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A beginner's two-stage transmitter

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The "Longfeller" - on wood or metal.

At a recent hamfest in Rochester, N. Y., the writer mentioned that he had just finished a two-tube beginner's c.w. transmitter built without tools other than pliers, a soldering iron, a screwdriver and a hacksaw blade. The toastmaster, WRATH, commented that he could not see how an old timer like W2OEN could have gotten by, all these many years, without discovering long ago that ham gear was usually built without any tools other than Mom's butcher knife and the two-bit pliers from the OM's Model T.

The real truth is that with the modern trend toward complexity, the simple things in amateur radio have been somewhat neglected, with the result that a beginner is faced with the immediate problem of learning how to become sheet-metal worker, journeyman-assembler, and wireman before he can put his newly-acquired (but sometimes limited) radio knowledge to work. He either has to learn those trades and obtain some almost-impossible-to-find tools, or use the "good old breadboard" style of construction. And even breadboards are reported to be in short supply these days.

So, with this situation in mind, the problems of the genuine beginner (who has neither tools, or much knowledge of amateur design) were recently approached anew by Don Mix, W1TS, and the writer. Don had already started the ball rolling with his article aimed at the Junior Constructor in QST for June, 1941. Then in July of the same year W1CTW described a two-stage portable-emergency transmitter having neat circuit features. What about a 1946 combination of the simple wooden-frame construction with W1CTW's circuit features? "Can do?" asked WITS, and W2OEN replied, "Wilco." Thus was born the "Longfeller," a neat-appearing two-stage beginner's transmitter and power supply for the amateur without a drill or a socket punch.

Specifications for the "Longfeller"

Basic specifications included ease of construction with a minimum of tools, a noncomplicated circuit furnishing two-band output from a single crystal, a built-in antenna tuner that would feed anything from a piece of wet string to a farmer's wire fence, and a layout utilizing low-cost components that could be purchased now - without any back-orders.

The chosen circuit has proven reliable in many varied forms. It consists of a 6V6G Pierce oscillator operating on the crystal frequency with an untuned plate circuit, capacity coupled to a 6V6G or 6L6G output tube having a shunt-fed plate circuit that may be operated either on the fundamental frequency or on the second harmonic of the crystal in the 3.5- or 7.0-Mc. bands. Operation on the higher frequencies is not recommended for the "Longfeller." Although the transmitter will work on the higher harmonics, their output is not recommended for use on the air in order to assure ease in tuning and afford sure-fire operation.

Photo 1
The complete "Long-feller" - lacking only an antenna to make it a whole transmitting station. This photograph depicts the construction and layout of the wooden-chassis transmitter, power supply and key filter. Shunt feed eliminates high-voltage d.c. from the top of the chassis. Coil and both condensers have r.f. present under "key down" conditions.

Two variable condensers in the output circuit permit proper loading to almost any length of wire. Shunt feed keeps d.c. off the coil and condensers. However, these components are "hot" with r.f. when the key is closed.

The transmitter crystal-output-frequency combinations are as follows:

 Output Operation and Frequency
Crystal Frequency
(Straight Through)
Second Harmonic
1.75 to 2.0 Mc.Not used3.5 to 4.0 Mc.
3.5 to 3.65 Mc.3.5 to 3.65 Mc.7.0 to 7.3 Mc.
3.65 to 4.0 Mc.3.65 to 4.0 Mc.Not used
7.0 to 7.3 Mc.7.0 to 7.3 Mc.Not used

Break-in operation is provided as the cathodes of both tubes are opened by the key. A key-thump and key-click filter are included in the complete transmitter layout. (See Fig. 1.)

Fig 1
Fig. 1. Schematic diagram of the "Long-feller" - a two-stage beginner's transmitter built on two wooden chassis.

