Amplifier's Builder Guide - Wire Recorder & Amplifier
By planright Jun 10th, 2009
Adjustment of controls when recording
List of Parts
RI, 3—2-meg resistors
R2—1,500-ohm resistor
R4, 83—500,000-ohm resistors
R5—500,000-ohm potentiometer
RG—1,000-ohm resistor
R7—1-meg resistor
R8, 13—250,0<:0-ohm resistors
R9, 10, 11, 12—4,000-ohm resistors
R14—1,500-ohm resistor
R15—100,000-ohm resistor
R16—250,000-ohm resistor
R17—200-ohm, 10-w resistor
R18, 1ft—30,000-ohm resistors
B20—100,000-ohm potentiometer
K21—20,000-ohm, 10-w resistor
R22—25,000-ohm resistor
R24, 25—25,000-ohm potentiometers
(All resistors % w nnless otherwise stated)
CI, 4, 11, 13—20-/it, 50-v cond.
C2, 3, 5. 12, Sh— .06-/rt, 600-v cond.
C7, 10, 14—.25-
f, 600-v cond.
C8, 9—.006-f, 600-v cond.
C15, 16—8-f, 450-v cond.
C17, 18—S0-f, 450-v cond.
C19—.0005-f mica cond.
C20, 22—.01-f, mica cond.
C23—.03-f, 600-v cond.
CHI, 2.—125-mh r.f. chokes
CH3—10-20h, 120-ma filter choke
L1—Oscillator coil
L2—No. 1 secondary
L3—No. 2 secondary
L4—Erase coil
L5—Recording and reproducing head
SI, S2—Single-pole single-throw switches
S3—6-pole double-throw switch
T1—Power transformer, 600 v, ct., 120 ma; 6.3 v, 3 amp; 5 v, 2 amp
T2—Output transformer, 4,500 ohms pri.to voice coil.
Wire Recorder & Amplifier
Using parts lying around the workshop, any amateur, serviceman or experimenter can build this magnetic wire recorder and special audio amplifier-oscillator, to record voice and music on wire about the diameter of a human hair.
The amplifier used is a conventional circuit capable of 5 watts output with a few modifications and the addition of a low-frequency oscillator. The oscillator may be on a separate chassis if necessary. The recording head, which is also the reproducer, can be constructed from old audio transformer laminations which must be filed to fit the coil being used in the head. The coil can be found in old magnetic phonograph pickups or magnetic speakers.
The wire puller, the spool that winds the wire, is best powered by an old electric phonograph motor. The spools or reels on which the wire is wound can be cut out of solid wood or laminated boards. The wire guides should be of nonmagnetic material such as pulleys taken from old radio dial assemblies. These components may be mounted in any number of ways to suit the individual builder.
The wire used for recording must be a steel wire with .5- to .9-percent carbon content. Piano wire .014 inch in diameter and smaller can be used. The smaller the wire, the better the quality.
The audio amplifier is a four-stage resistance-coupled job using two 6SJ7's, a 6N7 and a 6L6. An 80 or 5Y3-GT is used for the rectifier. The amplifier differs from conventional ones only in the addition of an audio filter to attenuate the low frequencies when recording. The filterconsists of two resistance-capacity-inductance networks connected between the second and third stage of the amplifier. Fig. 50 shows the circuit. The first network consists of an .006-f condenser C8, shunting two series-connected 4,000-ohm resistors R9 and RIO. Series-connected from ground to the junction of R9 and RIO are a 125-mh r.f. choke CHI; a 25,000-ohm variable resistor R24; and a single-pole single-throw switeh SI. Circuit and parts values of the second network (C9, Rll, R12, R25, CH2 and S2) are identical with the first.
f condenser C8, shunting two series-connected 4,000-ohm resistors R9 and RIO. Series-connected from ground to the junction of R9 and RIO are a 125-mh r.f. choke CHI; a 25,000-ohm variable resistor R24; and a single-pole single-throw switeh SI. Circuit and parts values of the second network (C9, Rll, R12, R25, CH2 and S2) are identical with the first.
