©2004-2020 F. Dörenberg, unless stated otherwise. All rights reserved worldwide. No part of this publication may be used without permission from the author.

Latest page update: 4 October 2020 (added Fig.11 & 12, associated text )

Previous update: June 2020 (added Fig. 10B and text, added notes above Fig. 10B about interfacing with a 15WSEb transmitter); 25 October 2018

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The Hell Feldfernschreiber was designed to be operated with tone-pulses over standard phone lines and "voice/telephony" radio, and with tone-less pulses via CW "telegraphy" radio. The schematic below shows a simplified schematic with the available interfaces:


Fig. 1: simplified schematic of the Feld-Hell with its interfaces

The next figure shows a standard Luftwaffe interface configuration of the Feld-Hell with a Torn.E.b. receiver and a 100W.S. transmitter. Both are AM and CW capable, but the equipment configuration below is for "AM" modulation.


Fig. 2: system configuration of a Feld-Hell with a "Torn.E.b." receiver and "100.W.S." transmitter

(source: ref. 1)


Fig. 3: Generic system configuration of a Feld-Hell with a mobile receiver and a transmitter

(source: Fig. 3 in ref. 10)

The various options for interfacing with an AM/SBB/FM transceiver and with a CW transceiver are discussed below.


The nominal amplitude of the tone pulses at the Feld-Hell's output is 2 Vpp. Most modern transceivers only require (and can only handle) a small signal at the microphone input - in the 10 millivolt range. So, an appropriate voltage divider (e.g., a potentiometer) must be installed between the Feld-Hell's output and the microphone input. The tone-pulses are output via two interfaces of the Feld-Hell: the "Mithören" ( = monitoring) output for a high impedance headset (4000 Ω), and at the "La Lb/E" phone line jacks (600 Ω). Using the phone line interface is preferred, as here, the amplitude of the tone pulses is fixed. The signal level at the "Mithören" output depends on the manual setting of the volume control knob.


Fig. 3: interface from the Feld-Hell's phone line jack to the transmitter's microphone input

Obviously, the transmitter must be turned on ("keyed") when the Feld-Hell is sending tone pulses. There are three options:

  • Transmitter with built-in VOX ("Voice-operated control"): make sure that the off-delay ("hang time") is sufficient. The first and last pixel-column of each Hell-character is blank (2 x 1/7 x 1/2.5 = 114 msec), though a few characters have a pixel at the bottom of the second and at the top of the sixth column. So, the off-delay  should be at least double that, to be safe. A delay of about 1 sec (the equivalent of 2.5 characters) will work fine. The on-delay is not an issue, if a pre-amble is used for each transmission (e.g., standard "RRR", or "...").
  • Transmitter without built-in VOX:
  • use a simple manual switch, connected to the "push to talk" (PTT) pins of the transmitter's microphone input, or
  • use an external VOX-detector, with its output connected like a PTT switch. See ref. 2 for some circuit examples. Note: I have not built these circuits, so I cannot endorse them.

Interfacing the audio from the receiver to the Feld-Hell is also straightforward:


Fig. 4: interface from the audio output of the AM/SSB/FM receiver to the receiver input of the Feld-Hell

Note that all audio interfaces of the Feld-Hell are transformer isolated. So, there is no need to provide galvanic isolation with additional transformers, to protect a solid-state transmitter/receiver.


The keying contacts of the Feld-Hell's character drum are readily accessible via the round 12-pin connector on the front of the amplifier box of the machine. Note that there are Feld-Hell machines without this connector! Those machines were intended only for use over phone lines ("Drahtbetrieb"), although they can also be used with "voice/telephony" transmitters and receivers. The pin-out of this connector is provided in Fig. 3 below.

Note that when the 12-pin round plug is inserted, a contact behind the connector is opened. This interrupts the path of the keyed 900 Hz tone, such that it is no longer coupled to the "Leitung" (phone-line) coupling transformer. This way, the Hellschreiber circuitry is fully isolated from the potentially high voltages at the keying input of the CW transmitter. This also implies that own-text is no longer printed during transmission, unless the side-tone of the CW transmission is fed back to the "Empfänger" ("receiver") or "Leitung" input of the Hellschreiber.


Fig. 5: pin-out of the 12-pin connector of the Feld-Hell's amplifier box

The keying contacts (pin 3 and 4) are simply connected to the keying input of the CW transmitter, just like a standard "Morse" telegraphy key. Note that the Feld-Hell's keyboard also has a "Morse" key. It is marked with a big green dot on it.