Partlist fig. 1
C1,C3.002 µF mica.
C247 pF mica.
C4470 pF mica.
C5,C6,C7220 pF mica.
C8.001 µF mica.
C9,C10220 pF variable.
C11,C1216 µF 475 V electrolytic.
C131.0 µF 400 V paper.
C140.5 µF 400 V paper.
R1,R347,000 ohms, 1 watt.
R2,R6100 kohm, 1 watt.
R422,000 ohms, 3 watt.
R5,R10330 ohms, 1 watt.
R7,R815,000 ohms, 2 watts.
R920,000 ohm 10 W wirewound.
L1Tank coil for 3.5- or 7-Mc. band depending on output frequency desired.
3.5 Mc. - 32 turns No. 20 enam., 1½ inches long.
7.0 Mc. - 16 turns No. 20 enam., 1½ inches long.
Both coils are wound on 5-prong Hammarlund coil forms, 1A inches in diameter.
Manufactured coils (five-prong 80- and 40-meter "End-link") may be used.
L2Filter choke, 10 H, 130 mA, 100 ohms (Stancor C-2303).
I160 mA 2 V pilot lamp.
P1Chassis-mounted power plug, five prong.
P2Power-cable socket, five prong.
P3A.C. line-cord plug.
RFC2.5 mH r.f. chokes.
TPower transformer. 350 volts each side of centertap; rectifier filament winding, 5 volts, 3 amperes; r.f. filament winding, 6.3 volts, 4.5 amperes (Stancor P-4080).
V16V6G or metal equivalent.
V26V6G or 6L6G or metal equivalent.
V3Rectifier tube, Type 80.

After the "Longfeller" (wooden version) was completed a tally of the costs showed that all parts for the complete transmitter and power supply (with all tubes, a $2.80 crystal, one coil and an inexpensive key) could be purchased at amateur net prices for approximately $26.00. Standard components were used throughout. The quality and type of components used permits their re-use in a future and possibly more elaborate transmitter. The metal-chassis unit would cost a couple of dollars more.

Chassis construction

The wooden chassis was made from pieces cut from a length of ¼ × 1_3/8 inch white-pine strip, known locally as "lattice stripping." It cost 2½ cents per linear foot. The width of this stripping varies but this does not affect the construction of the chassis provided the lattice is at least 1¾ inch wide. The only critical dimension on either the transmitter or power-supply chassis is the spacing which should be the exact width required to clear the holding ring of a tube socket. For the sockets used, this dimension is 1¼ inches.

To make the transmitter chassis, four pieces 15½ inches long were cut for the top, front and back sides (Detail A in Fig. 2). Two more pieces were cut 4 inches long to make the ends (Detail B in Fig. 2). These pieces were nailed together, using 1-inch wire brads, to form a frame with an open space running the length of the top, leaving just enough room between the top-inside edges to place the tube sockets crosswise on the chassis. (See Fig. 2.)

Fig 2
Fig. 2. Working plan of the wooden chassis for the transmitter. (The chassis for the power supply is constructed in the same manner, and its dimensions are the same except for the length, which is 9¾ inches instead of 15½ inches.)
The functions (and direction of the socket key) of the various top-chassis-mounted components are:
1 - Male power plug, key toward left end of chassis.
2 - Crystal socket, key toward front of chassis.
3 - Oscillator V1, tube socket, key toward right end of chassis.
4 - Output tube, V2, socket, key toward front of chassis.
5 - Tuning condenser, C9.
6 - Coil socket, key toward left end of chassis.
7 - Loading condenser, C10.

After assembly, the chassis were given several coats of grey lacquer.

The power-supply chassis was made in the same manner, using 9%-inch sides and top strips (Detail A) and 4-inch end pieces (Detail B).

Assembly and wiring

Fig. 2 shows the center-line spacing for the various components mounted on top of the chassis. Thin M-inch round-headed wood screws fasten the sockets to the strips. The variable condensers were mounted with 6/32 machine screws inserted from underneath the top strip, and shimmed with small pieces of wood cut to shape. (See the photograph for a close-up view.) Fig. 2 also includes the function of each chassis-mounted device. A 5-prong male chassis plug was used on the transmitter to permit an enclosed (female) receptacle on the end of the power cable, in line with the ARRL Safety Code. An octal socket was installed for use with the 3%-inch-spaced crystal holders.

After fastening the sockets and condensers in place, the wiring was begun. A bare bus wire was placed in first and connected to power-plug Pins 4 and 5, to form a "ground" along the chassis. The "hot" filament wires were run in next. Bypass condensers were placed in position and connected between their respective socket lugs and the ground wire. Starting with crystal-socket Pins 1 and 3, the various circuits were wired progressively to the right. Tie-points were used to advantage as shown in the photographs. The r.f. leads connected to the grids and plates were kept short and direct. Resistors carrying d.c. were arranged to place them in suitable locations and avoid crowding. Unused tube-socket pins were not used as tie points, thus avoiding complication and difficulty in servicing the unit.

