Home:  Semiconductors: MULLARD OA5 AND OA10 GERMANIUM DIODES, 1950's


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

Nortel Collection

Your comments:

  • 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:
    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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