IMPORTANT: do NOT apply more that 200 volt to these contacts, and draw NOT more than a couple of milli-amps through them. Ref. 3 specifies a maximum combined voltage and current of 2 watts. Ref. 4 (page 164) actually even states limits of 0.1 mA and 100 volt. This is no problem whatsoever with solid-state transmitters. Note that nearly all German telegraphy transmitters used "grid-block keying" (as opposed to cathode keying): simultaneously blocking the control-grids in all stages. Ref. 5. This allows for near full break-in.

The round connector at the lower right-hand corner of the amplifier box mates with a plug that has 12 signal pins and a keyed center pin. The latter actuates a switch behind the socket. The connector and plug are actually borrowed from the Luftwaffe. Hence, they carry an Fl-number : Fl. 32620-1 for the "Steckerdose" socket, Fl. 32620-2 for the bell-shaped shell of the "12-pol. Stecker" plug, and Fl. 32625-1 for the Bakelite insert of the latter. They were made by Siemens-Halske.


Fig. 6: the matching plug for the 12-pin connector


Fig. 7: the insert of the 12-pin plug


Fig. 8: dimensions of the pins of the plug

Original cables and connectors are often even harder to find than Hellschreiber machines and radios of that era. They appear to have been made of a material called "un-obtainium". For several years, I improvised with a home-built plug insert:


Fig. 9: an improvised home-built insert

The diagram below shows the simple keying interface:


Fig. 10: interface from the Feld-Hell's keying contacts to the keying input of the CW transmitter

It is easy to verify correct connection: insert the plug into the round connector, connect an ohm-meter across pins 3 and 4 of the plug, and push down the Morse-key of the keyboard ( = the key with the green dot). The ohm-meter should show close to zero ohms.

Note: pins 3 and 4 of the 12-pin connector are used for keying of a CW transmitter keying. As stated above, when the 12-pin plug is inserted into the connector, the center pin of the plug opens a contact between pins 2 and 3 of the connector. See Fig, 11 and the detailed Feld-Hell schematic (Fig. 15) on the "Electronic circuitry & schematics" page. This is necessary, so as to avoid the constant 900 Hz signal from the Feld-Hell's tone oscillator from being connected to the keying input of the CW transmitter. Without interrupting the 900 Hz signal connection, the transmitter (e.g., a 15WSEb) could be keyed "on" permanently. It is connected to pin 3 via the switch contact, such that the character drum can key the 900 Hz tone. The 900 Hz signal is always present at pin 2. It was forseen to use the constant 900 Hz output (amplitude 2 Vpp) in an external CW keying device. In that device, the 900 Hz was up-transformed to 400 Vpp and rectified for use as a blocking bias voltage to key the CW transmitter. It would be pulled to ground by the Feld-hell's character drum when sending a pixel. See Fig. 14 and 15 below.

Feld-Hell CW transmitter interface

Fig. 11: Connections of the 12-pin connector on the front of the Feld-Hell

Normally, the Feld-Hell prints prints not only received texts, but also the text that is sent. As a matter of fact, if a Feld-Hell machine receives signals while sending, both are printed simultaneously, on top of each other. As described in the note above, the 900 Hz tone signal is disconnected from the character drum when plugging in a 12-pin plug for keying a CW transmitter. So, when keying a CW transmitter, the sent text is not printed locally, unless the connected transceiver outputs a side-tone via its audio output. However, some CW transmitters (such as the WW2 15WSEb in "FF" mode) do not output a side-tone. In this case, there are several ways to ensure local printing, see methods A-D in Fig. 12. Method A is taken straight from the WW2 manuals (ref. 10, ref. 11). D. Bölke (DJ1LP) has shown that without the jumper between pins 4 & 5 (i.e., method B in Fig. 12), local printing is still active. He also found that jumpering pins 3 and 8 with a small capacitor also works (method C). The capacitor value is chosen such that there is no continuous keying of the CW transmitter. H. Fykse (LA6NCA) uses method D. Note that the volume setting on the Feld-Hell amplifier box may effect the  results.

Feld-Hell CW transmitter interface

Fig. 12: Four methods for obtaining local printing when keying a CW transmitter with a Feld-Hell machine

As stated above, when the 12-pin plug is inserted into the connector, the thick center pin of the plug opens a switch contact that is part of the connector. However, I have come across a Feld-Hell amplifier box (from 1941) that is different: its round connector does not have the integrated switch contact that is actuated by the center pin of the plug. Instead, it has a separate, manual switch. The amplifier box must be opened (and the module with the vacuum tubes removed) to access this switch and toggle it. It is easy enough to identify such an amplifier box: the shaft of this switch is mounted into a through-hole the front of the amplifer box, and is visible from the outside; see the yellow circle in the left-hand image of Figure 10B below.