Photo 2
Looking underneath the "Longfeller." That "clean" look is attributable to careful arrangement of components and leads. Machine screws (shimmed by small blocks of wood, whittled to size) hold the variable condensers in place. The pilot-lamp bulb is located in a protected position where it can be seen but not touched, as there is 350-volts d.c. on its terminals. The ground bus bar extends from the power-supply socket across the chassis and connects to the frame of the loading condenser. All ground connections are made to this bus bar. The right-hand r.f. choke is in the plate circuit of the oscillator tube and the choke on the left is in the plate feed to the output tube. Components are grouped around their respective sockets for convenience and for short r.f. connections.

The wiring around the tubes was placed first. The r.f. output circuit was then completed using bare tinned wire. The pilot-light bracket was mounted on an insulated tie-point and connected between the cold end of the r.f. choke in the output tube's plate circuit and B +. A 60 mA 2 volt pilot bulb serves as the plate-current indicator for the output tube.

By placing the coil socket as shown (with Pin 3 towards the right end of the chassis) the r.f. leads are short and the link winding on the coil is at the "cold end" of the circuit, if desired for use with a low-impedance line.

The stators (fixed plates) of the two variable condensers, C9 and C10, are connected to the tank coil, L1, and the rotors (frames) are grounded to the common-ground bus.

A Fahenstock clip is fastened to the stator of the antenna loading condenser C10 for an antenna connection. Another such clip is fastened to the frame of this condenser for a ground connection.

Key-thump and key-click filter

A key-thump filter, shown in Fig. 1 and in the photograph, was made up on small pieces of the lattice strip for installation directly at the key. On-the-air tests revealed that more effective thump and click elimination was obtained by connecting the filter right at the key and not in the transmitter. The ground side of the filter circuit should be connected to the frame of the key. The values used in this filter were found after a series of tests with various circuit combinations, using the "Longfeller" and a broadcast receiver under actual operating conditions. As a result, keying is clean and free from clicks.

Power supply

The power supply includes a broadcast replacement-type transformer (350 volts each side of center plus 5- and 6.3-volt filament windings) and a condenser-input filter circuit comprised of a 10-henry choke and two 16 µF 475 V electrolytic condensers. A 20,000 ohm wire-wound 10 watt bleeder resistor was used. Approximately 350 volts d.c. is available with a load of from 50 to 60 mA on the output tube.

Photo 3
The complete metal "Longfeller" with dials and everything. This transmitter and associated units incorporate the same circuit and group of components as those included in the original wooden "Long-feller" but it has a few added conveniences. The key and its shielded filter are mounted on a sheet of aluminum which may be screwed to the operating table. The pilot lamp protruding from the chassis (through a rubber grommet) serves as an output-tube plate-current indicator. The toggle switch is the on-off control in the a.c. line. The insulated antenna binding post stands out like a miniature "lighthouse in the fog." Fahenstock clips (not shown) on the rear apron serve as ground and key connections.

Bench-testing the transmitter

After the unit was completely wired, two 6V6G tubes, a crystal and a coil were inserted in their respective sockets. As 3.5 Mc.-band output was desired, both coil and crystal were chosen for that frequency. The loading condenser, C10, was turned so that it was completely meshed and the tuning condenser C9 was set about one-third in. The line cord was plugged in and after a 30-second wait for the tube filaments to heat the key was closed. Fortunately, no wiring errors had been made and the pilot light glowed at about half brilliancy indicating that the output 6V6 plate was drawing current. Rotating the tuning condenser C9 tuned the plate circuit to resonance and the pilot bulb went out indicating the normal condition of a low value of plate current.

Photo 4
There's nothing much to the underside of the power supply, just a pair of electrolytic condensers and a wire-wound bleeder resistor. The leads of the transformer were curled up to eliminate cutting them short, just in case they should be needed "next time." The cable connecting the power supply to the transmitter is terminated on an insulated tie-point strip.
No - the transformer does not normally hang in midair. We wanted you to be able to read the labels on the electrolytics without standing on your head!

A "loop and a lamp" was then made up, consisting of a pilot-lamp socket connected in parallel with a one-turn coil about 2 inches in diameter. A 6-8 volt 0.250 amp. bulb was inserted in the socket and this absorption loop placed near the transmitter tank coil. Maximum brilliancy of this absorption-loop lamp was obtained when the plate circuit was in resonance and loose coupling was required to prevent burning out the pick-up lamp. About one-third of the capacity of Cg was required to hit resonance on the 3.5 Mc. band.

