Modified For Use With Twovolt Tubes

Components have the values given in Fig. 713-A tcith the following additions: Au, 50,000-ohm potentiometer; R12» 5000 ohms, 1-watt rating; Ru, 10-ohm rheostat; RFCi, same specifications as given in Fig. 707. Ril and Rl2 constitute the gain control circuit in the r.f. amplifier; fiia is used to make certain that a small amount of grid bias will be applied to the tube even though Rn is set at its minimum-bias end. Filament supply may be from an Air-Ceil battery or from two dry cells connected in series.

To prevent "B" and "C" battery discharge through the voltage-dividers when the receiver is not in use, switches may be installed in series with the " —22.5" and "+45".

operation from an a.c. power pack which will deliver 2.5 volts a.c. for the filaments of 58 and 56 tubes, or 6 volts d.c. for 6D6 and 76 or similar 6-volt types, with 200 volts d.c. for the plates. Voltages for the screen grids are obtained by means of voltage dividers and series resistors. If "B" batteries are to be used resistors R:> and Re may be omitted and a separate lead brought out from Bio to the 45-volt tap on the "B" battery, as shown in Fig. 713-B.

Resistor R3 of Fig. 713-A, or Ru of Fig. 713-B, controls the amplification of the r.f. tube by varying the bias applied to its grid. The advantage of such a control is that it permits reducing the strength of strong signals and thus prevents the detector from "blocking" or "pulling in." A strong signal will occupy much more space on the dial than a weak one unless its strength can be reduced. The sensitivity control does this and thereby greatly increases the effective selectivity of the receiver.

The antenna input has been arranged so that a doublet antenna can be used with the receiver. With an ordinary antenna and ground, one of the antenna posts should be connected to the ground post to complete the circuit.

Should the set not work right at the first trial, check over the wiring and apply the tests outlined elsewhere in this chapter. These tests also apply to the two-tube re-ceiverspreviously descn be d.

Noticeably weak signal response will result with an open antenna coupling coil or open connection in the antenna-ground circuit. A shorted grid condenser, either in a detector circuit or an r.f. amplifier using capacitive coupling, will have the same effect. This may be checked by removing the grid resistor, which should cause the periodic clicking sound in the output. Shorts of this kind can be caused by a blown condenser or by soldering paste smeared between the terminals. Needless to say all soldered connections should be thoroughly wiped with a clean cloth to prevent such leakages.

A regenerative receiver may "howl" just as the detector starts to oscillate. This "fringe howl" is most likely to result with transformer or impedance-coupled detector output and the best precaution against it is to use an audio transformer or choke of the better grade rather than one of the cheaper type with inadequate primary windings. If it does occur with the transformer that must be used, however, it


RECEIVER Li and £3 — See coil table of Fig. 708. 1.2 and L4 — Primary windings. All are close-wound with No. 36 d.s.c. wire, at bottom end of coil form. No. 1 has 20 turns; No. 2,15 turns; No. 3, 10 turns; and No. 4, 5 turns. Ls — 1080-henry detector plate coupling choke (Thor-

darson T-2927). Lg — Output plate choke (30-henry 50-ma.). Cl,C2—Double-gang band-spread tuning, 100-^fd.

per section (National 2SE-100). ('::. C4 — Band-set tuning, same as above. C5 — 75-fififd. midget antenna coupling condenser

(National VM-75). Ce — 100-unfd. mica condensers. C7 —- 0.01-fxfd. tubular paper condensers. C& — 0.001-[tfd. r.f. by-pass condenser. C9 — 1- to 4-tifd. screen-grid by-pass (400-volt paper or electrolytic). c,0 — 10-nfd. electrolytic cathode by-pass. cll — 0.5-nfd. 400-volt output coupling condenser (paper).

Rl — 300-ohm \'2-unit cathode resistor, i?2 — 10,000-ohm variable resistor (r.f. gain control).

R4 — 100,000-ohm V2-watt screen voltage dropping resistor.

R5 — 75-ohm filament center tap resistor. Rq — 0.5- to 5-megohm grid leak (to give desired sensitivity and smoothness of regeneration control).

R~ — 50,000-ohm potentiometer (regeneration control).

R& —20,000-ohm 10-watt voltage divider resistor. ffl — 2500-ohm 1-watt divider resistor. R10 — 0.5-megohm Vi-watt audio coupling resistor. Ril —400-ohm 1-watt audio cathode resistor. RFC—2.5-mh. r.f. choke (National Type 100). S'Pi — S.p.d.t. toggle switch (limiter control).

can be reduced or eliminated by connecting a resistor across the secondary of the audio transformer. In most cases a resistance of 100,000

ohms will be sufficiently low. A grid leak of lower value also may help in some cases. These expedients reduce the receiver output, of course, and must be considered as less desirable than the substitution of an audio coupler having better characteristics.