The output of the amplifier is fed through a .25f condenser C14 and a 6-pole selector switch S3, which connects to a pickup coil on the oscillator when recording. When playing back, the selector switch grounds the oscillator pickup coil and connects the output to the voice coil of the speaker. For recording, the magnetic pickup is connected across the oscillator pickup coil to ground. The magnetic recorder head is connected by a selector switch from the input to the output of the amplifier, depending on whether you are recording or reproducing.
f condenser C14 and a 6-pole selector switch S3, which connects to a pickup coil on the oscillator when recording. When playing back, the selector switch grounds the oscillator pickup coil and connects the output to the voice coil of the speaker. For recording, the magnetic pickup is connected across the oscillator pickup coil to ground. The magnetic recorder head is connected by a selector switch from the input to the output of the amplifier, depending on whether you are recording or reproducing.
The oscillator circuit is a conventional Hartley using a 6V6. The oscillator coil is wound on a form 3 inches in diameter and 3% inches long. The primary has 260 turns of No. 33 s.s.e. closely wound. The two secondaries can be wound with No. 32 enamel wire (from an old speaker field coil). Tap the coil at 45 turns and then at every fifth turn up to 80 turns. The plate supply is fed into one of these taps. In the original model, the oscillator worked best connected to the third tap. At the terminations of the windings 2-56 screws are used as terminals. Apply a liberal coating of coil dope. After this dries, wind L2, the No. 1 secondary. This is the coil used in the audio circuit in recording. This coil consists of 27 turns which are also brought to two 2-56 screws as terminals. After doping this winding and allowing it to dry, wind the second secondary L3, which has 120 turns terminated in the same manner as the previous windings. This second secondary is the pickup coil for the erase coil.
The oscillator is tuned with an .01-f mica condenser and should produce a signal between 27 and 30 kc. The action of this supersonic frequency added to the signal current is not well understood but it is essential to good recording.
f mica condenser and should produce a signal between 27 and 30 kc. The action of this supersonic frequency added to the signal current is not well understood but it is essential to good recording.
As previously stated, when recording, the output of the amplifier is fed through the oscillator coil L2 to the recording head L5 and then to ground. The selector switch opens the oscillator cathode connection to ground when the machine is used to play back or reproduce.
The next step is the construction and selection of parts for the recording head. Here each individual must use his imagination and ingenuity. We have constructed about 12 heads. Each one used a coil different in either physical size, shape, number of-turns, or d.c. resistance. Therefore we will describe the construction of only one, the one that has worked best.
The coil used came from an old discarded magnetic speaker, the d.c. resistance measuring 700 ohms. The physical dimensions were % by 25/32 by y2 inch. The core was 3/16 by 5/16. Refer to Fig. 51 for the shape and manner of assembling. Almost any coil will work as long as the gap in the core is kept between .001 to .003 inch. Use a piece of brass shim stock to maintain the gap at .002. Use a jeweler's hack saw with the finest blade obtainable to cut the slot for the wire to run through. After filing the core to shape, assemble the pieces without putting them on the coil. Clamp them together to permit drilling the holes for 2-56 screws. Then string the pieces on a wire, being careful to observe the order in which they were assembled so they may be reassembled in the exact way they were when the holes were drilled. With the wire strung out so the individual laminations are well separated, heat them red hot with a blow torch and then let them cool gradually. We also have tried cooling them in an a.c. magnetic field—using an old speaker field for the purpose. This seemed to improve the efficiency of the core.
After cooling, assemble the laminations on the coil. Hold this together with 2-56 screws and nuts. Before tightening, dip the whole thing in dipping varnish, tighten and allow to dry.
Next get a small piece of phenolic tubing 1/8-inch diameter or some similar material, to wind the erase coil L4 on. The wire will be passed through this coil to clean it magnetically so it will be ready for another recording. We used No. 28 enamel wire, but the size or number of turns should not be critical. The tubing was about 1 1/2 inches long and the coil on the tube about 1 inch long, layer-wound about 200 turns. Mount this between two pieces of 1/4-inch bakelite with holes drilled for a close fit so the tubing fits into the holes in the bakelite ends. Cement the coil in place with ordinary coil cement.