A long-wire antenna was connected to the antenna post and a ground connection made to the frame of the loading condenser. By adjusting the loading condenser C10 and then resonating the circuit by means of the tuning condenser, C9, a point was reached where only a slight dimming of the plate-current lamp was noticed. As the loading-condenser capacity is decreased (plates unmeshed) the loading is increased. If the circuit is loaded too heavily (indicated when the bulb will not dim on resonance) increasing the capacity of the loading condenser slightly will reduce the loading on the circuit. A pilot lamp (6-8 volts, 0.250 amp.) may be inserted in series with the antenna and the loading and tuning condensers adjusted to give maximum brilliancy of this lamp, indicating that maximum antenna current is being obtained. It is wise to disconnect this test lamp after the antenna has been tuned.

Both 6V6s and 6L6s and their glass equivalents were tried in the "Longfeller's" output stage. The 6L6s gave more output. Either a glass or a metal 6V6 worked well as an oscillator.

Harmonic operation

Tests were made using 3.5 -Mc. crystals with a 7 Mc. coil and slightly-less output was obtained on the second harmonic. 7 Mc. crystals and coils gave good output on the fundamental but when a 14 Mc. coil was used, the 7 Mc. crystals gave good output performance but with a slight chirp in the signal. The operation of the "Longfeller" on harmonics above the 7000-7300 band requires critical adjustment of the tuning and loading condensers. Also, an absorption wavemeter is required to determine to which harmonic the output is tuned. Therefore, operation above 7300 kc. is not recommended. However, the unit was intended primarily for operation on either the 3.5- or 7 Mc. c.w. bands, and it does that with excellent results.

Voltages and Currents

Measurements of plate and screen voltages and plate currents, taken under test, are as follows:

Table 1
Crystal (Band)Output (Band)Tube (Amp.)Plate VoltagePlate Current (Loaded)Screen Voltage
3.5 Mc.3.5 Mc.6V6G360 V40 mA*220 v.
350 v.50 mA220 V
3.5 Mc.3.5 Mc.6L6G350 V60 mA220 V
3.5 Mc.7.0 Mc.6V6G350 V50 mA**220V
3.5 Mc.7.0 Mc.6L6G350 V60 mA220 V
7.0 Mc.7.0 Mc.6V6G350 V50 mA***220 V
7.0 Mc.7.0 Mc.6L6G350 V60 mA220 V.
6V6G170 V 90 V

* 10 mA unloaded.
** 20 mA unloaded.
*** 5 mA unloaded.

The input (15 to 21 watts) varied with the type of tube and with frequency, depending on the loading of the output tube. In actual on-the-air use, about 15- to 18-watts input is normal with the transmitter properly loaded.

A simple antenna for the "Longfeller"

A simple easily-erected antenna for this transmitter is a "long wire," strung up as high as possible in the clear, and in as straight a line as the area will permit.

This end-fed antenna should be a half-wave long on the 3.5-Mc. band and a full wave on 7.0 Mc., and should load properly and easily using the built-in antenna matching network.

The antenna should be put up so that the length of the lead-in and the "flat top" (either "straight out" or put up as an inverted-L) will be approximately 133 feet long over all.

The "Longfeller" on metal

After the completion of the "Longfeller," a single-metal-chn'.sis job was built using the same circuit and values of components but with the addition of a few refinements such as dials, an a.c. on-off switch, and a versatile socket-andplug arrangement permitting the connection of an external source of power.

A 6 × 14 × 3 inch metal chassis was laid out and drilled and the components mounted as shown in Fig. 3. Drills and a socket punch were used to make the required holes. The dimensions shown may vary slightly with different components and are given merely to show a placement that worked out satisfactorily and one that may be used as a guide.

Fig 3
Fig. 3. Working plan of the layout of the 6 × 14 × 3-inch metal chassis for the complete transmitter and power supply.
The function (and direction of the socket key) and the location of the various holes are shown. Diameters are not given as they will vary with components.
1 - Crystal socket, key toward front of chassis.
2 - Oscillator tube, V1, socket, key toward right end of chassis.
3 - Output tube, V2, socket, key toward front of chassis.
4 - Rectifier tube, V3, socket, key toward front of chassis.
5-6 - Clearance holes for leads from power transformer.
7 - Antenna post, insulated bushing.
8 - Coil, L1, socket, key toward left end of chassis.
9 - Loading condenser, C10, shaft-clearance hole.
10 - Tuning condenser, C9, shaft-clearance hole.
11 - A.c. on-off switch, S1.
12 - Clearance hole for pilot lamp used as plate meter (in grommet).
13 - Clearance hole for a.c. cord, in grommet.
14 - Socket for connection of transmitter to external source of filament and plate power.
15 - Ground connection.
16 - Bey connection, insulated bushing.