"Stringy quality" or poor base-note response usually can be traced to an open or inadequate bypass capacitance in a detector or audio amplifier circuit. Too-small capacitance across a cathode resistor is a common source. An open or too-small grid condenser in a grid-leak detector also may be the cause of this trouble.

T.R.F. Receiver With Ganged Band-Sprea'd and Band-Set Tuning

• A compact receiver chassis in which band-spread and band-set tuning is accomplished by double-gang condensers is illustrated in Fig .715. The circuit diagram is given in Fig. 716. This particular layout is both electrically and physically balanced, making the controls symmetrical on the front. The circuit is adaptable to either the metal or equivalent glass type tubes. The tuning system uses the tapped-coil method of band-spread, as in the two-tube pentode receiver, and the coils are of the same specifications except for the primary windings. Both coils for each band are wound the same, although the cathode tap is not used on the r.f. coil. However, with the r.f. and detector coils interchangeable it is unnecessary to pick out each one separately when changing bands.

The 16-gauge plated steel chassis measures 7 by 12 inches and is inches deep. The double-gang tuning condensers are centered

2)4, inches in from each side, the band-spread gang on the left and the band-set gang on the right as viewed from the front. The coil sockets are positioned next to the band-spread gang with the band-spread tap terminals towards


the condenser so as to give short leads. Grid end connections of stiff bus wire run across to the band-set condenser stator terminals. The 1/16-inch aluminum baffle shield between the two stages is 5 inches wide by 4 34 inches high, extending about 1 inch above the tops of the coil forms. The coil sockets are mounted to give approximately %-inch spacing between the coils and the shield.

Primary leads and the cathode feed-back lead of the detector coil run through holes in the base directly below the respective socket terminals. The tube sockets, between the coil sockets and band-set condenser gang, are sub-panel mounted with their filament terminals toward the coil sockets. This gives a short connection for the detector cathode, which is important in this regenerative circuit. The grid condenser and leak for the detector, in front of the baffle shield, are soldered directly to the bus wire running from the detector coil grid terminal to the band-set condenser. As shown in the bottom view of Fig. 717, the r.f. gain control is to the left of center and the regeneration control to the right. Between these two controls is the audio amplifier screen voltage switch. This is a single-pole double-throw toggle which changes the screen-grid voltage either to full value for maximum power output or to about 30 volts for reduced output and limiting action, as described in the previous chapter. The reduced voltage is taken from the detector screen voltage divider, across R 9, with which the regeneration control is also in parallel. To the extreme right, as viewed from the bottom, is the detector plate audio impedance and to the extreme left is the output coupling choke of the audio amplifier. The antenna and ground terminals are at the back of the chassis, as is also the antenna or "trimmer" condenser connected in series with the primary of the r.f. coil. This condenser serves to line up the input circuit to suit different antenna characteristics, usually requiring only a single adjustment for each amateur band. The audio output jack, to which either magnetic or crystal headset, or a speaker, can be connected is at the rear-left. The detector screen-grid by-pass condenser C9 is in the rear-right corner. Other components are located to give short connections.

Although no individual trimming is provided for the different coils, no trouble should be experienced in getting proper alignment. There is actually considerable tolerance in the alignment of the single r.f. stage. Adjustment of the antenna coupling condenser for "peak" performance is adequate, provided the coils are wound in exact accordance with specifications. The tuning of this receiver follows the same procedure given for the two-tube pentode set. The audio screen voltage switch will usually in the low-voltage position for headset reception, especially on c.w. telegraph. A power supply especially designed for regenerative receivers, as described in Chapter 15, must be used to prevent excessive hum. It should be capable of 60 ma. or so at 250 volts for maximum receiver output.

Single-Signal Superhet With Regenerative I.F. Stage

# The six-tube regenerative single-signal superheterodyne shown in Figs. 718, 719 and 721 is illustrative of the design and construction of amateur high-frequency superhets. It has a preselector stage, first detector with separate oscillator, a stage of regenerative i.f., power second detector, and separate beat oscillator.

The photographs show the general arrangement and Fig. 720 gives the wiring diagram. The left-hand shield in Fig. 718 contains the high-frequency oscillator. Directly behind the drum dial is the 2A5 second detector. In the center compartment is the first detector and its tuning circuits, with the oscillator coupling condenser, while in the right-hand compartment is the r.f. preselector-amplifier.

On the back deck, at the extreme left, is the c.w. beat oscillator coil and condenser unit, 7's, with the beat control knob projecting at the top. Next is the c.w. beat oscillator tube. The center can contains the i.f. transformer assembly) Ti, with the i.f. amplifier tube to its right. At the extreme right is the regenerative i.f. transformer assembly, T\.

