• [Introduction]
• [Electronics box]
• [Base unit]
• [Printer module]
• [Gear box]
• [References]

Latest page update: 5 March 2023 (inserted Fig. 16 & Fig. 17 and text).

Previous page update: 1 May 2021 (replaced Fig. 14); 27 August 2018.

©2004-2023 F. Dörenberg, unless stated otherwise. All rights reserved worldwide. No part of this publication may be used without permission from the author.

INTRODUCTION

The Hell Feldfernschreiber (Feld-Hell) was manufactured from 1935 through 1944 (though, based on my database of about 200 machines, 1937 appears to have been skipped). The 1939 and 1940 version of the Siemens-Halske sales brochure "Der SH-Feldschreiber" (ref. 1 and ref. 2, respectively) show a clear transition regarding elements on the front of the electronics box. See Figure 1:

Fig. 1: change-over to the new knob shape, binding-posts, connector block, round connector, and label

(source: ref. 1 from February 1939, and ref. 2 from January 1940)

The actual transition may have already been introduced in 1938, based on photos of actual equipment labels etc., that I have captured in my database (see the "Markings on Feld-Hell machines" page). Clearly, as the war progressed, materials and skilled labor became increasing scarce. Hence, the design and manufacturing was rationalized, starting in 1943. In general, none of the changes that are documented here affect form, fit, functionality, or performance of the machine.

The resulting design variants are discussed below. In particular, this affects the following aspects:

• electronics box ("Verstärker- und Anschlußsatz" = "Amplifier & Interconnect Unit"):
• equipment label
• 12-pin round connector
• signal lamp of the main switch
• jacks for audio input from a radio receiver ("Empfänger")
• binding-post terminals for the La - Lb/E phone line and for the case-ground (earth) connection
• power source toggle-switch
• base unit ("Unterteil", below the motor-generator and the keyboard/drum unit): the lower drawer for a roll of paper tape
• printer module: number of solenoids, solenoid DC-resistance, hammer-height adjustment
• gear box: worms and worm gears on various drive shafts
• ball bearings.

Note that there are also derivative Feld-Hell models, that are not regular design evolutions of the standard Feld-Hell: model Tbs 24b, built by the Hell company rather than Siemens-Halske, and post-WW2 model HTG-1, built in Hungary.

DESIGN EVOLUTIONS OF THE FELD-HELL: THE ELECTRONICS BOX

Before the transition ca. 1938, the equipment label only comprised either the equipment designator or the drawing number, the serial number, and the year of manufacturing (see Fig. 1 and 2). After the transition, the label had both the equipment designator and the drawing number, and was expanded with the operating voltage, and the logo or identifier code of the manufacturer. See Fig. 1 and 3.

Fig. 2: data plate format in 1935 (possibly even  as early as 1934) and 36

Fig. 3: data plate format from 1938 onward

There is a round 12-pin connector ("Zwölffachsteckdose", "Steckerbuchse", "Anschlußbuchse") located at the lower right-hand corner of the Feld-Hell's amplifier box. This connector provides access to the contacts of the character-drum, for keying a CW transmitter. There are also outputs for accessories such as a remote control unit and a keying device for high-power CW transmitters: the constant 900 Hz tone, 12 volt DC and 165 volt DC.

This connector has a brown, dark brown, or black Bakelite insert. The natural color of the Bakelite resin is brown. Black is obtained with a molding powder, hence more expensive. I have seen at least one Feld-Hell machine where the connector had a white insert. This may have been a very early model, with a ceramic insert (note that the first RV 12 P 4000 vacuum tubes (1935) had a white porcelain base).

There are basically four connector configurations (in chronological order):

• round connector with a pivotable cover (up to the 1939/1940 transition),
• round connector with wire-retainer for the mating male connector (after the 1939/1940 transition),
• no round connector - a square plate covers the hole in the front of the amplifier box,
• no round connector, and no hole in the front of the box.

