Lost to time: Check out the Fairchild 627 Equalizer circa 195nvr

fairchild_627_equalizerApologies for the poor image quiality; these came from the research department of a long-gone British broadcast-gear-manufacturer via some dampish papers that I purchased recently from a rare-books-dealer in London.  I had never heard of the Fairchild 627 ‘Variable Equalizer,’ even though I recently sold an example of its (even-more-rare) predecessor the model 540, which in retrospect seems to be the ‘boost’ sections of the 627 plus a mic preamp plus a 20-watt cutter-head power amp.  Well shit.  Now i wish I had measured the values of the triple-ganged frequency-select pots.  Why?  We will get to that in a minute.

fairchild_627_equalizer_internals

vintage-unobtanium-fairchild-627-tube_1_99dc4f93df9c200f11996b06da8735f3image source

A quick google search suggests that there are thought to be only 5 Fairchild 627 equalizers in existence.  One seems to have sold on eBay recently in the $7000 region.  The 627 is a line-level EQ with continuously variable low and high bell boost sections as well as high and low roll offs. Unlike a Pultec, though, it is an active EQ, and it uses some pretty unusual circuitry – especially the high-cut section, which I don’t comprehend at all.

fairchild_627_block_diagram

fairchild_627_equalizer_schematicOne point to note: T2 should be wired to reverse phase.  I was v excited to find this schematic, because it seems like a pretty easy piece to build – there is no mention of the proper B+ voltage, but 250 is generally a safe bet; there are no weird inductors (the UTC S-23 plate-load choke is an off-the shelf part, so we know its specs  -5000ohm DC, 300 henries); the audio transformers don’t need to handle DC and the turns ratios are spec’d:  BUT:  But.  Those triple-ganged pots.  WTF do we do about them?

Can anyone crack this case? Any insights, pls let us know in the comments section.

On the plus-side:  at very minimum, this schematic does reveal an excellent and easy way to implement a 6E5 seeing-eye tube simply paralleled to a grid in the audio path.  So at very least, now i know how to use those v excellent things in my mic preamps.  Way more evil than a VU meter.

Also: the 6SN7 output section is a neat little self-contained module; def worth trying it with a Hammond 156C choke and an Edcor 15K:600 1/2 watt.   The 156C is the closest modern part to the S23 spec; it is rated 150h and 3.7K ohm, seems close enough?  I used onna these on a hunch in my OP6-semi-clone and it worked just fine in that application, so I am guessing it may work fine here as well.  fairchild_627_boost_curvesfairchild_627_high_low_frequency_rolloffsfairchild_627_curves_2

 

3 thoughts on “Lost to time: Check out the Fairchild 627 Equalizer circa 195nvr”

  1. Hi Chris, did you get AML to make the kit of this old EQ? So cool man! What is your email? Thanks

    Forrest

  2. Hi Chris,

    probably expensive as hell if you can even get a small qty, but there are options for the triple-taper pot it seems:

    http://www.potentiometers.com/seriesS127.cfm

    Any mention of what value they should be?

    Thanks a lot for posting this; looks like a very cool piece!

    By the way, i really dig your videos on Youtube, would love to see you do more; especially regarding equipment builds and studio techniques!

    Cheers,
    Steve

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