e43 (43a) Signal detectors in ex-German receivers

Other pages:
e12 Data for German communication receivers
e86 Thermionic Valves (tubes)
e. List of German radio communication

Mw.E.c

Torn.E.b

Detectors and audio amplifiers in ex-German communication receivers

The well known series of ex-German receivers use a standard pentode (RV12P2000).
The number of valves is seldom over 11 in order to keep the number of valves as low as possible, but still perform well.

Some receivers have another vari-mu pentode (RV12P2001) for [the] AGC controlled stages, but usually it was compromised by using only a RV12P2000. The 2V filament version of standard pentode is the RV2P800, and the 2.4V version is the RV2,4P700 – the AGC version is the RV2,4P701.

Wehrmacht equipment for 2V supply often have an audion detector. It seems this is desirable to pull weak signals out of the noise when diode detectors have insufficient drive level to operate satisfactorily - a similar effect can be achieved using quasi-synchronous AM-detectors using an IC (TDA1576, TBA120, MC1351, NE604 etc). The grid-bias for the audion-valve must be adjusted somewhere between f- and f+ for optimum reception, but it seems that different receivers have different settings, some have variable settings, while most have fixed resistors. So there may be different parameters to bear in mind.

Torn.E.b. regenerative detector. LA7MI, Stein Torp wanted to investigate practices which might have been forgotten in constructing good regenerative detectors for a wide frequency range. He started his 'industry espionage' to test some components in Torn.E.b:

RF choke (59) in anode circuit of detector of the Wehrmacht version of the receiver (seems to be a resistor in the version exported to Sweden) has a resistance of 480 ohm. It is an RF choke wound with thin resistance wire with a pot-core. He found it difficult to measure the inductance, but managed later to define the value as 170mH. Self resonance is 150kHz, it has a capacatative
effect on the higher frequencies. As he wasn't tempted to desolder the very thin wire from the RF choke, he used the following procedure to measure the inductance. He removed the detector valve, and set the sets of coils to a neutral position. The reaction control was set to minimum. Capacitance seemed to be 27pF, but if he used 170mH @150 kHz the value was 6.6pF.

Audio filter. He also measured the inductance of the LF-choke in the anode circuit of the detector stage. He first removed the 500pF capacitor. With "Tonsieb" set to "mit" you may measure between pot.7 and 27. It measured 85Henry.

MwEc BFO
A complex series resonnance type. Note the link which goes to the cathode circuit of the 2nd IF amplifier

Mw.E.c detector
Pentode applied as duo-diode-triode, not widely known, but some reports exists (G3VA technical topics, Radcom), and on an occasion 6AM6/EF91 was used by accident as 6AT6/6AV6/EBC90/EBC91 and functioned properly, the "fault" was only discovered several years later. In the next circuit (T8PL39) only one diode function is used, while in E52 one diode is used as detector and the second is used in AGC-delay circuit.

Note that AF gain control is bypassed for A1A reception. This is not practical for amateur radio (and may easily be changed), but it must be considered that the German operator needed easy as possible operation which suited his often limited experiences with radio equipment. You'll find similar mode switch in E52 and Kw:E.a. For SSB reception it is impossible not having the possibility to decrease the RF gain (to avoid overloading the detector) at the same time as you increase AF gain (since the level from the detector has been reduced far below normal).

T8PL39 Detector
See the note above (Mw.E.c) for this type of application as duo-diode-triode with RV12P2000

T9K39 BFO
Similar circuit to the one shown for Mw.E.c, but one position as free-running oscillator for opposite sideband selection.
Note that the xtal control is used as calibrator function, too.

E52 Detectors
See the notes for Mw.E.c for application of RV12P2000 as duo-diode-triode. Here the 'anode' is used in to delay the AGC such that a certain amount of detected voltage must be present before it is some variation in voltage to AGC-Line. For Kw.E.a the AGC-detector diodes are biassed with +4V, but it is not possible here as the cathode is grounded. In Lorenz 6P203 the cathode (EBC91/6AV6) has +5,5v level and gives a similar delay.

Note that AF gain control is bypassed for A1A reception. This is not practical for amateur radio (and may easily be changed), but it must be considered that the German operator needed easy as possible operation which suited his often limited experiences with radio equipment. You'll find similar mode switch in Mw.E.c and Kw:E.a. For SSB reception it is impossible not having the possibility to decrease the RF gain (to avoid overloading the detector) at the same time as you increase AF gain (since the level from the detector has been reduced far below normal).

Kw.E.a (detectors).

Detector circuits and modifications are shown on page 22b

My T9K39 was somewhat amateur'ized, so the connections for anode and screen-grid were reversed In the audio output stage, it may be a good idea to repeat the socket connections for those valves once more.

e/m

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