Fig. 3: the 12-pin connector of the Feld-Hell electronics box

It appears that the connector (and associated internal wiring) was eliminated some time during 1944. The majority of field operations was probably over phone lines, in which case the connector is not used. Eliminating the connector saves material and labor. Once the connector was eliminated, is was also no longer necessary to machine a hole for it in the front of the die-cast box, and install a cover plate. Further savings!

The shape of the knob of the main switch was changed in the course of 1938: from asymmetrical to symmetrical. Possibly the latter is more rugged, as force is not applied off-center. At the same time, the knob material was changed from cast metal to Bakelite - possibly a cost reduction. Note that the actual switch is not a rotary switch at all: its is a simple slide switch that is actuated by an eccentric lever on the shaft of the knob!

Fig. 4: Old and new shape of the main switch knob

My machine from 1938 has a dark brown rectangular knob, whereas my 1939 machine has a black Bakelite knob that is rounded and narrower at the top:

Fig. 5: the knobs on my 1939 machine (left) and on my 1938 machine (right)

There is a toggle switch to the left of the volume control knob. It switches the 900 Hz tone filter on/off. Some early amplifier boxes (ca. 1936) appear to toggle between a 900 Hz and a 1500 Hz tone filter:

Fig. 6: Tone filter toggle switch - 900 Hz On/Off vs. 900 Hz/1500 Hz

There is a red indicator lamp, just above the main switch. It is illuminated when the main switch is in the upright "Bereit" position ("Standby").

Fig. 7: amplifier box with the signal lamp (1938, left) and without (1944, right)

At best, the lamp is a "nice to have" feature, as it does not illuminate when there is no circuit-continuity through the motor's carbon brushes and commutator. Other than that, it provides no useful information - the voltmeter already indicates the 12 volt power level. A number of units from the final production year (1944) do not have the lamp installed (nor pre-wired) and there is no a hole at its place in the front of the box. Material and labor savings!

The Feld-Hells have three binding-post terminals ("Polklemme"): one for case-ground (earth), and a dual binding-post for the La - Lb/E phone line wires.

In the 1936 model, the binding posts are all-metal. The dual binding-post is on the same rectangular Bakelite block as the phone line jack, the radio and headset jacks, and the 12 volt power connector. The case-ground binding-post can only accommodate a wire, whereas the dual binding-post can also be used with banana plugs.

The binding-posts of the 1939/40-1944 models have a Bakelite knob for (un-)tightening. The dual binding-post is mounted on a separate Bakelite block. The terminal for case-ground is also mounted on a Bakelite block, and can also accommodate a banana plug. Also see Figure 1 above.

Fig. 8: all-metal bindings-posts (1936, left) and Bakelite binding-posts (1938-1944, right)

The connector block of the amplifier box has jacks for connecting the loudspeaker/headphones output of a radio receiver ("Empfänger"). Some units that were built during the final production years (1943/44) have a small Bakelite plate that covers these jacks. I do not know whether the jacks (and wiring) are not installed, or only covered up. For operation over phone lines ("Drahtbetrieb"), this interface is not required. The Feld-Hells in which this modification was made, typically also do not have the 12-pin round connector for interfacing to a CW-transmitter.

Fig. 9: connector block with "Empfänger" jacks (1938) and with covered jacks (1943)

The connector block is made of light brown, dark brown or (more expensive) black Bakelite. In my database, there appears to be no consistent switch-over between colors. Brown and black connector blocks are found in pre-war machines as well as late-war machines. There also appears to be no correlation between the Bakelite color, and the manufacturer of the amplifier box (Siemens-Halske vs. Mende Radio).

There is a toggle switch on top of the electronics box. It is labeled "Sammler – Netzgleichrichter". That is: "battery – transformer-rectifier unit". In the "Netzgleichrichter" position, a 5 Ω resistor is placed in series with the 12 volt DC power input when the main switch is in the "Bereit" position. This is to support an older model Feld-Hell-specific external power supply, that had poor voltage regulation under low-load conditions.

