The first audion which I really like

 

Last update: December 12, 2005

 

Russian tubes from military stock are offered by several mail-order companies, often for bargain prices (i.e. 50 dollarcents). Such tubes can be used to build radios ranging from simple regenerative receivers to very complex superheterodyne circuits (see references 1-4). The unusual shapes of the tubes and their low prices were very tempting. I decided to build the “KW Röhrenaudion mit 12V Anodenspannung” published by German radio amateur Lutz Höll (callsign DK3WI). A 12V battery can power this whole receiver. Current consumption is about 100mA. The schematic is presented in Fig.1.

 

 

 

Circuit description

On his website (ref.3), Lutz gives the following circuit explanation (in German): “It is a Hartley-ECO Audion, in which the plate is grounded for RF. Positive feedback from cathode to grid occurs via the coil tap. The amount of feedback is determined by the 100k pot; this controls the voltage at the screen grid and thereby the transconductance of the valve (higher voltage resulting in greater transconductance). The screen grid is decoupled for RF and AF by the 100nF capacitor (a 1 µF Mylar cap with a 4.7 nF ceramic in parallel would be even better). The 100pF capacitor at the plate is a short circuit for RF, the following RC-filter (1k and 100pF) suppresses remaining traces of RF even more. The value of the plate resistor (180k) was experimentally determined (largest audio volume). The audio signal is led to the 1M volume control via a 10 nF capacitor. The 27k grid stopper prevents oscillation of V2 in the VHF region. Purists who like a more correct configuration of the volume pot may add another 10nF cap to the wiper and a 1M resistor from signal grid to ground. It may seem strange that the screen grid of V2 is connected directly to the 12V supply, but this setup was chosen after several trials – the addition of a screen grid resistor resulted in less audio volume”.

 

Tank circuit

My (Aren’s) coil L1 is wound on a plastic bobbin which originally held 250 grams of solder. The coil is made with enamel wire (AWG 18). Diameter of the coil is 28 mm (1.1 inch);  length is 25 mm (1 inch). The antenna tap is at 2 turns, the cathode tap at 3 turns from ground. As tuning capacitor I used a good quality air variable with 3 sections and  reduction gear. With all sections in parallel, the capacity variation is 38-90 pF. One may also use a more common 365 pF or 500 pF air variable with a silver mica, polystyrene or NPO ceramic capacitor in series (Lutz Höll employed a 500pF varco with a 110pF series cap). My circuit tunes from 5,800 to 9,000 kHz. Lutz points out that the optimal value of C in the LC circuit of a valve audion is: 100pF for 3.5 MHz, 50 pF for 7 MHz and 25pF for 14 MHz.

 

Output transformer

Old-fashioned valve audions would require high-impedance (2000-4000 Ohms) headphones, connected between the plate of V2 and the 12V supply. For 32 or 60 Ohms (Walkman-style) headphones, one needs an output transformer. Since good audio transformers are rare, I followed Lutz’advice and used a miniature power transformer for this purpose (220V:9V, 2.4 VA). Since the current flowing through the primary is quite small (less than 1 mA), this is a perfectly adequate (and cheap) solution.

 

Tubes

V1 (a 12SH1L) is an RF pentode with a Loctal socket and a normal glass bulb which is completely covered by an aluminum shield. The shield can be removed for better looks, although this will make the circuit susceptible to hum (I left the shield intact). V2 (a 1SH29B) is a subminiature pentode. Datasheets for both tubes can be downloaded from various websites (see below, refs.5-7). If it proves difficult to acquire them: drop me a line and I will send them by e-mail. Please note that Cyrillic characters on Soviet tubes are not always transcribed in the same way. The tubes used in this circuit may also be sold as 12J1L and 1J29B, 12S1L and 1S29B, or 12Z1L and 1Z29B.

 

Construction

The handdrawn construction plan (Fig.2) shows my layout of  the audion. It is built on a piece of plywood (16.8 x 11.5 cm, or 6.5 x 4.5 inches). The sides of my “chassis” are also made of plywood; front and back of the receiver are of unetched PC board. Tubes, output transformer, tuning capacitor and coil are on top, the other parts are mounted at the bottom. Three strips of unetched PC board are glued to the plywood (B+ supply, plate circuit of V1, and earth node of AF stage, respectively). Each valve stage has a single earth node (shown hatched in the figure). A thick ground bus connects these to the central earth of the whole receiver (stationary meshes of the tuning capacitor). For the sake of clarity, the ground bus is not shown in Fig.2.