Looking at the front of the panel, the upper row of knobs are, left to right: h.f. oscillator tank, C5; first detector tank, C4, and r.f. tuning condenser, Ci. At the bottom of the panel, the left-hand switch, SW1, controls the high voltage supply to the receiver. Next is the c.w. beat oscillator "on-off" switch, SW2, cutting the screen voltage. The knob below the illuminated dial is the main tuning control operating the ganged condensers C2 and C3, with the gain control, R3, next. The knob at the right operates the i.f. selectivity control, the regeneration attenuator R9.

Doublet antenna connections are made to insulated binding posts on the outside shield of the r.f. stage, with the ground binding post nearby on the main deck. With a conventional single-wire antenna connected to one insulated post, the other is connected to ground. Of course the doublet antenna should be used if possible, since it makes possible considerable additional gain.

Insulated 'phone tip jacks on the left end of the chassis provide connections for 'phones and speaker.

Once the tank condensers have been set for a given band, the selectivity adjusted to the desired degree, and the c.w. beat note fixed, the receiver is in effect single-dial tuning with operating controls for volume, frequency and c.w. note convenient for one position of the hand.

The structural part of the receiver is all of sheet aluminum. The chassis or main deck is made from a piece of 3/32-inch aluminum 21 inches by 12 inches. From two corners on one long side of this piece, 2-inch squares are cut out and then three sides are bent down at right angles so as to form the sides and back of a deck 17 by 10 inches and 2 inches high.

All of the inter-stage box shields are cut from 1/16-inch aluminum. The six sides are 7 inches long by 4% inches high, while the three ends are 4 J<£ inches wide by 4% inches high. The shields are held together at the corners by }4-inch square brass rods drilled and tapped for 6/32 machine screws. The corner posts are fastened to the main deck by screws into their lower ends.

The front panel is of K-inch thick aluminum, 18 inches long by 7 inches high. It is fastened by screws to the front posts of the shield boxes. A cover fitting over all the shields is a sheet of 1/16-inch aluminum 16 inches by 7 inches held in place by flat springs on its under side, pressing against the sides of the shield boxes.

The Isolantite five-prong coil sockets are mounted above-deck on pillars long enough to clear the contacts. Similar tube sockets (six-prong) are mounted below the base under their 1 J^-inch holes. With this arrangement a minimum of wires need pass through the base. Complete tube shields are provided for all tubes. A J/2-inch length of J^-inch rubber tubing slipped over each grid wire, before soldering on the grid clip and afterwards pushed up on the clip, prevents any possible grounding of the grid on the grid-cap shield.

The National 500-kc. i.f. transformers each require minor alterations to adapt them to the circuit, and they should be removed from

FIG. 718 — A SIX-TUBE REGENERATIVE SINGLE-SIGNAL SUPERHETERODYNE RECEIVER A preselector stage, a separate high-frequency oscillator, and a high-gain i.f. stage with controllable regeneration make this receiver an outstanding performer, (W1EAO.)
Superheterodyne Band Spread
FIG. 719 — A REAR VIEW OF THE REGENERATIVE SINGLE-SIGNAL SUPERHETERODYNE This supplements the front view of Fig. 718 and shows more clearly the construction of the intermediate-frequency amplifier.

their cans for this purpose. The first operation is on the regenerative i.f. transformer, T\.

As supplied, the grid coil, I/7, is at the upper end of the dowel, nearest the condensers, and the plate coil at the bottom. In order to couple the tickler coil, to the grid coil, the external connections from the unit T\ must be changed so that the grid coil is the lower one. This means that one of the wires that normally passes out through the bottom of the can should be brought out the top through a piece of shield braid; and the wire originally at the top is brought out through the bottom.

A one-inch length of J^-inch dowel is fastened by means of a wood screw to the end of the dowel carrying the coils in the unit. At the lower end of the new dowel, the tickler L& is bunch-wound with 25 turns of No. 30 d.s.c. wire. If this tickler is wound in the same direction as the other coils, the final connections from Tiare as follows: Inside end of upper or plate coil to B+, outside to first detector plate through shield braid; inside end of middle or grid coil L7 to ground, outside through shield braid from top of can to grid cap of i.f. amplifier; inside end of lower or tickler coil Ls to i.f. suppressor, outside end through shielded lead to i.f. cathode. If the i.f. circuit cannot be made to oscillate with Bg in the maximum resistance position or disconnected, then the tickler connections should be reversed at the coil terminals. If oscillation should fail with the tickler connected either way, the number of tickler turns should be increased a few at a time until oscillation is obtainable.

For T2 the connection out of the top of the shield is removed and brought down inside to the detector grid condenser and leak which are placed within the can. Plate and grid leads from T2 also should be shielded with flexible copper braid.

In the beat oscillator unit the grid condenser and leak are also mounted within the can. The only other operation required is to shield the grid lead from the top of the can to the oscillator tube.

