The Beatles' Abbey Road SECRET WEAPON - 1960s ALTEC SUPER 438A 436B Compressor Preamp Mods

In the late 1950s, engineers at EMI Studios, later renamed "Abbey Road Studios", purchased new Altec 436B compressors from the US to install in their control rooms. The original 436 compressors were designed for "set it and forget it" applications in music halls and public forums where various speakers of different vocal volumes might be taking turns on the same microphones, and thus an easy to use compressor was necessary.

The EMI engineers, realizing the potential and limitations of these units, set to work modifying them with additional controls and impedance selections. The resulting compressors were used on every Abbey Road Beatles record have been dubbed the EMI RS124 units. Similar vintage and reissue units are highly sought after today. Chandler Limited makes reissues that cost in the thousands and many recording plugins feature modelers of these units. Practically anything Beatles related fetches lots of money and attention, but these Altecs deserve both.

Here we will take a pair of 1960s 438A compressor/preamps, which are the big brother of the 436 series, and are an even better platform for mods than the 436, because they include an additional 12AY7 preamp tube not present in the 436B or 436C. In this video, we'll detail how to add additional controls for Threshold and Release to the 438A, while also introducing the capability of turning the 6AL5 dual diode compression tube OFF. We'll also add a second 50K dual gang volume control after the phase splitter transformer, which will allow more control over the gain of the preamp. Finally, we'll swap the 12AY7 and 6BC8 tubes for 12AX7 and 6AQ8, respectively. This will give the preamp more potential gain and allow the recording engineer to drive the tube preamps harder, if desired.

RESOURCES:
438A Manual: https://coutant.org/altec438a/index.html
436C Manual: http://www.dvq.com/hifi/images/436c.pdf
Abbey Road Official Altec Page: https://www.abbeyroad.com/news/rs124-compressor-gearthatmadeus-3174
RS124 Reissue Info: https://umbrella-company.jp/chandlerlimited-rs124.html

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