OA85 Germanium Diode

[Biggles]

Hi all, I have a few diodes type OA85 one of which is to be used in a crystal set I am constructing at the moment, but they don't seem to appear in any of the usual data books. I am just wondering if this type will perform ok in place of the usual OA79/81. The data available on the net seems to be pay only so I was hoping someone on the forum would have the data to hand. Was the OA85 developed for a particular role and this is why it isn't one of the more commonly found types? Thanks in advance.
Alan.

[paulsherwin]

http://www.alldatasheet.com/datashee...ETC2/OA85.html

Should be fine in a crystal set.

[Herald1360]

Just about anything will work(ish) in a crystal set. Ideally, get the lowest Vf diode you can find- OA47 is good but there's lots of opinion out on't interweb.

Base emitter junction on most small signal geraniums will also do quite happily.

[WME_bill]

OA85 point contact germanium diodes. It appears that it once was difficult to make diodes to an exact specification, so the makers would offer a selection, with better performance more expensive. Detectors for TV or AM/FM are low stress, so they used the cheapest end of the selection. Crystal sets even more relaxed. You could use any.
Type numbers for Philips changed as they were repackaged into smaller units. OA70 = OA90. 30v peak, 50ma, for low impedance detectors.
OA79 = AA119 was 45v, 4ma for TV detectors. Available matched pairs for FM.
OA81 = OA91 115v, 150ma, high back resistance. Forward voltage 2v at 30ma
OA85 = OA95 115v, 150ma, lower forward voltage. Forward 1.85v at 30ma.
OA86 90v high speed for switching
OA47 is gold bonded construction, for switching 30v 150ma, and much lower switch on voltage than the point contact diodes: forward 0.54v at 30ma. Similar are AAZ17 (75v) and AAZ15(100v). "Herald1360"s comment about low Vf is valid, but don't go and buy an OA47 specially. Any diode will do very well. germanium will start conducting at 0.2v, silicon at 0.6v.
Try DatasheetArchive.com. A very valuable free resource. Bill m0wpn

[Radio Wrangler]

There are 'Low barrier' Schottky diodes which are fully current parts and can rival the best Germanium point contact types.

HSMS-2820 is a low barrier Schottky in a tiny SOT23 package with about 250-300mV threhold

HSMS-2850 is its weird cousin. Virtually no threshold; it is a zero-bias part with a square-law region down to almost nothing. Don't use it above -20dBm RF input say the makers, so it's really for lower levels than listenable crystal sets (but interesting with an audio amp and speaker) If you've ever wondered what's in those HP/Agilent power meter sensors which go down to -70dBm well, now you know. (they use unpackaged die of this sort on a sapphire substrate)

Just to say that low threshold diode technology didn't stop with the OA81

David

I remember snipping (non specific) diodes out of dead dual standard TVs, you would get five, two for the FM, one for AM and one each for negative and positive modulation. They all worked in my crystal sets. The procedure was usually performed on a landfill with a penkinfe.

Oh happy days!

[Biggles]

Thanks for the info. I will just try one in and see how it performs when I finish building the set. I was just wondering if the OA85 had been developed for a particular role which would make it unsuitable as a straightforward signal diode but I think we can now discount this. I have designed the layout so that the option exists to either use a glass encapsulated type across two brass terminals on the front panel or a "proper" crystal using a piece of galena that can be commonly found on lead mining spoil heaps not far from here. I have rigged a springy contact and a clamp for the galena which fits nicely on the front panel so a bit of experimenting beckons!
Alan.

[Valvepower]

Originally Posted by merlinmaxwell

I remember snipping (non specific) diodes out of dead dual standard TVs.
Oh happy days!

Hi,

Reading that brought back the same memories of diode removal. Mine were late 50’s 405 sets back in the very early 70’s when Colour and ‘625’ TV took off. Mine manly came from the local council tip and the diodes removed in a similar ‘surgical’ manner in the back garden!

Sadly I feel a Bush TV22 came to a sticky end in the pursuance of bits to build radio projects - Guilty as charged, mi' Lord!

Terry.

[Biggles]

I seem to remember finding a GEX34? glass diode with a yellow plastic sleeve around it and removing it from a TV to use in a crystal set which in the end could only pick up Radio Moscow and I think Radio 4 from a local transmitter about 5 miles away with a bit of rewinding of the coil. Anyhow, it got me hooked on radio tinkering. I think I would be about ten at the time. Happy days.

[John_BS]

germanium will start conducting at 0.2v, silicon at 0.6v

If you ever want to build a simple log amp, the humble 1N4148 will come in handy!

John

[Fidelius]

Biggles, I hope you still have some of those OA85 germanium diodes from your November 2012 query. I believed I had 2 working OA85s when I designed my TRF valve radio; I even just finished the printed circuit to accept some of the components that won't fit on the receiving valve base, which would have included these two OA85, and discovered to my horror that one of them is a dud. Any chance we can trade ?

[jim_jobe]

You'll need to listen after dark for best reception of MW Biggles unless you're very close to the transmitter. During daylight try winding a SW coil. I used to get terrific SW reception in Newcastle during the early 60's!
Jim

[Boater Sam]

When we sold up and moved I skipped a full box of new GEX34s, about 1000 of them.
Fool.