Fig. 12: Manual switch - instead of the switch contact actuated by the center pin of the 12-pin plug

Of course, another option for keying a CW transmitter is to use the 900 Hz tone-pulses and convert them back to on/off-pulses. See ref. 6 for some circuit examples. Recovery of the on/off pulses inherently causes some reduction in pulse duration. This is not all that critical, but can be compensated with some simple circuitry as described on the  "Software & PC-interfaces" page. Also see the Note below.

As with the AM/SSB/FM receiver, interfacing the audio from the receiver to the Feld-Hell is also straightforward:


Fig. 13: interface from the audio output of the CW receiver to the receiver input of the Feld-Hell

Again: all audio interfaces of the Feld-Hell are transformer isolated. So, there is no need to provide galvanic isolation with additional transformers, to protect a solid-state transmitter/receiver.

Note: direct keying of a CW transmitter is no problem with solid-state CW transmitters or tube transmitters with a solid-state keying relay. However, older tube transmitters may have significant negative or positive voltage at the key input connector (as much as several hundred volt!). This depends on whether grid (grid-block) keying, cathode keying, or anode keying is used. Key-down current draw may also be significant. This may exceed the voltage, current, and power limits of the Hellschreiber's character drum and slip contacts. Rudolf Hell and his co-workers (ref. 3, ref. 7) recommend such direct keying only for low power tube transmitters.

For medium power (up to 100 W) they describe a simple keying device ("Tastgerät") between the drum contacts and the transmitter's key input. It basically generates a high DC voltage. The circuit takes the continuous 900 Hz tone of the Hellschreiber (available at the 12-pin round connector), transforms its 2 Vpp amplitude up to 400 Vpp, which is then rectified. Ripple smoothing is done with a simple RC-filter. The resulting voltage is passed through a large current-limiting resistor (1 MΩ) to the keying input of a grid-keyed transmitter via the Feld-Hell's drum contacts (accessible at the 12-pin connector). They short the keying input when pixels are transmitted. 12 volt DC heater-filament power for the rectifier tube is also available at the 12-pin connector.


Fig. 14: Hell company's concept of a keying circuit for medium power CW transmitters

(source: adapted from Fig. 13 in ref. 3, Fig. 7 in ref. 7)

For high power CW transmitters, a real keying device was recommended. It works the same way as the Feld-Hell's keying circuitry for the printer solenoid: the tone-pulses from the Feld-Hell machine (not available at the 12-pin connector!) are amplified, rectified, and filtered. The resulting DC pulses are fed to the control-grid input of a keying tube:


Fig. 15: Hell company's concept of a keying circuit for high power CW transmitters

(source: adapted from Fig. 14 in ref. 3)

I have not (yet) been able to determine if the Hell company or Siemens-Halske ever manufactured such active keying devices. As with CW, an electro-mechanical relay (EMR) could be used for keying a transmitter. Rudolf Hell recommended against this (1940), based on relay reliability during long term operation. Note that 1 hour of full-time transmission = 60 x 60 x 2.5 = 9000 characters. Assuming an average 10 distinct pixels per character, this corresponds to 10 x 9000 = 90,000 switching cycles per hour. Modern EMR typically have a life expectancy of a million cycles. I.e., the equivalent of only 11 hours of continuous Feld-Hell transmission! Reed relays may have up to 100x longer life expectancy. Mercury-wetted reed-relays are an option, but not exactly eco-friendly when they break, and not usable below -39 deg (°F and °C) as the mercury freezes. But at such low temperatures, the grease in the Feld-Hell's drive train may also seize...

Siemens-Halske did manufacture at least one relay-based transmitter keying-device ("Relaistastgerät"), specifically for the Feld-Hell. See the "Feld-Hell peripherals" page.

Modern keying circuit designs use high-voltage/high-current MOSFETs, that can handle the high keying voltages of tube transmitters. Designs are available on the web and in kit form (e.g., towards the end of ref. 8, and references therein). The 2008/2009 ARRL Handbook for Radio Communications also contains an excellent Universal Keying Adapter design that is available in kit form (ref. 9).


External links last checked: March 2016, unless noted otherwise.

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