Such are the breadth of functions delivered by integrated circuits, it’s now rare to see a simple small-signal transistor project on these pages. But if you delve back into the roots of solid state electronics you’ll find a host of clever ways to get the most from the most basic of active parts.\
Everyone was familiar with their part numbers and characteristics, and if you were an electronics enthusiast in Europe it’s likely there was one part above all others that made its way onto your bench. [ElectronicsNotes] takes a look at the OC71, probably the most common PNP germanium transistor on the side of the Atlantic this is being written on.
When this device was launched in 1953 the transistor itself had only been invented a few years earlier, so while its relatively modest specs look pedestrian by today’s standards they represented a leap ahead in performance at the time. He touches on the thermal runaway which could affect germanium devices, and talks about the use of black silicone filling to reduce light sensitivity.
The OC71 was old hat by the 1970s, but electronics books of the era hadn’t caught up. Thus many engineers born long after the device’s heyday retain a soft spot for it. We recently even featured a teardown of a dead one.
I remember removing paint from this transistor to use it as a photo-diode.
And I just learned that this is a myth :D
I was just a kid… what do I know.
It’s not a myth, I did it too and there were more than a couple of magazine projects that used ‘scraped’ OC71s.
I even remember seeing construction projects that suggested you could alter the sensitivity by scraping more or less of the paint off or make the response more selective by using different colour gels/paints/nail varnish (usually with an ‘of the era’ sexist comment along the lines of ‘don’t let the little lady know’)
in my experience the little lady is always eager to give up a little enamel for the workbench :)
Not so strange, you reminded me that my first digital camera used a delidded DRAM chip as sensor :)
I went to a very techie high school. I did a roaring trade in knocking the lids off without breaking the wires for fun ans profit
I did this to use as a fake CCD on my telescope :)
It isn’t aa myth. I used one to make a light pen for my CBM PET.
No, that was definitely not a myth. As a kid, I collected OCs from old radios, scraped off the paint, and connected them in parallel to create a small “solar cell”. It only produced a few millivolts, but my youthful inquisitive mind was thrilled ;-)
My passion for electronics began with OC transistors. Therefore, I still have a soft spot for them .
As do I! They had what looked like a light blue paste inside them. Presumably for heat dissipation.
Quite right I did same to hmm I should still have some OC71 some where in a parts draw
Still have some in my “junk box”. Scraped, unscraped and the official unpainted OCP71 Mullard photo transistor.
and I did it as a kid, so not a myth..
And me. According to the article only later ones were filled with black stuff, supposedly to avoid problems with the paint flaking off in service. But the fact that they sold a special OCP71 optical one at a higher price might have something to do with it!
They filled the inside of the case with silicone grease for better thermal transfer from the die to the case because beyond 70°C they exhibited runaway leakage, gain loss, or thermal runaway.
What’s inside a OC71 transistor from 1957 (PWJ277)
https://www.youtube.com/watch?v=L7GaFaqFGR8
Internal diagram – a slice of n-type germanium with the indium collector and emitter lead bonding “solder” serving to p-dope their area of contact:
https://www.thevalvepage.com/trans/manufac/p1_fig1.gif
The first transistors I bought were ‘Red Spot’ reject AF devices from Henry’s Radio in Edgware Road, London. 10 shillings each, IIRC. I think they were by AEI.
10 shillings !!
You were robbed, I got them way cheaper. That was about 1960.
Henry’s Radio, now there’s a blast from the past, I believe they’re still in business as Henry’s Electronics and much changed.
So many electronics shops lost to history (and it was happening pre WWW as well), so many memories,
Henry’s was but a dream to me. I was too young to go there on my own. But, I was euphoric to find that an electronic components shop was hidden away behind a newsagents shop really near my school. It was called Chromasonic Electronics, and was accessed by walking through the newsagents, which I think was called ‘Raymond’, in Muswell Hill Broadway. It had previously been in Fortis Green Road, but the newsagent moved, and the Aladdin’s cave component emporium moved with it! It was about 50 years ago, so might have been the other way around! Wasn’t a problem – I could still walk there after school.
I went there at least once a week. And bought many OC71s and LEDs to make flashers. I still recall the feeling of profound joy when I had saved up enough money to buy myself a ’17” Veroboard’. I probably still have some remnants of it in my boxes.
I was introduced to the OC71 by the Ladybird ‘Making a Transistor Radio’ book. I still have the book. And some of the OC71s. Sadly not the Jackson Dilecon variable capacitor!
Such fabulous memories.
There were LEDs the same time as OC71’s??? I remember getting my first LED. Was a very new thing that cost me something like 26 bucks! Took me weeks to save up for it! But at the time I’m pretty sure I also had a silicon transistor to drive it with.
Yes, fabulous memories. I may had the same book. Remember using an OC71 to amplify a crystal radio! Think I paid 1 shilling 9 pence at Lings radio store in Wolverhampton. Circa 1964.
Edgware Road, an electronic hobbyist’s paradise when I was young!
And Lisle Street. GW Smith at one point had 2 stores there. Lots of military surplus stuff. For my 11th birthday I got a Hartley dual beam oscilloscope. I felt like Doctor Who.
