The electronic circuitry of the Hell-Feldfernschreiber comprises four stages, each with an RV12P4000 vacuum tube ("valve"):

	Tonsummer – AF-oscillator stage (here: 900 Hz tone)
	Vorstufe – (pre)amplifier stage (here: 900 Hz bandpass filter and amplifier)
	Endstufe – final stage (here: tone detector and amplifier/driver for electro-magnet solenoid control)
	Reglerstufe – regulator stage (here: motor speed regulator)

These stages are shown left-to-right in the schematic below. The Feldfernschreiber is powered by 12 Vdc. The power input is passed to the 3-position main switch via a line filter (not shown in the schematic below) and a fuse ("Schmelzsicherung", S). A 5 Ω series-resistor can be switched in series with the fuse, while the main switch is in the "Bereit" (ready/standby) position. This voltage-drop resistor accommodates an early model of a Hellschreiber-specific transformer-rectifier power supply unit that had poor voltage regulation with small loads. According to the 1941 manual, the switch is not necessarily installed in Feld-Hells that were built later.

The heater filament of each tube is powered by 12 Vdc (hence the "12" in the tube designator RV12P4000), as is the motor-generator. The generator provides an anode voltage of 150-180 Vdc (165 V nominal, 25 mA) to the tubes.

 

Simplified schematic of the Hell Feldfernschreiber
 

The 900 Hz tone is generated with an LC-oscillator. The "L" part of the "LC" is provided by the inductance of the oscillator's output coupling transformer T1.
 

Schematic of the 900 Hz tone oscillator
 

It is a variation on one of the two standard Hartley oscillator configurations (named after its inventor Ralph Hartley, who was awarded the patent in 1920). It has two series-connected (and coupled) inductors and a single capacitor (here: C18). The two inductors are formed by the tapped windings on one side of coupling/isolation transformer SÜ ("Summer-Übertrager"). So this transformer cleverly fulfills two functions.

 

Hartley oscillator configurations
(note that the series-fed circuit arrangement (a) is less suitable for radio applications)

 

When a key is selected via the keyboard, the continuous 900 Hz tone is keyed by the associated track of the character drum ("Geberwalze"). See the "raster can" section of the "how it works" page. The tone can also be keyed with the "Morse" telegraphy key of the keyboard.

 

The keyed tone is output via transformer T2 to the telephone line I/O port ("Leitung" connectors La and Lb/E), and the telephone jack that is connected in parallel to this port. The "Leitung" port has an impedance of 800 ohm (at 900 Hz). This may at first appear to be incompatible with what many people consider to be the (fixed) impedance of standard POTS telephone land-lines: 600 ohm. However, keep in mind that phone lines only have 600 ohm impedance at one single frequency: about 1300-1400 Hz. For 900 Hz (the Hellschreiber tone), the impedance of a standard phone line is actually 800 ohm! At the high end of the phone-voice bandwidth (3400 Hz) the line impedance drops to about 350 ohm. See ref. 1. The impedance of the connected line is not at all critical, as long as the end-to-end attenuation of the 900 Hz tone is less than 46 dB (5.3 Neper). The "Leitung" port is bi-directional: keyed tones are output to the phone line, and tones from an other Hellschreiber are received from this phone line. The latter are, therefore, also transformer coupled. The output level of the keyed 900 Hz tone is at least 2.2 V at the La-Lb/E port, and at least 6.1 V at the "Empfänger" port (ref. 13).

 

The same transformer T2 provides coupling of the keyed tones to the bandpass filter / pre-amplifier stage. This causes the text that is transmitted by a Feld-Hellschreiber, to be printed locally, as it is transmitted. This creates a record of the transmitted text. Note that this allows full-duplex communication (except when using the character drum to directly key an CW transmitter)! Obviously, simultaneously sent and received text are printed on top of each other. This was actually used during the standard speed adjustment procedure upon establishing a communications link (ref. 54).

 

Most Feldfernschreibers are equipped with a 12-pin round connector on the front panel. If the corresponding plug is inserted, the keying contacts of the character drum are diverted to this connector, and the keyed tone is no longer coupled to the pre-amplifier stage or the line output.

 

All transformers have a static shield that is connected to ground. Additionally, T1 is placed in a shielding box that is also connected to chassis ground. Shielded wiring is used where it is prone to picking up noise or hum.

