A compact efficient three-tube receiver, exceptionally high sensitivity is obtained through the use of a special electron-coupled detector circuit which has several unique features. Primarily designed for 5-meter work, the 56 and 60 MC band is spread over 75 dial divisions and regeneration is constant over the entire scale. The interruption frequency oscillator is coupled to the screen grid of the detector, effectively isolating it form the audio circuits, and giving the optimum superregenerative action. A phone jack is connected to the detector output by means of an impedance matching transformer, the audio output tube being automatically disconnected by insertion of the phone plug. The 89 pentode gives ample power for loud speaker operation. and a potentiometer connected in the grid circuit furnishes smooth and full range control of volume.
|Low Frequency Oscillator||237|
If maximum economy of operation is desirable, as in portable work, it may be advisable to omit the 89 tube and operate the receiver with headphones. The circuit is such that the 2-volt tubes cannot be employed.
The heater circuit requires approximately 6 volts at .9 amperes. The voltage is not critical and may be between 5.5 and 6.5 volts. The supply may be either A.C. or D.C. except as noted under instructions for the Low Frequency Coils. The plate supply voltage normally required is 180 volts and this may be obtained either from B-batteries or from an A.C. operated power supply. The NATIONAL TYPE No. 5886 AB power unit fulfills these requirements and is supplied with a suitable receptacle for the 4 prong cable plug. 135 volts of B-battery may be used with good results, provided the 25,000 ohm resistor, mounted near the center of the chassis (underneath), is changed to 10,000 ohms. Fair results may be obtained with 90 volts of B-battery, in which case the resistor should be shorted out entirely. Coils are available for the 10, 20, 40 ,80 and 160 meter bands. When operating on these bands the heater circuits must be supplied from a D.C. source, such as a storage battery, in order to eliminate A.C. hum. If A.C. operation is desired on these bands, it will be necessary to change the tubes to the 2.5 volts type. A 224 may be substituted for the 236; a 227 for the 237; and a 2A5 for the 89- altogether this last substitution will require some re-wiring of the output tube socket. The bias resistor required for the 2A5 tube is approximately 500 ohms and should replace the 1,000 ohm resistor used for biasing the 89.
The output tip jacks for speaker operation are located at the back of the receiver on the right-hand side. The speaker requirements are not at all critical and any conventional magnetic or dynamic type of unit will give good results. The output impedance of the receiver is approximately 7,000 ohms, requiring a speaker impedance of between 3,000 and 15,000 ohms. The phone jack for headphone operation is located in the left-hand side of the front panel and is connected to the plate circuit of the detector tube by means of a step down auto-transformer. CAUTION: At all times when the heaters are lighted and when B power is connected to the receiver, either the headphone jack must be plugged in or a loud speaker connected to the output terminals. If this is not done, the 89 tube may be seriously damaged.
The left-hand knob actuates the audio gain control, which varies the signal input to the 89 tube. It has no effect when headphones are being used. The dial is the main tuning control. The 56-60 megacycle band normally covers approximately 125 to 80 on the dial. The calibration of individual receivers will vary somewhat and the size of the antenna and degree of antenna coupling will likewise alter the above figures. The right-hand knob is the detector regeneration control. The toggle switch located near the bottom of the front panel on the right-hand side opens the B+ circuit and is useful in shutting off the receiver during transmission periods where duplex operation is not desirable. If B-battery plate supply is employed the switch should be thrown to the "Off" position at all times when the receiver is not in use.
The antenna binding post is located at the left-hand side of the receiver, the lead being brought through the rubber bushing beside the post. A series antenna coupling condenser is located directly below the antenna post near the chassis. The success of any 5-meter work depends largely upon the receiving antenna and antenna coupling employed. In most cases it is advisable to experiment with several antenna arrangements, but as a general rule the antenna described should be as high as possible and may be a single vertical wire approximately 8 feet in length. The lead-in consists of a single wire connected to the antenna 13 1/2 inches from the center and should be run at right angles to the antenna for a few feet before being brought down to the receiver. The length of the lead-in is not critical in any way, but is should be well insulated and sharp turns should be avoided. It should not be shielded. When the receiver is put in operation with certain types of antenna, it may be found the detector will not oscillate over certain portions of the range. The is indicates too much antenna coupling and the coupling condenser plates should be spread apart slightly until the dead spot just disappears. When the more conventional type of untuned antenna is used, the coupling condenser plates should be moved closer together for best results. As a general rule, a ground connection is not necessary but under certain conditions its use may be beneficial.
LOW FREQUENCY COILS
An important feature is the use of plug-in inductance coils, enabling the receiver to be operated on the 10, 20, 40, 80 and 160 meter bands as a conventional "detector and one stage" with the interruption frequency tube removed. Regeneration is smooth throughout the range and each band is adequately spread over a large portion of the dial. The flexibility of this arrangement makes the Type SRR an ideal receiver for the experimenter. Low-loss R-39 and Isolantite insulation is used in all H.F. circuits
Cabinet dimensions 7 3/4" x 7 3/4" x 7" high. Weight with tubes 8 1/4 pounds.
|Type SRR Receiver, completely wired, with 56-60 MC coils, less tubes||$25.00|
|10 Meter Coil||$1.25|
|20 Meter Coil||$1.25|
|40 Meter Coil||$1.25|
|80 Meter Coil||$1.75|
|160 Meter Coil||$1.75|
Source: National Radio Products Descriptive Price List No. 210, Installation and Operating Instructions For The National Type SRR Receiver
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