This short wave receiver was manufactured for the Kriegsmarine and is built like a battleship as well. It weighs a hernea-producing 65 kg (140 lbs). It was used on battleships like the Prinz Eugen or the Bismarck, and it was used in u-boats. Many coastal ground stations also used this receiver. Rommel's Afrika Korps used the Lo6K39a as well because of its incredible sensitivity to intercept enemy transmissions. This is one of the most sensitive short wave receivers ever built, and it must have cost the Germans a large sum per unit. Like with many other radio sets produced specifically for the military, the amount of intricate hand assembly and precise engineering used here is unequaled for that time period. This Lo6K39a was meticulously restored by Vladimir Dvorkin, from whom I purchased this radio.

The purpose for this receiver's design is very unique. It is a direct amplification receiver for a reason. On super-heterodyne receivers, the oscillator generates a weak signal at the required intermediate frequency. There is a danger, however small, that this signal escapes the receiver through the antenna, and will be triangulated by enemy direction finders. Whether or not this is in fact true, the Germans took no chances, and used a TRF receiver on their battleships.

Lorenz Short Wave Receiver Lo6K39a (=E03237)
The Ultimate Tuned Radio Frequency Set? 
by Dick Rollema, PA0SE 

(borrowed from the LA8AK web site)

'Lo' stands for 'Lorenz'; '6' means there are six tuned circuits, 'K' means 'Kurzwellen' (short wave), 'L' means Langwellen (long wave); '39' is the year the set came into operational use and 'a' means that capacitors of standard types were used. 

The German forces used quite a lot of TRF sets, though superheterodyne receivers with single-knob tuning (ganged variable capacitors for the input tuned circuits and local oscillator) were known since about 1932. It is sometimes said there was fear that radiation from the local oscillator in a superhet would enable the enemy to locate the set by direction finders. The German superheterodynes were always very well screened and featured extensive selectivity before the mixer with three or more tuned circuits. It is difficult to imagine how the oscillator signal could reach the antenna via these circuits which are tuned to a frequency that differs from the oscillator frequency by the intermediate frequency. 

Radiation could just as well be feared for a TRF receiver, which for telegraphy in the A1A mode (keyed carrier) is operated with an oscillating detector. True, the amplitude of the signal is much smaller than of the oscillator signal in a superhet but in the TRF set the input circuits resonate at the frequency of the oscillating detector, so transmission of that signal through these circuits to the antenna is more likely. It has been reported that a German raider during WWII indeed managed to locate merchant ships sailing on their own, not in convoy, by direction finding on the signal radiated by their receivers. Presumably these ships had rather old-fashioned equipment, perhaps with an oscillating detector directly coupled to the antenna. 

The German Navy (Kriegsmarine) had another reason to dislike the superheterodyne receiver: its spurious frequency responses (second channel or image frequency and higher order products of the mixing process). On board ship, transmit and receive antennas are close together. Transmitters may be active with reception on other frequencies taking place at the same time, so the risk of spurious responses is a very real one. 

The Lo6K39a was not only used on shipboard but was also the main receiver in the land-based stations that were in contact with the German submarines. 

GENERAL DESCRIPTION OF LO6K39A 
Lorenz developed the Lo6K39a according to a specification issued by the Kriegsmarine in 1937. The author had access to the technical manual for the set and the first line in that comprehensive booklet neatly summarises what it is. In translation it says something like 'The shortwave receiver Lo6K39a is a five valve tuned radio frequency receiver with six ganged tuning circuits. A sixth valve serves for frequency calibration'. 

Photo 1 shows the front panel of the receiver. The dimensions are 538mm high, 500mm wide and 320mm deep (21.2 x 19.7 x 12.6in approx.). That is without the shock absorbing feet, on which it was normally mounted for shipboard use. The radio is built like the proverbial battleship and that is reflected in its weight, a solid 65kg (1431b approx.). The radio covers 1.5-25MHz in eight sub-bands that cover the following ranges according to the specifications: 

In practice there is some overlap between ranges. The relatively small frequency segments of each range, together with the 50:1 slow-motion drive makes for very easy tuning, up to the highest frequencies. But it is made even easier by the outer one of the two concentric knobs to the left of the dial window, which varies the tuning of the detector circuit over plus or minus 3kHz around the frequency set by the main tuning control. The inner knob varies the gain of the receiver. The main tuning knob is the left-hand one on the bottom row. The tuned frequency can he read from a separate scale for each sub-hand. There is no risk of reading the wrong scale because a window in a mask that rotates with the band selector control displays only the scale to be used. Below it is a logging scale and on top a blank scale on which marks can be entered by pencil through a hole in the glass cover. The mask also clearly shows the frequency range of the selected sub-band and by means of arrows even the direction the selector knob has to be turned for the adjacent subbands. Band switching by means of the crank on the right of the middle in the bottom row is very positive and imparts a sturdy feeling, like opening the door of a safe or an expensive old-time motorcar. 

The knob on the extreme right at the bottom controls detector regeneration. The German designers certainly knew how to make a superb regeneration control. The detector moves smoothly in or out of oscillation without any backlash or fringe howl. The round object at the top left of the front panel covers a neon lamp that protects the receiver against high voltages on the antenna, e.g. from a neighbouring transmitter. Immediately below it to the left is a three-position antenna switch whose function will he explained later. Top right is a meter with a yellow and a red pushbutton. Operating the yellow button makes the meter indicate the 12.6V heater voltage; the needle should be in the yellow sector of the meter dial. Depressing the red button makes the meter show the anode DC voltage; the needle should now be in the red sector of the dial. The knob below the meter allows measurement of the anode current of each of the six valves; when currents are normal the needle is in the blue sector of the meter dial. For this the gain control must be at its maximum position and the frequency dial at zero on the logging scale (the reason for this will become clear later). 

The meter switch is spring-loaded; when released it automatically returns to the starting position. The knob in the middle on the right operates an audio filter, tuned to l000Hz and with a bandwidth of 200Hz at -3dB. The pushbutton below right activates a quartz-controlled oscillator for frequency calibration.

Finally below the dial window are outputs for a headphones and an 'Einheitsbetriebsgerät' (Remote control unit).