 

Tones can also be received from an other Hellschreiber via a radio receiver. The associated "Empfänger" (receiver) input port passes through a line filter (not shown in the schematic above), and is tied across the secondary side of transformer T2. This is sort of a "transformer-coupled wired-OR". This port has an impedance of 4000 Ω (at 900 Hz).

 

At this point, a potentiometer provides audio input volume control. The potmeter output is passed through an LC "Tonsieb". I.e., a bandpass filter. As for the tone oscillator, the "L" part of the "LC" is formed by a transformer. In this case, the T3 coupling to the pre-amplifier tube. The filter is turned off by manually switching-in a "spoiler" resistor between the audio gain potentiometer and the filter.
 

Schematic of the 900 Hz filter

Per ref. 2, this filter has a bandwidth of 150 Hz; per the 1941 manual (ref. 3 and 4), the bandwidth is 100 Hz. Note that filter bandwidth may originally have been defined as 0.5 Np (4.34 dB) down from the filter's center-frequency (ref. 55). I measured a 3 dB bandwidth of 130 Hz for my machine (see bandpass curve on the maintenance page).

The pre-amp has an impressive gain - as to be expected: the RV12P4000 tube has a μ of 4000. At the monitoring port ("Mithören" - "to listen in"), the 900 Hz tone begins to clip at 15-20 Vp (36 Vpp !). A signal of 55 mV at the line-input produces at least 1.8 V at the "Mithören" output (ref. 13).

The output of the pre-amplifier is coupled to the tone-detector via transformer T4. A separate secondary winding of this transformer is output to the "Mithören" port. This enables monitoring of the received radio signals that were passed through the 900 Hz bandpass filter and pre-amp, and tune the receiver to change the pitch of the tone with respect to the filter's center frequency.

Note that all interfaces are transformer-coupled, so no isolation transformers are required when interfacing a Feld-Hell machine to a PC soundcard or solid-state transceiver.

The detector and printer-solenoid driver of the Feld-Hell

 

The detector is a full-wave rectifier, followed by RC-filtering to reduce the ripple of the rectified signal. The diode rectifier comprises two "Kupferoxydul-Gleichrichter": cuprous-oxide-on-copper diodes, see the Components and Construction page. These first-generation solid-state diodes have an excellent low forward-voltage (0.2 volt, similar to germanium diodes); germanium and silicon diodes did not yet exist in those days. However, they also have a low reverse-voltage of 6 volt. The Feld-Hell uses Siemens-Halske Sirutor 5b diodes, in which 5 such diodes are put in series.

 

The rectified and filtered input signal goes to the grid of the "normally off" final amplifier tube, and controls the tube's anode current. The grid current passes through an RC-network (R_g, R_v, C in the diagram below). This network provides a simple automatic gain control ("Schwundregelung", "selbsttätige Pegelregelung"; ref. 51), with a range of 5 Neper (≈ 43 dB). The voltage drop across R_g  provides a current-limiting negative grid bias. The time-constant of the parallel circuit of R_g and C is consistent with the duration of several Hell-pixels. As a result, only the highest momentary signal amplitude opens the grid of the tube. Signals with a smaller amplitude (e.g., atmospheric interference, echo, reverberation) are blocked, and short fading is compensated. The filter R_v + C suppresses pulse type signals that have a duration that is shorter than that of Hell tone-pulses. Ref. 2 (section 7, p. 7), ref. 3 (section IIIa, p. 16), ref. 51, ref. 52 (pp. 22-23).

 

Automatic Gain Control via variable grid bias
 

The final amplifier tube is the solenoid driver. The solenoid is connected between the 165 Vdc high-voltage rail, and the anode of the tube. When a tone is received, the tube is keyed "on" by the detector. This causes anode current to flow, which energizes the solenoid. In turn, this causes the armature to push the paper tape up against the turning spindle of the printer mechanism. Note that the solenoid is not pulled down to ground level. The printer solenoid comprises 15000 windings of 0.06 mm enameled copper wire ("lackisolierter Kupferdraht", CuL) and has a nominal DC-resistance of 4090 ohm; the one in my machine is within 0.5%: it measures 4075 ohm. According to Rudolf Hell (section 6 (p. 6) in ref. 2), a dissipation of at least 0.4 watt is required to reliably actuate the printer hammer. This implies 10 mA at 4 kOhm. Other literature (e.g., p. 328 in ref. 56, 1939) states that the solenoid measures 4200 ohm and requires 10 mA at 165 V. This appears to exceed the 3 mA anode current rating of the RV12P4000 tube (and 1.5 W max anode dissipation)...

