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McMurdo Silver Company


DESCRIFTION:  Model 701 Transmitter is a modernized, post-war example of the basic transmitter  featured in most recent editions of the A.R.R.L. Handbook.  It consists of a Tritet crystal oscillator driving an 807 beam power amplifier. It goes well beyond the early design in that it incorporates recent tube and circuit improvements, includes 6-meter operation, plus a push-pull modulator for phone operation. Compactly and expertly constructed to size only 10" long, 5" high and 5" deep in cabinet, it is not only a transmitter of extraordinary flexibility and effectiveness, but probably the most powerful, size for size, so far available.

Model 701 circuit consists of a 6AQ5 miniature beam tetrode fuctioning as Tritet crystal
oscillator to deliver output upon crystal frequency, or optionally upon 2nd or 4th harmonic
of crystal frequency. This oscillator drives an 807 beam tetrode power amplifier to a maximum of 75 watts plate input on CW, or to 30 watts input for 100% modulation in phone work.  The push-pull modulator employs two 6AQ5 tubes to deliver 14 to 15 watts audio output sufficient to 100% modulate 30 watts plate input to the final amplifier.  Any good carbon microphone may be plugged into the MIKE jack upon the panel, the transmitter supplying microphone current to this, the most efficient and generally satisfactory type of speech microphone available to the amatuer.  Key jack is provided oscillator cathode circuit to allow break-in operation. Fixed antenna coupling coil allows direct feed to a suitable antenna, or through an antenna tuner.

ACCESSORIES REQUIRED:  Accessories required to put Model 701 in operation, but not supplied
3    6AQ5 miniature beam tetrode tubes (omit 2 for CW operation only)
1    807 beam tetrods power amplifier tube
3    coils (see "coils & Crystals")
1    Carbon microphone with 2-circuit plug (for phone operation only)
1    Telegraph key with 2-circuit plug
1    Crystal in FT243 holder (" pin-center spacing), of frequency for desired band.
1    Antenna and feeder system (see A.R.R.L. Handbook for details)
1    Power supply furnishing 6.3 volts, a.c. or d.c., at 2.25 Amperes for heaters and
300 to 350 volts at 130 MA. d.c. for C-W operation only, 300 to 350 volts at 200 MA. for phone operation.
1    S.P.S.T. switch (or relay) for SEND-RECEIVE control
1    S.P.S.T. switch to cut off modulator power in CW operation.

INSTALLATION:   The transmitter should be removed from its cabinet for tube installation and initial examination. Insert tubes in sockets as in  Fig. 1, placing clip on flexible lead upon top-cap of 807 tube after inserting same in its socket.  Pull antenna feeders through hole in right rear of cabinet and solder their ends to the two free contact lugs found at the upper left (seen from rear) of the upper left 5-pin coil socket.  Pull power cable through cabinet rear hole and remount transmitter in cabinet.

ANTENNA:  Full data upon various types of antennae will be found in the A.R.R.L. Handbook,
as well as upon antenna tuners.  A suitable antenna for operation in the desired band, or bands, should be erected, preferably of a type which will allow use of Amphenol Twin-Lead transmission line for feeders, either 70, 150 or 300 ohm size. For single band operation an antenna cut to proper length may be fed directly from the transmitter itself.  It is preferable, however, to use an antenna tuner to insure feeding all available power into the antenna, where it alone does any good.  The effectiveness of this, or any other, transmitter can usually be increased from two to possibly ten or more times if care is taken to secure proper impedance match between transmitter,  transmission line and antenna. This can be easily and simply accomplished through use of the Model 908 MICROMATCH standing-wave-ratio meter and power meter.

POWER SUPPLY:   A single or dual power supply may be used, connected to the multi-colored leads of the power cable per Fig. 2.  Basically the oscillator (BROWN wire B+) requires 300-volts d.c. at about 30 MA.  The 807 power amplifier (RED B+) requires 100 MA. d.c. at 300 to 350 volts per phone operation, but will take up to 750 volts at 100 MA. for full 75-watt CW telegraph input.  DO NOT OPERATE THE POWER AMPLIFIER AT MORE THAN 30 WATTS PLATE INPUT IN PHONE OPERATION.  Both of these tubes must operate in either phone or CW work. The modulator (BLUE B+) used in voice operation only, requires 70 MA. at 300 volts d.c. For economy's sake the modulator plate supply may be cut off in CW operation, as by an S.P.S.T. switch in series with the BLUE power cable lead.  If this is not done, be sure to remove microphone plug from MIKE jack in telegraph operation to prevent possible simultaneous transmission of voice or room noise.  Removing the key plug from the KEY Jack, or closing shorting-switch of the key energizes the oscillator for voice operation.

