GEX34 diode?

[Neil Purling]

Anyone know what the equivalent would be from Mullard?
I assume this is a GEC component.

[oldticktock]

OA90(50V) or 91(115V) depending on Voltage required

[mickjjo]

My 1968 Mullard data book gives the GEX34 equivalent as the AA119 germanium point contact diode, for simple radio circuits most other germanium diodes would probably work fine, 0A81, 0A91 etc.

Regards, Mick.

[geofy]

Or a B_E B_C junction of a germanium transistor.

[Neil Purling]

I bought a bunch of 'surplus' diodes, but they have the Service CV7130 reference code.
Has anyone got a guide to it? I was told they were one of the types (OA81, 90 or 91), but I can't remember which. The component itself is marked with the same CV7130.
What inverse voltage was the GEX34 rated at?

[oldticktock]

I think a GEX34 is 50V ..............Daten/electr.data: I F: 1 mA; U F: 1 V; I sp: 250 µA; U sp: 50 V; Imax: 30 mA; Umax: 60 V; tmax: __ °C.

CV7130 is a OA91

[Neil Purling]

CV7130 = OA91, brilliant equivalent.

[rherber1]

Originally Posted by mickjjo

My 1968 Mullard data book gives the GEX34 equivalent as the AA119 germanium point contact diode, for simple radio circuits most other germanium diodes would probably work fine, 0A81, 0A91 etc.

Regards, Mick.

Looking at the data on the OA81,90,91 these typically have a Vf of around 0.2V at If = 0.1mA and Vf is only around the 1V mark for If = 10mA and above. This contrasts markedly with the GEX34 data provided by oldticktock which quotes Vf = 1V at If = 1mA, which is typical of the maximum current one would see in detector circuits. The OA81 etc are more general purpose diodes whereas the GEX34 is designed for detector use. They would work to some extent (depending on the application) but if the application specifically requires the characteristics of GEX34 or equivalent, then circuit performance would be degraded with the OA's.

I was after the data on the GEX34 myself because it is used as the detector diode MR1 at the ouput of the meter amplifier of the AVO Transistor Analyser Mk 2 which I am restoring.

http://www.radiomuseum.org/schemavie..._mk2_sch3b.pdf (right hand side VT3, 4, 5))

Incidentally, this amplifier looks ideal for translation to silicon transistors in place of the Ge OC75's and CV2400 without too many problems. A readjustment to the values of the base biasing resistors should be all that is required.

The collector dc rail is nominally -8.2V and in all 3 transistor stages the resistors are 120k (upper) and 10k (lower) and the Ge transistors have an Hfe of approx 100. My thinking was to change the bias resistors to 180K (upper) and 27K (lower) and use a BC178 (Hfe typ 180) as the replacement for all 3 Ge transistors. I would appreciate any input from others as to their thoughts on whether this seems a reasonable modification.

[David G4EBT]

I wonder if the GEX34, is equivalent to the 1N34, the data for which is:

Piv 60v max.
Reverse current: 30ua at 10v,
max forward volt drop 1 at 5ma

If so, new 1N34s are widely available.

EG, an ebay supplier is currently offering packs of ten for £5.15 inc P&P.

I'd rather not post the link, but google 'ebay+1N34' will throw up several sources.

The 1N34 is a germanium dode, specifically for signal detection - not a rectifier diode.

Note: It's number 1 not letter I. ('one en' not 'eye en'!)

(The topic of GEX34/1N34 has been debated in another radio forum).


Hope that helps a bit.

David.

[jim_jobe]

PM me if you want a GEX34
Jim

[paulsherwin]

Originally Posted by rherber1

The OA81 etc are more general purpose diodes whereas the GEX34 is designed for detector use. They would work to some extent (depending on the application) but if the application specifically requires the characteristics of GEX34 or equivalent, then circuit performance would be degraded with the OA's.

The OA81 is the standard detector diode in practically every British made transistor radio of the 1960s. The exceptions mostly use a GEX34

[Electrical]

Hello
I am not sure what application you wish to use the GEX34 or OA91 diodes. They are both Germanium point contact diodes and are suitable for use as AM detectors and have a very low junction capacitance of around 0.9pF at 45MHz.
I have scanned the GEC data on the GEX34 and the Mullard data on the OA91 diodes.
Please find attached PDF’s for the diodes. I hope this information is of some help.
Regards Stan.

[rherber1]

Originally Posted by paulsherwin

The OA81 is the standard detector diode in practically every British made transistor radio of the 1960s. The exceptions mostly use a GEX34

I agree the OA81 was widely used as a detector mainly because they did work in that application. Probably because they were easier to obtain and cheaper than the GEX34. In the datasheet provided by Valveman49 the graphs show expanded Vf Vs I curves from around 15uA to 6mA for the GEX34which is specific to the very low forward current region typically encountered by specialised detector diodes. A typical OA81 datasheet from Philips is far less informative in that region. This graph concentrates mainly on the upper regions of performance and would tend to indicate the general purpose application of the OA81. The GEX34 was physically much smaller than the OA81 but that was solved with the introduction of the OA91.