Fig. 10: the "Sammler - Netgleichrichter" switch on the top of the electronic box of my 1938 Feld-Hell

According to the 1941 Feld-Hell manual (line item 44), "newer" Feld-Hell versions do not have this switch. So far, I have found only three amplifier boxes without the toggle switch. Both were all manufactured in 1943. In one case, the switch (and associated wiring) are simply not installed. There are no indications that the switch was removed after installation. In the second case, there are no legends for the switch positions, and the hole has a round cover plate. In the third one, the hole is completely eliminated.

Fig. 11: "Sammler - Netzgleichrichter" switch: a) not installed, b) hole covered with plate, c) no hole

DESIGN EVOLUTIONS OF THE FELD-HELL: THE BASE-MODULE ("UNTERTEIL")

The Feld-Hell has a compartment for two drawers with a roll of paper. It is located below the keyboard. The compartment has the following parts:

• a hinged cover plate
• a push-button in the front of the frame of the keyboard, to release the latch of the cover plate
• two drawers, each holding one roll of paper tape
• tracks, for sliding each drawer in & out
• two push-buttons, to release the latch of each drawer
• a slit to the left of each drawer, where the paper tape leaves the drawer compartment, towards the printer module
• a cover plate underneath the drawer compartment.

Fig. 12: the front of the paper drawers compartment - cover closed (my 1938 machine)

Fig. 13: cover opened - two drawers for paper tape rolls, tracks on the left & right, release buttons

When the upper paper cassette of the Feld-Hell runs out of paper, the lower cassette must be opened, the paper tape pulled through a slit on the left of the cassette, and the paper passed underneath the printer spindle. Exactly the same procedure is followed when simply putting a new roll of paper in the upper cassette. So, in fact, the second (= lower) paper cassette of the Feld-Hell is just a convenient place to store a spare roll of paper tape. Pure luxury!

Towards the end of the war, this luxury became unaffordable. Some of the machines built in 1943/44 dispensed with the compartment cover plate, the lower paper tape cassette, the mounting hardware and release button for the lower cassette, and the cover plate underneath the compartment.

Fig. 14: A 1943 model - lower paper tray drawer and associated tracks & button not installed

(source: adapted from eBay article nr. 164838071878, May 2021)

In a final rationalization step, the release button for the (no longer installed) compartment cover plate was no longer installed. Also, drilling and machining steps for for the paper exit slit and the (no longer installed) cassette release button were skipped.

The 1944 machine shown in Fig. 12 shows the final rationalization step: the release button for the (no longer installed) compartment cover plate was no longer installed in the frame of the keyboard. Also, drilling and machining steps for for the paper exit slit and the (no longer installed) cassette release button were skipped.

Fig. 15: A 1944 model - additional parts not installed and several machining steps eliminated

A small spring-loaded rod brake rides on the side of the turntable disk. It is mounted in the inside wall of the the paper tape drawer. Its purpose is to apply a small drag to the turntable, to maintain a slight tension on the paper tape. At some point, the design was rationalized:

Fig. 16: The small rod brake in the paper tape drawer

In my 1938 Feld Hell machine, the surface of the turntable disk of the paper tape drawers is completely smooth. It appears that sometime between 1938 and 1942, the surface treatment was changed. In my 1942 machine, and a 1943 machine of which I have detalied photos, the surface of the disk has a regular pattern of tiny dents - which makes it look a bit like ostrich skin! It is unclear why this extra manufacturing step was introduced. It reduces the contact surface area with the paper tape roll, which reduces friction. This implies that the paper tape roll can unroll without the turntable turning...