We are now ready to assemble the wire puller, head and erase coil. We mounted the motor below the panel with an extension on the drive shaft. The two wire-pulling drums used were about 5 inches in diameter with a rim on each side to keep the wire from running off. Use microphone cable to connect the recording head and erase coil to the amplifier-oscillator assembly. A word of caution here in regard to running the wire puller: use your hand as a brake when stopping the reels! The wire may become hopelessly snarled if the reels are allowed to coast after the power is turned off. Rewind the wire by hand. Incidentally, when recording or reproducing, the wire should move through the head at a constant speed of about 180 feet per minute. Whatever your speed is (it will work at slower speeds but not as well), it must be constant all the time.
Adjustment of controls when recording
You will have to find by trial and error how high to run the volume control when recording. Each set will vary, of course. The audio filter is used only when recording, the switches being opened when playing back. The setting of the controls when recording is found by experiment. If the reproduction sounds too bassy and garbled, it indicates you haven't filtered out the low frequencies enough; it will be necessary to set the controls for less resistance in the circuit when recording. By connecting a pair of earphones across a small coil similar to the one used in the recording head and holding it close to the recording head you can monitor the record continuously.
Before you have taken the "bugs" out you will have broken your recording wire many times, unless you're exceptionally fortunate—and we've never yet met a radioman that lucky. So remember this suggestion: When the wire breaks, anneal the ends with a match flame, the heat from a cigaret, or bring out a tap from a filament supply and hold the wire across the supply until it changes color. Then tie the ends together with a square knot and cut the surplus ends off. Apply a little more heat after tying the wire. The very small steel wire will burn if a match is held too close to it, so be reasonably careful.
A neon bulb is used as a volume level indicator as shown on the schematic. The point at which this bulb will flash can be controlled by R20. It should be adjusted so the bulb flashes only on the amplitude peaks.
List of Parts
RI, 3—2-meg resistors
R2—1,500-ohm resistor
R4, 83—500,000-ohm resistors
R5—500,000-ohm potentiometer
RG—1,000-ohm resistor
R7—1-meg resistor
R8, 13—250,0<:0-ohm resistors
R9, 10, 11, 12—4,000-ohm resistors
R14—1,500-ohm resistor
R15—100,000-ohm resistor
R16—250,000-ohm resistor
R17—200-ohm, 10-w resistor
R18, 1ft—30,000-ohm resistors
B20—100,000-ohm potentiometer
K21—20,000-ohm, 10-w resistor
R22—25,000-ohm resistor
R24, 25—25,000-ohm potentiometers
(All resistors % w nnless otherwise stated)
CI, 4, 11, 13—20-/it, 50-v cond.
C2, 3, 5. 12, Sh— .06-/rt, 600-v cond.
C7, 10, 14—.25-
f, 600-v cond. C8, 9—.006-
f, 600-v cond. C15, 16—8-
f, 450-v cond. C17, 18—S0-
f, 450-v cond. C19—.0005-
f mica cond. C20, 22—.01-
f, mica cond. C23—.03-
f, 600-v cond. CHI, 2.—125-mh r.f. chokes
CH3—10-20h, 120-ma filter choke
L1—Oscillator coil
L2—No. 1 secondary
L3—No. 2 secondary
L4—Erase coil
L5—Recording and reproducing head
SI, S2—Single-pole single-throw switches
S3—6-pole double-throw switch
T1—Power transformer, 600 v, ct., 120 ma; 6.3 v, 3 amp; 5 v, 2 amp
T2—Output transformer, 4,500 ohms pri.to voice coil.
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Beautiful
I'm still new to this sort of thing but this actually looks like a pretty simple setup. I'll have to rip apart some of my old equipment and snatch some piano wire from the local music shop. But yeah, thanks, really interesting!
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