Germanium Diodes and possible alternatives?

Thread Starter

Ed Bray

Joined Feb 1, 2017
46
Hello,

I am looking at building the simple signal tracer at PastTimesRadio.co.uk

Here is the schematic:
signaltracer.jpg
And here is the component list:
Components List
Resistors (0.25W Carbon)
R1 22K
R2 4.7K
R3 3R3 (3.3 Ohms)
VR1 22K Linear with switch

Capacitors
C1 1nF Ceramic Disc (300V)
C2 0.1uF Ceramic Disc (300V)
C3 10uF 25V Electrolytic
C4 68nF Polyester / Milar
C5 0.1 uF Polyester
C6 470 uF 25V Electrolytic

Semiconductors
D1 OA79 germanium signal diode
IC1 LM380N

Misc.
Miniature L.S. 8 Ohm 300 mW
Veroboard, connecting wire,
9V PP3 battery, probe,
suitable enclosure.

I have pretty much everything required, but do not own any Germanium Diodes, I have been looking for them in the UK, but many places do not stock them anymore and the diodes on ebay such as 1N60Ps are disputed as being Shottky diodes and those that aren't are quite expensive and only have 2nd class delivery (and I am not a patient man).

In the build blurb for the circuit, it states the following "The detector or demodulator diode should be a good quality germanium type, OA79 or similar. Don't be tempted to use a silicon diode - it won't work !"

So now to my questions:

I have gone through my diode box and have checked a selection from each of the types on two digital multimeters and also on my cheapy Chinese component tester, almost all show between 0.5V and 0.6V forward voltage. However, one set of diodes (which I can't fully confirm the number which begins 1N581X, where X is smudged and unreadable on all the diodes) all show a forward voltage of between 0.175V and 0.185V on the digital multimeters and show a forward voltage of 0.285 or so on the component tester (see below):

multimeter diode.jpg


Multitester diode.jpg

So my questions are, would these diodes work in place of the required OA79 given that the have such a low forward voltage, or is there something else that Germanium offers as well as a low forward voltage that would negate their use? If so what is it please?

TIA, Ed.
 

Thread Starter

Ed Bray

Joined Feb 1, 2017
46

Thread Starter

Ed Bray

Joined Feb 1, 2017
46
Thank you again Eric, I have just managed to get 10x 1N34A with first class postage for £4 on ebay, I will build the circuit and when I have it working I will try it with both the 1N581X and the 1N34A to see if there is any difference.

Other than forward voltage, is there any other reason why Ge might be preferred over Si? It seems what was available previously does not seem so prevalent now, is it just down to cost, or have improvements in Si components caused the death of Ge types?
 

GopherT

Joined Nov 23, 2012
8,012
Thank you again Eric, I have just managed to get 10x 1N34A with first class postage for £4 on ebay, I will build the circuit and when I have it working I will try it with both the 1N581X and the 1N34A to see if there is any difference.

Other than forward voltage, is there any other reason why Ge might be preferred over Si? It seems what was available previously does not seem so prevalent now, is it just down to cost, or have improvements in Si components caused the death of Ge types?

What frequency are you looking at. I think frequency is more of an issue than voltage drop. A 1N4148, 1N914 should be tried. What IC is that?
 

Thread Starter

Ed Bray

Joined Feb 1, 2017
46
What frequency are you looking at. I think frequency is more of an issue than voltage drop. A 1N4148, 1N914 should be tried. What IC is that?
Thanks, did not realise about the frequency aspect, the IC is an LM380N which is just a 2W amplifier to drive the output speaker.
 

Willen

Joined Nov 13, 2015
300
Maybe power of source signal is critical with voltage drop of diode and choosing between Ge-diode and 1N581X is critical with frequency of the signal.

In my batteryless crystal radio (around 1MHz) the 1N581x is performing better than Ge-diode because of its low voltage drop. Because of low drop, the headphones gets much power than Ge-diode. (An xperiment: 1N4148 was not working just because of high Vdrop. I heated the 1N4148 so much with soldering iron and the voltage drop decreased from 525mV to 170mV, then the 1N4148 were performing better than Ge-diode (voltage drop around 300mV.)

But if frequency could go around GHz then 1N581x power schottky might not rectify as Ge-diode can.
 
Last edited:

Alec_t

Joined Sep 17, 2013
11,979
Some of the low current (30mA) Schottky diodes look promising. According to LTspice the RB705D and RB715F, for example, have a 3dB bandwidth of ~70MHz. Don't know if they're actually used as RF detectors.
 

Reloadron

Joined Jan 15, 2015
5,875
The first germanium diodes I used as a kid here in the US was the 1N34 and as far as I know they are still available as 1N34A or an NTE 109 I think. I am not sure of availability in the UK.

Ron
 

Thread Starter

Ed Bray

Joined Feb 1, 2017
46
The first germanium diodes I used as a kid here in the US was the 1N34 and as far as I know they are still available as 1N34A or an NTE 109 I think. I am not sure of availability in the UK.

Ron
I managed to order 10x 1N34As with 1st class post in the UK earlier for £4 ($5), however, have also just ordered a further 30x 1N34As for £4.36 including shipping from the USA, so won't run out anytime soon.
 

Reloadron

Joined Jan 15, 2015
5,875
Glad it worked out for you. Thinking back to the late 1950s was my first crystal radios using those 1N34 diodes. They have been around for awhile. :)

Ron
 

Thread Starter

Ed Bray

Joined Feb 1, 2017
46
Thanks again all, just for future reference I built the circuit on a breadboard yesterday and tried both the Schottky and Ge diodes and it worked fine both ways. At least I know now.
 
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