MULLARD OA5 AND OA10 GERMANIUM DIODES, 1950's

Early Germanium Junction diodes from the 1950's made by Mullard. The OA5 is 'Gold Bonded'. Advantage or Sales pitch!

Nortel Collection

Your comments:

• I remember going to the local tip 2 miles out of town, back in 1969 and scavenging 0A81 and 0A79 germanium diodes from old TV IF transformers, and biscuit tuner modules. I'd come home with a swagger of them, the IFs and all the litz wire. Old valve radios too, for the tuning capacitor, trimmer caps, IF 455 khz transformers for the litz wire and ferrite rods. With an earphone from an old rotary telephone, I'd have enough parts to build a basic crystal radio set, using mum's best cutting board, of course!
  .......... Austin Hellier, Brisbane, Queensland, Australia, 2nd of March 2025
  
  
• There are tens of thousands of OA5 diodes in the ICT1301 computer, one of which is at The National Museum of Computing.
  
  There are many OA10s too but far less. The other comments about optimising the OA5 for core memory selection is interesting as the 1301's core memory uses many of them, though probably less than are used in the computer's logic.
  
  Each 2D core plane of 40 x 50 bits is surrounded by, I think, 180 diodes, two for each of the column and row wires. A fully equipped 1301 has 5 'barn doors' each with ten core planes, two on each side.
  
  In the main logic, two or more OA5s are used to form AND gates which feed into a GET872 transistor to amplify the signal.
  .......... Roger Holmes, Medway,Kent, 8th of February 2025
  
  
• Remember making crystal radios with oa70s and oc71 amplifier. Currently work as a design engineer for London Underground, and a lot of the signalling equipment still in use using core store is packed with oa10s.
  
  
  
  .......... Roger cawley, Hemel Hempstead england, 9th of January 2020
  
  
• I worked at Mullard Semiconductor Applications Lab,
  in Totton, near Southampton, until 1964,when I took
  decided to take the plunge, and emigrate to the USA
  where I worked for Tektronix designing nice scopes.
  I remember the very first days of the OA5, which at
  the time (early /60's) was being optimized for core
  memory selection.I still have a stack of them along
  with hundreds of other early transistors, and right
  now I am very busy preparing for the Keynote speech
  at this year's BCTM (in my home region of Portland,
  Oregon). The talk has much to say about the history
  of the bipolar transistors,so if you have any ideas
  to add to the talk (it's next Monday, 6th Oct 2012)
  You can send your comments to this e-mail address:
  barrie.gilbert@analog.com
  Happy Memories!
  .......... Barrie Gilbert, aloha, OR,USA, 25th of September 2012
  
  
• I used OA5 and OA10 diodes in the 1960s while working at the Post Office Research Labs at Dollis Hill. I still have a number and indeed I am about to use some in a circuit with germanium transistors. The heading talking about gold indicating a 'cats whisker' is, I believe, wrong. The OA5 and OA10 came after cats whisker diodes and are germanium junction devices. The OA5 is a switching diode and for fast operation (for those days!) the semiconductor material was gold doped to reduce the hole storage time and hence the time to come out of saturation - you will know what that means if you understand the physics of semiconductors. Switching transistors were also gold doped, whereas those for analogue applications, such as amplifying, were not. Devices with an apparently higher frequency capability (fT, transition frequency), but which were not gold doped, were of less use for switching.
  .......... William Jones, Billingshurst, 10th of February 2012
  
  
• Ahhh the memory of my childhood in the 1960's fabricating my first Radio crystal sets using OA series Germanium Diodes (Mullard) for RF signal rectification , winding ferrite coils and stringing outdoor long wire antennas with paxolin isolators ! Not a battery in sight (using high impedance headphones)until I moved on to transistor audio amplification and speakers !
  .......... Hugh Brazier, Hitchin , 25th of November 2010
  
  

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