with a charging voltage of 14 volts from the alternator.

The transformers shown are surplus. There are many similar ones available from the surplus dealers who advertise in 73.

A small replacement transformer rated at 350 volts each side of center at 50 ma, 6-3 v at 4 amp and with a 1)5 v primary is used to furnish bias and screen voltages, The 6,3 volt winding becomes the primary and is connected to phase A at the center tap point of this transformer. The 115 voll winding becomes the bias source.

The diodes are standard TV replacement types.

This amplifier is a little unusual. It is a grounded grid tetrode amplifier. The 1CX250B tube is not suitable for regular grounded grid operation (that is? with the control and screen grids both grounded), for screen and grid currents will run too high and ruin the tube, But they work fine as tetrodes with drive applied to the cathode, the grid grounded and the screen at about 350 volts. As in true grounded grid, about 8035 of the drive shows up in the output. Drive required is higher than class AH, but less than grounded grid. An exciter with 65 watts output will drive the pants off this linear on all bands—even ten.

You've probably noticed from the schematic that I didn't include eighty meters in the linear. If you want to, you'll have to use larger pi network capacitors or switch in some extra capacitance on this band.

I didn't use on L network in the cathode circuit of the linear. An RF choke worked fine. II your exciter output is less than 65 watts PEP, you probably will need a tuned input.

The amplifier was neutralized to improve its stability, but this turned out to be unnecessary.

You probably could use 100 \xf electrolytics with slightly poorer regulation. Each of the 750 volt outputs is shunted by a 50 k, 20 watt resistor that acts as a voltage equalizer and bleeder.

Te high voltage supply is switched into action by energizing two six volt auto started solenoids connected in series. They complete the circuit of phase B-C to the transformer. Phase A leg will not supply any voltage without one of the other legs connected. I used six volt solenoids since they were cheaper than twelve.

The filaments and blower are designed to be switched on at least three minutes before the plate supply is energized, the neutralizing capacitor is a piece of brass Hat stock V by 7" mounted away from the plates of the final tubes. It is supported by three steatite stand-offs one inch long.

The screen and cathode currents are read with the flip of the meter switch. When the amplifier is properly loaded, cathode current is about 550 ma at 2000 volts. The screen current is 50 to 60 ma at 350 volts. This current should not be exceeded; if it is too high, increase loading. If the current is too low or negative reduce loading. These readings are with full carrier.

4cx250b Schematic Amplifier Coil

The side view of the linear shows the 4CX250B's mounted horrzontalfy with the blower fastened to the back of the cabinet.

4cx250b Amplifier Schematic

The 4CX250B linear. Operation is very simple, The only controls are tune, load, bandswitch, meter and bias set.

Bias should be set for about 100 to 150 ma resting current with 2000 volts on the plates, Once set, this current does not need adjustment.

The 40 cfm squirrel-cage blower is probably a little husky for this job, but the extra air helps to keep the tubes real cool. Incidentally, the amplifier helps heat the car on cold mornings with its 200 watt dissipation!

Checks with a Collins watt meter and the old fashioned RF ammeter and voltmeter shows that the amplifier gives excellent efficiency, One kilowatt DC in gives about 650 watts RF out.


1965 Ham Radio

Yes, you are correct in wondering what commercial mobile antenna will handle two kilowatts, I have tried a variety of commercial and home brew designs: helical wound, base loaded, center loaded, top loaded and trap. I found that the center loaded type with three foot base section, loading coil and five foot top section was quite effective. The loading coil was of heavy B & W stock two and a half inches in diameter. A separate coil was plugged in for each band. This antenna was used for about four years of mobile hamming.

But I am curious, so I wanted to try the Newtronics Hustler top loaded antenna. I did. But, as 1 feared, the loading coil was not tie-signed for this power. The new high power coils would be fine, but this was before they were announced, so I made my own coil from more B & W stock. The one illustrated is for twenty meters. Be sure to coat the coil thoroughly with Krylon or epoxy if you make your own coil Otherwise, water between turns will short out part of the winding with attendant loading problems and fireworks,

I made a few tests and found that the top loaded antenna seemed to increase both received and transmitted signals over a center loaded whip in the same position on the car.

It is easy to find resonance with this antenna. Just slide the top tuning rod in and out a quarter inch at a time while checking the SWR, I used a 20£ foot length of RG-8/U between the linear and the antenna.

Corona loss at this high power is high. Even so, all signal reports have proven to me that the extra power has more than paid for itself, ni happv with rav high power mobile. I think you will be, too.

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