Fig. 17: Dented surface of the turntable disk in my 1942 machine

DESIGN EVOLUTIONS OF THE FELD-HELL: THE PRINTER MODULE

The printer module comprises the printer spindle (helix) and the printer hammer with its electromagnetic actuation. In early Feld-Hell machines, the actuation is done with a single solenoid (coil) that is installed on a rectangular laminated iron core. Starting around 1939/1940, a new printer module was introduced (compare the 1939 and 1940 issues of the same Siemens-Halske brochure, ref. 1 and 2). It is nearly identical to the printer module of the civil Presse-Hell printer model T.empf.14 that was introduced in 1940. The photo series below shows several side-by-side views of the original and the new Feld-Hell printer module. They are electrically and mechanically fully compatible and interchangeable.

Fig. 18: Front of the Feld-Hell printer module (left: up to 1939/1940, right: from 1939/1940)

In the photos above, note that the tabs for the mounting screws are red. This means that it was allowed to loosen the screws as part of field maintenance. Screws without such color coding were only allowed to be touched by qualified teleprinter technicians in a maintenance shop.

Fig. 19: Front of the Feld-Hell printer module - cover removed (left: up to 1939/1940, right: from 1939/1940)

The two photos above show one and two orange-colored solenoids, respectively. They are marked as follows:

The right-hand photo in Fig. 19 above shows an electrical contact that is actuated by the printer hammer. It is visible just below the copyright mark. This contact is closed when the solenoids are de-energized. It is wired to the connector "fingers" on the back of the module (also see Fig. 19 below). This contact is not used in the Feld-Hell. However, it is used in the Presse-Hell printer, as part of the circuitry for remote-controlled start/stop via long tone-pulses (ref. 4).

In the new module, the spindle is different: it has a larger diameter and the shape of the thread is simpler. This makes it easier to manufacture. Also, the small pinion gear wheel is now fixed with a screw through the spindle shaft, instead of with a taper pin. Again, easier to manufacture than drilling a tapered ( = conical) hole through the shaft.

Fig. 20: Left-hand side of the Feld-Hell printer module (left: up to 1939/1940, right: from 1939/1940)

Fig. 21: Top of the Feld-Hell printer module (left: up to 1939/1940, right: from 1939/1940)

The original module design has two contacts for the solenoid, whereas the new module has two solenoid contacts plus two switch contacts (not connected and not used):

Fig. 22: Rear of the Feld-Hell printer module (left: up to 1939/1940, right: from 1939/1940)

In the original design of the printer module, the height-adjustment of the printer hammer is not accessible from the outside. This adjustment is not something a Feld-Hell operator should be playing with... In the new module, the hammer-height adjustment has been simplified and can be done from the outside. It is unclear what the two holes in the bottom of the cover are for - they do not provide access to adjustable or verifiable parts. Note that the adjustment wheel and the associated lock screw do not have a red marking, so adjustment was only allowed by special technicians.

Fig. 23: Bottom of the Feld-Hell printer module (left: up to 1939/1940, right: from 1939/1940)

The bottom views in Fig. 22 and 23 clearly show the difference in the coil configuration:

Fig. 24: Bottom of the Feld-Hell printer module - cover removed (left: up to 1939/1940, right: from 1939/1940)

Fig. 25: one solenoid (left) vs. two on the same core (Feld-Hell starting ca. 1939/40, and Presse-Hell 1940)

As mentioned above, the new Feld-Hell module  and the T.empf.14 Presse-Hell module are nearly identical (other than the color):

Fig. 26:  printer modules - Feld-Hell (left; starting ca. 1939/40) and Presse-Hell model T.empf.14 (1940)

(source: ref. 2 and ref. 3)

The primary difference is the configuration of the two identical solenoids: in the new Feld-Hell module they are connected in series, whereas in the Presse-Hell they are connected in parallel. This has to do with the specified solenoid-current drive: 10 mA in the Feld-Hell and 20 mA in the Presse-Hell (18 mA minimum). So, with a simple change of the solenoid interconnection wiring, the Presse-Hell module is fully compatible and interchangeable with the new Feld-Hell module, and one-way interchangeable with the original Feld-Hell module. The Feld-Hell solenoid is dimensioned for an energization current of 10 mA, because that is the maximum anode current of the RV12P4000 vacuum tube that turns the solenoid on and off. The Presse-Hell keying amplifiers typically have an output stage with a vacuum tube such as the AL4, EBL1, EL11, or RL12P10. These tubes all have 36 mA drive capability - more than enough for the required 20 mA.