COILS & CRYSTALS:  Only fundamentally-cut crystals (not harmonic-cut) types may be used.  An 80-meter crystal, of suitable frequency for doubling to 40 meters or quadrupling to 20 meters, may be used for these three bands. A 40-meter crystal will give output on 40, 20 and 10 meters.  A 20-meter crystal will give output on 20 and 10 meters.  For 6 meter output a crystal ground to  the final frequency (50 to 54 mcs.) is required.  In emergencies output at the 8th harmonic of the crystal may be had by doubling in the 807 plate circuit, though that is not recommended or considered herein.  As herein, considered operation of the 807 power amplifier is always "straight-through", either at cryatal frequency or the 2nd or 4th harmonic thereof, with frequency multiplication occurring in the 6AQ5 Tritet oscillator plate circuit.

Coil selection is as follows:  Tritet cathode coil is always for crystal frequency.  It may be 125-E1a for 80-meter crystal, 125-E2a for 40 meter crystal or 125-E3a for 20-meter crystal. Oscillator plate coil (located above 807 tube base) should be selected for final ouput frequency, as should 807 plate coil.  These two coils are identical in number and construction, though the antenna coupling winding is not used in oscillator plate circuit position.  Thus, for L2 and L3, use one each 125-E1b for 80 meters; 125-E2b for 40 meters; 125-E3b for 20 meters; 125-E5b for 11 and 10 meters; and 125-E6b for 6 meters.

TUNINC:  With tubes, coils and crystals in sockets and power supply connected, temporarily  disconnect RED wire from power supply. Turn on power, set OSC.P knob to 100, and tune CATH. knob for maximum glow of XTAL lamp.  This glow will vary from dull for 80-meter crystals, to fairly bright for 20 meter crystals, is a measure of crystal r.f. current. Advance CATH. knob just a bit above the point of maximum XTAL lamp glow-to insure stable operation and clean keying.  Now, tune OSC.P. knob to produce minimum glow of XTAL lamp.  Turn off power, connect RED wire to power supply and turn power on, and tune AMP.p. knob for minimum glow of AMP.Ip. lamp.  If a 40-watt lamp is connected to the output of the transmitter (two unused contacts at upper right, seen from front, of upper right coil socket), a good estimate of the power output of the transmitter may be had from the lamp's brilliancy.  It is best to first test and tune up with such a lamp as a dummy load.  Insert microphone plug in MIKE jack and talk into microphone.  Note that dummy antenna lamp increases in brilliancy upon whistling. indicating upward modulation. Do not talk into- microphoe loud enough to cause AMP.Ip. lamp to flicker. Insert key plug in KEY jack and see that keying is effective, clean and free of clicking or chirping as heard upon a nearby receiver.  Adjust CATH. knob for chirpless keying.

Power output upon crystal fundamental frequency, or its 2nd harmonic, will closely approximate 66% of actual d.c. plate input.  It will be somewhat less upon 4th harmonic of crystal.

Attach antenna feeders to transmitter and proceed to operate.


C1   			50 mmfd.  tubular ceramic 20%
C2   			6 mfd., 350 volt tubular dry electrolytic 20%
C3, C3a			.002 mfd. molded mica  2O%
C4			100 mmfd. air capacitor
C4, C4a			120 mmfd. air capacitor, wide-spaced
C5, C5a, C5b, C5c C5d	.02 mfd,  600 volt 20% (C5d across heaters)
R1    			50 kilohms,  watt 20%
R2			21 kilohms, "   "   "
R3			43 kilohms, 2   "   "
R4			2500 ohms,  "   "   " 
R5			120 ohms,   "   "   "
R6			50 kilohms, 4   "   "   (2-100 K, 2 W. resistors in parallel)
R7    			120 ohms,   2   "   "
R8			100 ohms,   1   "   "
M1, M2			Mazda #47 Panel Lamps


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