The high-frequency oscillator coupling condenser C^ is made of two brass angles, having faces about % by % inch, mounted on a small piece of bakelite in the detector compartment with the faces spaced inch. The connection from the plate of the h.f. oscillator to C7 is in shielded braid but may be left unshielded.

The coils are wound on National 5-prong forms according to specifications given in the

FIG. 721 — A BOTTOM VIEW OF THE SIX-TUBE SUPERHET By-pass condensers and resistors are placed in the most convenient locations. The detector output transformer is mounted on the side wall of the chassis, and can be seen in the lower right-hand corner.
2004 Cruiser Wiring Diagram


Center tap résister ond neqative connection , tn Power Pock



Center tap résister ond neqative connection , tn Power Pock


Dotted lines indicate shielded leads.

Lq — 500-kc. beat oscillator coil. (See text.) Ci — 140-nnfd. midget condenser (Hammarlund MC-140M).

C2, C3 — 25-fiufd* midget condenser (National SE-50

cut down to 3 stator plates). C4, C5 — 100-nnfd. midget condensers (Hammarlund MC-100M).

Ce — 70-nnfd. midget condenser (ire National i.f. units).

C7 — Zf./. oscillator coupling condenser. (See text.) Cg — 0.01-fifd. r./. by-pass condensers, tubular paper. C9 ami C10 — 250-npfd. mica grid condensers. Cn — 1-fxfd. audio by-pass and coupling condensers. Cjs — 250-tififd. plate by-pass condensers, tubular paper.

Rl — 50,000-ohm 1-watt oscillator grid leak. /?2 — 5,000-o/im 1-watt first detector cathode resistor. — 12,000-ohm variable resistor, right-hand taper (Electrad). «4 — 100,000-ohm 1-watt. R5 — 10,000-ohm 5-watt. Re — 7,000-ohm 2-watt. R7 — 3,000-ohm 2-watt.

table. No attempt has been made to make the tuned circuits track exactly. The over-all gain of the receiver is high enough so that, by-judicious use of the gain control, c.w,. reception is possible throughout an entire amateur band without touching the tank condensers. Better tracking can be secured easily by removing a

R$ — 2,000-ohtn variable resistor, left-hand taper {Electrad).

RiO — 300-ohm 1-watt (i.f. amplifier cathode resistor).

/?12 — 1-megohm fy2-watt second detector grid leak.

Rl3 — 50,000-ohm 1/2-watt beat oscillator grid leak (.Integral with National oscillator unit).

R17 — 20-ohm center-tap resistor (in power supply).

Ti and T2 — National 500-kc. air-tuned i.f. transformers. (See text.)

T3 — National 500-kc. beat oscillator assembly.

I4 — Universal push-pull output transformer (Ken -yon).

RFCi — 2lA-mh. sectional choke (National No. 100).

RFC2 — 10-mh. single-section universal wound r.f. choke.

RFCz — 60-mh. single-section universal wound r.f. choke.

SW\ and SWu — Single-pole panel switches.

few turns of wire from the oscillator coils A further refinement would be to gang an additional condenser, similar to C2 and C3, for the r.f. amplifier.

The power supply leads are brought in through a flexible cable in the rear. The B-f-voltage is conveniently distributed from a terminal strip attached to SWi- Although only four wires are essential to the power supply cable, cables with four wires having two which are of suitably low resistance for heater currents are not readily available. Accordingly, a standard 8-wire cable is used with three wires in parallel for each of the heater leads. By this means the filament voltage drop from power supply to set is kept to a value of less than 0.1 volt. Care must be taken, however, that all the paralleled wires are securely soldered to the terminal plug at the supply end of the cable.

The power supply may be of the superhet type described in Chapter Fifteen. The filament winding of 2.5 volts should be capable of delivering the 8 amperes necessary for the tubes and dial light. High voltage under 50-ma. load should be approximately 180 volts.

To align the i.f. amplifier, set the selectivity control at minimum selectivity, and apply a 500-kc. signal to the grid of the i.f. tube. The second i.f. transformer is then adjusted to resonance as indicated by maximum second-detector output, an insulated socket wrench being used to tune the condensers C$ at the top of the can. The oscillator is then coupled to the first detector grid and the same procedure is used to tune the first i.f. transformer. The beat oscillator may be isolated from the second-detector circuit and used as a signal source, but preferably a separate test oscillator should be used. If a modulated signal is used, the output can be judged by ear. For an unmodulated signal a 0-50 milliammeter should be placed in the plate circuit of the second detector, when resonance will be indicated by plate current dip to minimum.

After aligning the i.f., the high-frequency circuits are aligned, using an oscillator or frequency meter giving a signal in an amateur band. The three condensers Ci, C2 and C3 will have nearly the same settings, although the oscillator (being tuned 500 kc. higher than the detector) will have a somewhat lower capacity setting.

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