Fig. 27: printer module of one my T.empf.14 Presse Hell machines

Fig. 28: inside and bottom view of the printer module of my T.empf.14 machines

The table below summarizes the characteristics of the original and the new Feld-Hell printer module, as well as the T.empf.14 Presse-Hell:

Fig. 29: characteristics of the Feld-Hell and T.empf.14 Presse-Hell printer modules

(source: my Feld-Hell and Presse-Hell machines and ref. 1-4)

To the uninitiated, the printer spindle may look like a single thread that is wound twice around the spindle hub - but it is not: there are two separate threads that are shifted by 180 degrees, and each only makes one turn! There has also been a printer module does not have a spindle with two separate threads, but a 4-start spindle that has four separate 1-turn threads that are shifted by 90 degrees:

Fig. 30: printer module with the standard 2-start spindle (left) and with a 4-start spindle

What was the purpose of such a spindle? Clearly, when used to print Hellschreiber signals, it will print four parallel half-height lines of text, instead of the standard two parallel lines with full height:

Fig. 31: prints of Feld-Hell signals with a standard 2-start spindle and with a 4-start spindle

(source image b): ©2017 R. Caspers, used with permission)

What were printer modules with the four-start spindle used for? Clearly, not to print Hellschreiber signals. They were used to record Morse telegraphy signals! For Hell Morse-printers with a different type of spindle, see the "Feld-Morseschreiber built by the Hell Co." page.

Fig. 32: prints of Morse signals with a 4-start spindle and with a standard 2-start spindle

(source: ©2017 R. Caspers, used with permission)

The next figure also shows Morse signals (here: the word "SIEMENS") printed with a two-start spindle. However, this was not printed with a Feld-Hellschreiber machine, but with a Presse Hell machine. Its spindle turns twice as fast as the Feld-Hell spindle. This has the same effect as using a four-start spindle, as shown in Figure 30 c) above. The speed of the paper tape of a Presse Hell printer is also higher than that of a Feld Hell printer, which accounts for some minor differences between Fig. 33 and Fig. 32c).

Fig. 33: prints of Morse signals with a standard 2-start spindle at Presse Hell speed

(source: Fig. 668 in ref. 5)

Siemens & Halske did patent a self-adjusting spindle-and-hammer arrangement, with flexible elements (ref. 6). It does not appear to have ever been industrialized.

DESIGN EVOLUTIONS OF THE FELD-HELL: THE GEAR-TRAIN

The gear box of the Feld-Hell comprises four drive shafts:

• input shaft. It is installed vertically, and coupled to the motor. This shaft has two worms.
• character-drum drive shaft. It has one worm gear (a.k.a. worm wheel) that meshes with the larger (upper) worm of the input drive shaft. One end of the shaft has a large metal gear, that meshes with a pinion gear on the character drum. A worm at the opposite end of the shaft drives the paper transport shaft. Note that the term "worm gear" is often incorrectly used to denote the combination of a worm and a worm wheel.
• paper transport drive shaft. It has a worm gear that meshes with the worm at one end of the character-drum drive shaft.
• printer spindle drive shaft. It has one worm gear that meshes with the smaller (lower) worm of the input drive shaft. The crown gear at the opposite end of the spindle drive shaft meshes with the pinion of the actual printer spindle.

Fig. 34: the gear train of the Hell Feldfernschreiber

All worms are made of steel, and are single-start. All worm gears are made of "Hartgewebe" (HGW): cotton-reinforced phenol-formaldehyde resin. All worm gear wheels are helical gears: the teeth are not parallel to the gear's axis of rotation, but set at an angle. The teeth are actually very slightly curved, and are in fact a small segment of a helix. Helical gears run more smoothly and quieter than simple "straight cut" spur gears. However, they also exhibit more sliding friction between meshing teeth than spur gears - which is why they are lubricated.

