NTX-30

The National NTX-30 is an extremely flexible crystal-controlled transmitter having an RF output of 30 watts on the 10, 20, 40 and 80 meter amateur bands. It is complete and self-contained for c.w. operation, and terminals are provided for connecting an external modulator for phone use. The output stage, which consists of two 6L6G's connected in parallel, is operated at 300 volts, and the normal power input under load is approximately 60 watts. The 30-watt output rating is, therefore, very conservative, and if the output circuit is properly loaded maximum RF power will be as much as 35 or 40 watts. Excitation to the final amplifier is supplied by any one of the four 6L6 tubes. Three of these tubes are employed as doublers following a crystal-controlled oscillator which normally operates in the 3.5- to 4-Mc. band. The doublers will, therefore, provide excitation in the 7, 14 and 28 Mc. bands, and the desired excitation frequency is selected and is automatically applied to the final amplifier by means of a lo w-loss push-button type switch.

FREQUENCY CONTROL

The crystal oscillator is a conventional circuit wherein the crystal current does not normally exceed a few milliamperes. Under such conditions, there is no possibility of injuring the crystals themselves, but as a further safeguard a 2-volt 60-ma. pilot light is connected in series with the crystal holder. Normally, this lamp does not light, but if for any reason the crystal current should become excessively high, the lamp will burn out before the crystal could become overheated. A special National type 4-in-1 crystal holder is supplied as standard equipment. This unit is plugged in horizontally on the front panel, so that the crystal selector switch is in the same position as the other panel controls. Any crystal holder which is built to fit in a five-prong tube socket can if the operator desires, be used in place of the 4-in-1 holder.

BAND SELECTION

Three 6L6 doubler stages follow the crystal oscillator. The outputs of these doublers are on 40, 20 and 10 meters. By simply switching the grid circuit of the final amplifier to the proper tank circuit in the exciter line, and by plugging in the proper output coil, the transmitter, as a whole, may be put on any desired frequency depending, of course upon the frequency of the crystals. The tank condenser of the final amplifier is tuned from the front panel by means of a type "O" dial; it is not necessary to retune the various doubler stages when changing to different frequencies within any of the amateur bands.

METERING

A dual range, illuminated meter, used in conjunction with a five-position switch, serves to check the plate current and excitation of all stages. The meter itself has a 1-ma. movement and is connected through suitable multiplier resistors to any of the cathode circuits. The meter switch positions are numbered from 1 to 5 corresponding with the oscillator, first, second and third doublers and final amplifier, respectively.

KEYING

Two key jacks are provided; one in the cathode circuit of the crystal oscillator and one in the cathode circuit of the final amplifier. Keying the final will provide the cleanest signal and is recommended for this reason. Where break operation is desired, it is necessary to key the crystal oscillator, and the panel control of oscillator tuning will be found very convenient in obtaining the exact adjustment necessary to eliminate keying chirps. The operator should remember that is quite impossible to obtain good oscillator keying in any transmitter if the crystal is the least bit sluggish, or if the holder is improperly adjusted.

CIRCUIT DESCRIPTION

Much of the material which might properly be presented here has been covered in the previous section, and it remains, therefore, to comment upon the unusual design features with which the reader may not be familiar. The Exciter: As previously indicated, the fundamental circuit of the NTX-30 is quite conventional. The method of band-switching is unique, however, and may be described briefly as follows: Reference to the circuit diagram will show a four-section push-button switch. Each section of this switch has two positions and all sections are inter-connected mechanically by an automatic tripping mechanism. For instance, suppose the 20-meter button is pushed in to provide 20-meter excitation to the final amplifier, inspection of the diagram will show the excitation to the 10-meter doubler is now disconnected,the compensation condenser C-24 being switched in. The grid circuit of the output tubes is connected to a tap at the proper point on the 20-meter tank coil. Suppose that it is now desired to shift to the 40-meter band, it is only necessary to push the 40-meter button. When this is done, the 20-meter button will be automatically released, disconnecting the final amplifier excitation lead from the 20-meter tank. The 40-meter button will connect 40-meter excitation to the final and will, at the same time, disconnect the grid of the 20-meter doubler.

FINAL AMPLIFIER

There are several points of interest in connection with the final amplifier, which,as previously stated, consists of two 6L6G tubes connected in parallel. The parallel arrangement was chosen in preference to push-pull for a number of reasons. To begin with, only half of the excitation voltage is required and the excitation can be obtained directly from the various exciter plate circuits with no intermediate link circuits or grid tuning required. The most important point, however, is that the parallel connected tubes have comparatively low plate impedance, making it possible to maintain constant output from 80 to 10 meters in spite of unavoidable losses which would normally decrease output considerably at the high frequencies.

OUTPUT COUPLING

Each of the plug-in coils used in the final amplifier is provided with a semiadjustable pick-up coil, designed primarily for coupling the final amplifier to a 72-ohm transmission line. The pick-up coil can be adjusted to provide varying degrees of coupling as may be required for any particular antenna or feeder system. The same coil can, of course, be used for a link pick-up where the NTX-30 is used as a driver for a high power final amplifier.

POWER SUPPLY

The NTX-30 is designed to operate on line voltages between 105 and 125 at frequencies of either 50 or 60 cycles. Even at a line voltage of 100 volts an RF output of 30 watts is still available, the total power input at 115 volts is about 240 watts. Two switches are provided; one is the AC line switch, while the other is connected in the rectifier output and is used to disconnect all plate voltages while allowing the heaters of all tubes to remain turned on. This is the stand-by switch and it is wired to a terminal panel at the rear of the cabinet marked BSW. This terminal panel provides a convenient means of connecting a relay, as may be required in any particular installation. The B-supply circuits deliver 500 ma. at 300 volts, and are quiet conventional except for the use of parallel rectifier tubes, which are required to rectify the comparatively high current. 600 volt oil impregnated condensers are used in a two section filter, which supplies the oscillator and doubler stages. The final amplifier, wh ich requires less filtering, is connected at the junction of the two chokes.

Shipping Weight Approx. 70 Lbs.

NTX-30, Table Model Transmitter, complete with all coils, tubes, and crystal holder, but less crystal for operation on 10, 20, 40, and 80 meter bands. List $195.00

NTX-30, RS, Rack Model, same as above but mounted on 1/8" steel panel. Black wrinkle finish. List $205.00

NTX-30, RA, Rack Model, same as above but mounted on 3/16" aluminum panel. Black leatherette or gray finish.List $210.00

Special combination NTX-30 Transmitter and NSM Speech Amplifier mounted in steel cabinet, black wrinkle finish. List $345.00

Source: Instruction Manual for the National NTX-30 Transmitter and NSM Modulator

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Photo from Niel Wiegand WA5VLZ