Probably an MXR Phase 90 or a Phase 100, possible an ElectroHarmonix Small Stone Phaser, or other brand. There were a lot of small shops that made them with derivative names like 'Phasor' ,'Phazor', 'Fazor', etc,etc.
Phase pedals were pretty much standard effects pedals back in the 70's, and a couple of manufacturers, (MusicMan and Peavey being the most obvious) even built them into guitar amps, superseding Vibrato/Tremolo effects. . Original MXR pedals and Small Stones are still highly sought after which makes the originals spendy, but Dunlop and EH makes reissues of them that have been upgraded with LED On/Off indicators as well as jacks for AC Adaptors so you don't have to worry about batteries dying in the middle of usage.
Also, I'm pretty sure that Garnet Gillies built a phaser circuit into the Garnet BTO 100 amps that he made for BTO back in the 70's. He still produced amps until he died a couple of years ago, and aside from Pete Traynor, was one of the best know Canadian amp maufacturers. --He OEM'ed a bunch of small tube instrument amps for Montgomery Wards after MW's contract with Supro/National/Danelectro wasn't renewed.
Also worth noting is that Boston's guitarist, Tom Scholz is an MIT Electrical Engineering grad who modified most of his gear to get the sounds that he was after, hence his Rockman line of headphone guitar amps and half rack effects in the mid 80's. I don't know what he was using, but before the guitar part that you pointed out on the Boston track, the BG guitar is using a Chorus or Chorale pedal of some sort, most likely something that he built himself.
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Last edited by Fogman on 21 Sep 2009, 3:47 pm, edited 1 time in total.