Now I know what the Red Spot denoted on those bad boys! Worked great though – mid ’70s
Wasn’t this just posted already? https://hackaday.com/2025/10/10/inside-a-germanium-transistor/
It was …
No it wasn’t. Same device, different video. Do keep up.
As an African teenager, I would smash open integrated circuits and probed them under illumination in the 80s. Once i even made an automatic light using one such unknown IC. That people used scraped OC71s’ as a phototransistor isn’t unimaginable.
In the 1960s, when I was in elementary school, I had a few OC70s and an OC71. When I made my first flip-flop with them, I thought I had invented it! Funny, but the excitement I felt has not left me yet!
The most valuable piece of equipment was a 0-6 V voltmeter. I still couldn’t afford a soldering iron, so I connected the components by wrapping wires.
I used to heat up a screwdriver on the gass cooker to do my soldering in the early 70’s as a kid oc71’s where hard to get had to look on rubish tips to find
Old broken radios and tv’s for parts
I bought my OC71’s from Home Radio, Mitcham UK. I also had a Saturday job there. I would have worked there for nothing. It was full of electonic components. Nurdy boyhood dream come true.
You say ‘Europe’ but I think you mean UK.
Is there anybody here from the mainland that ever heard if this transistor?
I’ve never seen it nor recall hearing about it, now the BC547 and BC557 and BC558 are that range are the old European classics I heard about, which came from the predecessors BC107, BC108 and BC109.
Not that it matters, but why not say UK though.
The OC71 and OC75 germanium – transistors were very common throughout Europe, and from various manufacturers. Typical applications for OC transistors were AM radios and small audio amplifiers in the early 1960s.
The “BC” transistors were silicon (!) – transistors and came onto the market many years later. The BC107 was as popullar as a silicon transistor as the OC71 had been as a germanium transistor ;-)
And still I don’t recall seeing it IRL (saw it on CuriousMarc possibly), but 1950 stuff is very dated of course, but I’ve seen the inside of tube radios though.
It must have fallen right between the very old stuff I saw and the regular old-but-post-tube era stuff.
I do notice a lot of comments about it are from Brits though, and there are mainland Euro commenters on HaD.
In the mid-60s, as an elementary school student, I received a Philips electronics kit for Christmas. It already contained two AC126s, also a germanium PNP transistor and basically the successor to the OC71. Shortly after that, I also received some OC71s (my father was technical director of an electronics company). I “disassembled” these and used one of the “unfilled” ones as a photodiode. A short time later, though, I also received some BC107s…
Oh yes, that was in West Germany.
Given that it was a Philips part from the Netherlands, I’d say yes.
I’ve piddled w electronics (mostly in my mind, such as it is) and would spend hours perusing several magazines from the 70s on. I’d never seen this bullet style transistor case before! Is it unique to the UK?
No, the OC types were also widespread in Germany, for example. They were the first generation of transistors – but were then relatively quickly replaced by the “AC” types in metal packages. These remained on the market for quite some time before being superseded by the “BC” silicon types in the 1970s.
On a side note: The last series of OC 7x transistors were encased in a metal housing – including their original glass packages. From the outside, they looked like a new generation of components, but inside they were still the old glass OCs ;-)
Then the nomenclature of germanium transistors in Europe changed from “OC” to “AC” (and “AF” / “AD” for higher frequencies / power).
Now the AF ones I’ve seen, and I have a circuit with one actually, an antenna amplifier.
If I’m not mistaken, and if you’ve worked on transistor radios of the period here in the U.S., you’d have come across this part as the 2N406, bearing a label from Motorola or RCA.
Another old timer from India. From back in mid 70s and well into early 90s, we could get a germanium complimentary pair AC187 and AC188 in an aluminum cylindrical casing with very flimsy legs. We cut open the top of transistors whose base leg broke off, wash a white colored heat sink compound with IP and used them as photo transistors.
Wow, takes me right back to my early years in the 60s’ learning how to use OC71s from my dad’s transistor collection. Still have a few in a tin somewhere. Thanks for the memories.
Plz put video abt oc44 and oc45
I built a “Practical Wireless 6 transistor superhet” when I was 11, used an OC44, two OC45s, an OC71 and two OC72s, all bought mail order from Henry’s.
Happy days….
Aye. I learned about the OC71 from that ladybird book. A 1 transistor medium wave radio. Moved on to the Zn414 IC in the early 70s.
Interesting and fun to read these comments. I pursued music after mid- 60’s and didn’t fully return to audio/radio engineering (et cetera) until mid- 80’s … my but Father-in-Law’s career as was an electronics engineer with Lear Siegler (spelling?), and I got his audio/radio electronics literature/magazines from the 30’s and 40’s; historically fascinating fun.
On the west side of the Atlantic, 2N2222 and 2N2907 were the king and queen of small signal transistors.
That was probably a bit later when the 2N2926 appeared in loads of UK magazine projects.
I began a 3-year radio technician traineeship here in Australia in 1965. The OC71 was a common item for us, as was the AC126. And, yes, I scraped the paint off to make photo-transistors. I still have OC71s in my parts drawers, and still with plastic sleeving on the leads to identify base, emitter, and collector.