At some point in time, the geometry of the worms and corresponding worm gears were changed. Their pitch (the on-center distance between adjacent teeth of the gears, and the coarseness of the thread of the worms) was increased. A the same time, the diameter of the worms was increased and that of the worm gears decreased decreased accordingly - the geometry and position of the drive shafts remained unchanged.

The gear ratio of a worm gear set is obtained by dividing the number of teeth of the worm gear-wheel, by the number of starts (threads) of the worm. In our case, all worms are 1-start. So the gear ratio is simply the number of teeth of the worm gear-wheel. Depending on the gear wheel, the number of teeth was reduced by 10-30%. I am still trying to figure out the technical reasons for these changes, and why it did not affect the rpm of the printer spindle and the character drum... These gear changes were not made for economic reasons.

Fig. 35: worms on the main drive shaft (coupled to motor) - 1938 (bottom) vs. 1944 (top)

(note the difference in the pitch of the worms ( = distance between the turns of the thread) - larger on the older shaft)

Fig. 36: worm-gear on the drive shaft of the character drum - 1938 (top) vs. 1944 (bottom)

(note the difference in the pitch of the worms gears - smaller on the older shaft)

Fig. 37: worm-gear on the drive shaft of the printer spindle - 1938 (top) vs. 1944 (bottom)

(note the difference in the pitch of the worms gears - smaller on the older shaft)

One more change was made to the worm gears of the drive shaft of the character drum and of the printer spindle. See photos above. Originally, these gears were affixed to the drive shaft with a tapered pin through the hub of the gear wheel and through the shaft. Making a tapered hole requires a special tool: a tapered reamer. Also, the taper of the hole through the shaft must match the taper through the hub of the gear wheel. This usually means that the worm gear must already be on the shaft when the hole is reamed. All this takes time during production, and a skilled machine operator. Removing the taper pin - without damaging the gear or the shaft - is also not easy. The entire process can be greatly simplified by mounting these gearwheels with a standard, cheap, mass-produced M3 screw. Yes, this is technically somewhat inferior. But now the holes through the shaft and the gearwheel can be made separately (time and location), with a simple drill and a tap, and without specialized labor. Note that the worm gear on the paper transport drive shaft is screw-mounted in all Feld-Hells. This has to do with the fact that the gear is installed after the shaft is installed in the gear box.

REFERENCES

• Ref. 1: "Der SH-Feldschreiber", Siemens-Halske AG, Berlin-Siemensstadt, SH 7535, 1.2.39. TT1., 11 pp. (courtesy Siemens Corporate Archives, München)
• Ref. 2: "Der SH-Feldschreiber", Siemens & Halske A.G., Berlin-Siemensstadt, Fernmeldetechnik, Wernerwerk, 1940, 14 pp., SH. 7535a, 1.1.40 TT1. N/1069
• Ref. 3: "Der neue Siemens-Hell-Schreiber" [Presse-Hell model T.empf.14], Wilhelm Heller, Technische Mitteilungen des Fernmeldewerks, Abteilung für Telegrafengerät, May 1940, Siemens & Halske A. G., Wernerwerk, SH7996, 1.8.40 T T1., 4 pp. (courtesy Siemens Corporate Archives, München)
• Ref. 4: "Remote control sequence of the Presse Hell printer", Frank Dörenberg, August 2014, 8 pp.
• Ref. 5: "Abtast-Telegrafen", chapter IV in "Taschenbuch für Fernmeldetechniker", H. W. Goetsch, Oldenbourg Verlag, 1940, pp. 411-427 of 787.
• Ref. 6: "Schreibsystem für Bildpunktschreiber" [printer head for dot printer], Paul Storch, Otto Steiner, Fritz Hennig, Siemens & Halske A.G., German Reichspatent Nr. 744828, patent applicable from 4 october 1940.