Experience With 7360 Tube

NOTE ALI VOLTAGES MEASURFD WITH HF AT W Kit VW M

Fig. 2. (Opposite page). Schematic of El Marrinero. A ,01.uF capacitor between the 2001J carrier balance pot ond the cathode follower.

because both signals could not be taken from the same point because of the by-passing effect of the .01 ^F in the ring modulator, it just shunted the signal on the detector to too weak an output. The solution of taking the product detector from the oscillator plate and varying the coupling capacitor i50 pF) value to the product detector, a 6-9 volt rf signal could be set. The 100 kQ resistor on the grid of tf le cathode follower does not load the oscillator down and the signal for the modulator can now be taken off from the 3.3 kO cathode resistor which is now a low impedance point. This better matches the diode modulator and we have 5 volts here to apply to the arm of rhe balance potentiometer.

Wliile we are talking about the ring modulator it could be mentioned some of the earlv er photographs of the rig showed the balance lot above the chassis on a bracket, it was ater moved to a panel control, which in conjunction with a pF trimmer capacitor nulls out die carrier. The pF balances out the fixed 30 pF added on the other side of the modulator to obtain a greater null. This is necessar> because the pot has a metal cover and being bolted to the chassis has unbalanced capacitance. it was found un-necessary to use the extra capacitors if the pot was insulated from the chassis.

The vfo

Because of the high value oi shunting capacitance across a small inductance this oscillator is mechanically and frequency stable. To me it is amazing having built many type of oscillators that seemed to drift forever, 'i icon is probably un-necessary since the 180 pF variable capacitor from the ARC-5 transmitter padded with a 50 pF APC type capacitor, and 12 turns of #20 en. wire wound on a % inch ceramic form just tunes the 3.75 to 4.0 MHz amateur phone band, 'he oscillator itself ol course is tuning 3345 to 3555 kHz,

A cathode follower was used after (he VFO to prevent any pulling effect on the oscillator which might or might not occur. If the VFO was just used in a receiver the follower would not be needed. It might seem strange that a 9002 tube was used for the oscillator in place of the 6C4. Either one will work, but the 9002 is smaller and could be fitted between the box and transformer with no room to spare.

No measurements have been made on the oscillator stability, but from a cold start it does not drift off from a SSB signal. It has been

VFO box. Note the coil wound on the ceramic form.

used in several receivers with excellent results without any temperature compensating.

Receiver

Pages have been writen in QST on tlie merits of using the 7360 tubes as a front end and mixer because of its low inter-modulation. We used it to do away with an rf stage. It was lound the grid coil should be an air wound large coil it possible for the best selectivity. The 4.5 MHz sound if cans were tried and found too broad. Because of room restrictions a compromise was used by winding 28 turns of #28 w ire on a XR-50 coii form with ten turns on the bottom for the antenna. It works half way between the two other coils and is satisfactory, During the experiments with this tube several rules must be followed, the supply must be set at 200 volts by adjusting the 10 kQ 2 watt value of resistor. Otherwise tiie 1200 ohm cathode resistor value might have to be changed, In some instances the plate load resistors have to be lowered to 27 kQ and the cathode resistor varied for best output mixed signal if a lot of output is desired, "he cathode value can go as low as 600 ohins. When building this mixer observe that the accelerator has 175 volts on it, the two plates 150 volts and 2 to 4 volts of rf from the V O to swing the beam. If these tolerances are held there should be no problem, AVC can be put on the lube after the receiver is finished and for preliminary adjustment the AVC bus can be taken off and the 100 pF coupling capacitor left out if desired. There seems to be some advantage in using AVC on this tube where strong local signals are heard.

The signal from the 7360 mixer is fed into the mechanical filter which is tuned by a 130 pF capacitor. Whether this value applies to all

7360 Mixer

IOÛ Al FILTER

Fig. 3- Layout of El Mar-rinero as built by Ed Marrinei W6BLZ.

IOÛ Al FILTER

Fig. 3- Layout of El Mar-rinero as built by Ed Marrinei W6BLZ.

Collins filters is unknown but should be very close to a practical va ue„ This one was found by putting a variable capacitor across the filter and tuning it for resonance and measuring the value.

If stages 455 kHz

When the signal comes out of the filter it goes into a 6BA6 if stage, Here is where most constructors run into trouble trying to prevent the stage from oscillating. In anticipation the cathode resistor was increased from its normal value of 68 ohms to 120 or 220 ohms and it is suggested that the new J. \V. Miller 913-C.T, transformers be used. The grid tap on these has been placed one-third of the way down

Final amplifier, tank coit and TR switch.

from the top of the coil to lower the grid impedance. You will notice that the coupling to the second if is taken oif from the primary ol the first if transformer by a .005 pF capacitor. Smile if you will, but it serves two purposes, it allows the mixer grid to return to ground and it gives more gain plus it stops oscillations. This coil incidentally was modified by squeezing the coils to within one-half inch of each other. This can be tricky but by scraping the glue from the rod and inserting a spacer one-half inch thick the coil can be pushed up against i( by taking a piece of wood and drill a half inch hole in it and slipping it over the rod. If this is not done the pi will push out or the coils will two-block and won't come apart, (voice ol experience!) It is necessary to push the coils together to obtain enough coupling to the 12AT7 mixer grid. The coupling from the primary to the second if does not effect the selectivity because we are using a mechanical filter.

Since the placement of the coils in this transceiver layout were six inches apart a piece of RG-I74 was used to bring the signal from the .005 to the grid of the second if tube. This extra capacitance was too much for the 100 pF tuning the first if can and 80 pF had to lie put inside the can in place of the 100 p'l\ The second // can tuned without modification.

Product detector

This product detector is becoming very

+2 0

Post a comment