In wiring the metal "Longfeller," the power supply was connected up first, followed by progressive wiring of the r.f. stages and ending with the tank-coil circuit connection to the antenna post. The complete circuit of the metal-chassis transmitter is shown in Fig. 4.

Fig 4
Fig. 4. Schematic diagram of the metal "Longfeller" - a two-stage beginner's transmitter and its associated power supply built on a single metal chassis. Components are the same as for the wooden version except for the wiring arrangement of the power socket, P4, and its associated plugs, P5 and P6. An a.c. on-off switch, S1, has since been added to this metal unit.

Soldering lugs were placed under many of the nuts holding down the sockets and were used to ground the components where necessary. Insulated tie-points were placed for proper and convenient location of parts.

The pilot lamp used as a plate-current indicator was connected to flexible leads and terminated on an insulated tie-point. This permits the ready availability of this inexpensive "hermetically-sealed vacuum-type current indicator."

A key-thump and key-click filter complete with key was built-up on a metal base. The circuit and components are the same as for the key filter in Fig. 1.

The metal "Longfeller" performed exactly like its wooden cousin and was immediately put on the air connected to a random hunk of wire running from the lab to another part of the building. With W1LOP at the key, signing W1INF on 3750 kc., W20E0 was worked giving him the second contact of his newly-begun ham career.

Photo 5
This "below-deck" view of the metal "Longfeller" depicts many of the components and most of the construction features. The black object on the outside of the lower apron of the chassis is the shorting plug in the power-supply socket. At the lower left can just be seen the "hot" connection to the key. The pilot lamp, connected by flexible leads to an insulated tie-point, is located near, but not connected to (as it appears) the a.c. "on-off" switch. The r.f. output circuit features short direct leads, ending in the insulated antenna binding post at the right of the photograph.
As r.f. and power-supply components are contained in this single chassis, it may appear more complicated than the wooden version. Actually, each section is separated physically for easier assembly, connection and servicing.

Connections for emergency use of the metal "Longfeller"

This unit was so arranged that, by the removal of the plug on the rear apron of the chassis and the insertion of a five-prong plug attached to a source of filament power (6 volts) and a 250-350-volt d.c. plate supply, the transmitter could be operated from heavy-duty B-batteries, a filtered vibrator pack or a d.c. generator.

The connections to the 5-prong emergency power-cable plug follow:

Pin 1No connection.
Pin 2"Hot" filament, 6 volt a.c. or d.c. at 1.2 A.
Pin 3No connection.
Pin 4Plate supply, B+, 250-350 volt d.c. at 70-100 mA.
Pin 5"Cold" (grounded) side of filament supply and B-.

An Alternate and More Versatile Connection for Emergency Use

Just in case it is desired to provide for all-'round emergency or portable usage of the "Metal Longfeller," a substitution may be made in the power plug thus permitting the connection of (a) the built-in a.c. supply (plug P10), (b) an unfiltered d.c. supply, such as some types of vibrator packs (plug P8), and (c) a filtered d.c. supply, such as B-batteries, a filtered vibrator pack or generator (plug P9).

A six-prong chassis power socket P7 is substituted in the transmitter in place of the five-prong socket P1 shown in Fig. 4. By wiring the transmitter power supply as shown in Fig. 5, the built-in filter components are made available when required and eliminated if not needed, to the junction of R7 and Rain Fig. 4.

Fig 5
Fig. 5. Modified schematic diagram of the power supply for the metal-chassis beginner's transmitter. Components are the same as in Fig. 1, except for the power connector P7 (a six-prong socket) and Ps, Ps and Pio (six-prong plugs), connected as shown. The lead marked "to point X" connects to the junction of R7 and Rs, marked "X" in Fig. 4.

Normal operation is obtained through the use of the shorting plug, P10, and either filtered or unfiltered supplies may be connected when properly terminated by plugs P9 or P8, respectively.

A. David Middleton, W2OEN.