 

A separate tube tightly regulates the speed of the 12 Vdc electric motor. The anode current of the tube passes through the motor's governor-field windings. One side of these windings is permanently connected to the +165 volt from the generator. The tube, and hence the current, is switched on/off by a centrifugal switch arrangement that sits on top of the the motor shaft. It comprises a  spring-loaded governor weight, and a switch-contact.

 

 

"Bang-bang control" motor-generator rpm regulation

By design, the unregulated rpm (at least 9000 rpm!) of this motor is much higher than the required nominal rpm (3600 rpm). A protection is built in, to avoid reaching this destructive rpm in case of failure of the regulator tube. A second contact of the centrifugal switch (C2) closes at a speed somewhat higher than the normal speed. When this safety contact closes, the regulator tube is completely bypassed, and the motor's governor field is tied directly to +12 V. Unlike the primary contact, the overspeed contact has no circuitry for arcing and EMI suppression.   Rudolf Hell patented this electrical centrifugal governor ("Elektrischer Fliehkraftregler") in 1949 (German patent nr. 803577)

Why not simply connect the nominal speed contact directly to the "cold" side of the governor windings? That is, hook it up just like the overspeed contact. This would save an expensive tube and associated parts. Indeed! But this would cause constant arcing of the switch contacts (as with the overspeed contact), and rapidly wear out these contacts. The unreliability would have been unacceptable. By using the tube, there is never more than 12 volt across the contacts and no inductive load. The overspeed switch has 165 - 12 = 153 volts across is, and the inductive load of the governor windings.

 

 

The detailed circuit diagram for Feld-Hell model 24a-32 (a1, a2) is here (7 MB high-resolution jpg file), and here (1.4 MB pdf file); a low-resolution version is shown below. Note: the diagrams may not look good in your web browser, but should be fine when the files are saved to your PC and opened with your favorite image viewer. My document "Schematic, Component-Layout and Connector Pin-Outs of the Hell Feldfernschreiber" contains all the information that the title suggests. It is available here.

The components inside the Amplifier and Interconnect Unit all have a small round sticker on them, with a number that corresponds to the component number in the schematic (W.. for "Widerstand" (resistor), C.. for "Condensator" (capacitor), D.. for "Drossel" (choke coil / inductor), GL.. for "Gleichrichter" (rectifier diode) etc.). Very helpful during troubleshooting and repair! Also, all the solder lugs on the pertinax circuit card, the interconnect blocks, and the tube sockets, have a number printed on them or next to them; it corresponds to a numbered point in the schematic. Note that numbering and naming is not necessarily consistent between all available schematics, and there is variation between models.

        Rear-view of the Amplifier & Interconnect Unit of the Feld-Hellschreiber - cover removed         
(the components and solder lugs are numbered per the schematic)


 Rear-view of the Amplifier & Interconnect Unit of another Feld-Hellschreiber
(this unit was manufactured by "Radio H. Mende & Co. GmbH" in Dresden, rather than by Siemens-Halske; note that in this unit, the component number stickers were placed at the edge of the components, so as not to cover the component value markings; C47and C18 appear to have been replaced with SiKaTrop (Siemens Keramik Tropenfest) capacitors. They are packaged inside a porcelain tube with soldered sealed (moisture-tight) metal end-caps, and were trimmed to the specified value, by removing metal deposit on the outside ("Ausserbelegung") of the tube, ref. 53.They are virtually "unkaputtbar", and remain reliable after 70 years)
 

As can be confirmed by tracing the circuit diagram below, when the Hellschreiber’s main switch is in the “Bereit” (ready/standby) position, the red signal lamp extinguishes when one or both of the motor’s carbon brushes is not in contact with the commutator, or there is an open connection in that circuit.

 

Schematic of the Hell Feldfernschreiber  (drawing nr. 24a-32 (a1/a2)) - issue: 23-may-2011
- a very high-res version of this diagram is here (7.3 MB) -