 

 

 

A Yankee friend used to say: “Build it like a brick and it will be stable as a rock”. This is certainly sound advice: avoid dangling wires since what you’re building is essentially a variable frequency oscillator. In my receiver, the (relatively) long wires running to the controls at the front panel are made of 2.5 mm (0.1 inch diameter) copper wire, which is very stiff and solid. The receiver must be fed from a stable, hum- and noisefree power supply (or a 12V rechargeable battery). An excellent PSU circuit can be found at the Ocean State Electronics website, in their downloadable PDF document on direct-conversion receivers (see ref.8).

 

Why I like it

My previous experiences with simple regenerative receivers were not very positive. So why is this the first audion which I really like?

1.      Absence of microphony. The commonly employed battery tubes in valve audions (e.g. 30, 1H5G etc.) tend to respond to mechanical impulses with a loud “ploink…” However, the Russian military tubes used in this receiver are dead quiet.

2.      Smooth regeneration. When the voltage on the screen grid of the 12SH1L is increased, there is a gradual onset of regeneration. First, there is an increase of background noise. The noise becomes louder when the regeneration control is turned clockwise, and only after a while the “Mexican dog” appears. However, I must confess that my 100k lin pot has a 3:1 reduction gear… (it was recycled from a vintage pH meter). One can also employ a multiturn pot.

3.      Regen control needs little adjustment when the  tuning is changed. In my previous audions, the regen control required continuous adjustment when the band was scanned. In this receiver, such adjustment is not necessary. At the low end of the band (5800 kHz), the correct setting is a little bit lower than at the high end  (6500-9000 kHz), but the difference is small. After the correct setting has been found, the major part of the band can be scanned without touching the regen control at all.

4.      Stable tuning. A properly built receiver can be lifted 20 cm or more and dropped on the workbench; it will still be tuned to the same frequency. Moreover, there is no appreciable warm-up drift. The 12SH1L has a very low heater consumption (75 mA) and it does not get more than lukewarm during use (10 degrees C above ambient?).

5.      Can receive SSB. If a “clarifier” is added to the tank circuit, SSB transmissions can be received. Lutz Höll used a LED with inversed polarity as a varicap for fine tuning (see Fig.3). This allows listening to radio amateurs, although  the audio output is really low. You may consider the addition of an extra AF stage if ham radio is your primary interest

 

 

6.      Good audio quality, no audible hum and very little noise. AM broadcast stations are received with good audio volume and excellent audio quality after proper setting of the regen control.

7.      Requires only a short antenna. A three-feet whip antenna is sufficient for serious listening. During the evening hours, detector overload may occur in the crowded 49 or 31 meter bands. Shorten the whip antenna in that case. After I had completed this audion, I received low-power coastal stations in CW as far away as China, Guam and Taiwan, US and Canada, Bahrein, Turkey and the Ukraine. Every major shortwave broadcaster can be heard as well. Lutz’radio is an ingenious design. Build it yourself – I don’t think you will be disappointed! Tubes can be bought from various dealers, see e.g. refs 6, 9 and 10.

 

 

References

1.      Burkhard Kainka, http://www.b-kainka.de (German language site, many radio projects, very interesting. This guy tells you how to make radios from junk – even discarded disc drives and video  tape boxes are useful!)

2.      Jogis Röhrenbude, http://www.jogis-roehrenbude.de (German language site, a goldmine of information about valve radios, amplifiers, measuring instruments, etc. Also about the history of vacuum tubes)

3.      Lutz Höll, KW-Röhrenaudion mit 12V Anodenspannung, http://www.qsl.net/dk3wi (German language site)

4.      Jochen Wander, A Tiny Regenerative Radio with the Russian 12J1L, http://www.qsl.net/dl1jwa (Jochen claims that the performance of the circuit is even better at normal plate voltages, i.e. 180 Volts. He presents an “all-wave” regenerative  receiver with two valves 12SH1L).

5.      Klausmobile website, http://klausmobile.narod.ru/indexe.htm

6.      Tubes.ru (Russian tube dealer), www.tubes.ru (Please note that the tubes are called 12J1L and 1J29B at this site)

7.      Some datasheets are also made available at Jochen Gittels (a.k.a. Jogis) “Röhrenbude” (see above) and at the website of  Lutz Höll.

8.      Ocean State Electronics, http://www.oselectronics.com

9.      Pollin Electronic (German tube dealer), www.pollin.de

10. Oppermann Electronic (German tube dealer), www.oppermann-electronic.de

 

Finally, a picture of the Audion which we affectionately call our “Howlicrafter”:

 

 

 

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