Originally Posted by: dlwalkeCan anyone tell which effect is being used on the Queen song Drowse.

Sounds like a phaser. There's also a slide being used on the electric guitar part. So that's adding to the effect. But the MXR Phase 90 was introduced in 1974, so it's possible May had one.

https://www.youtube.com/watch?v=OJfFt2i2Yd4

Also, Queen made wide use of physical phasing & flanging by slightly altering the tape speed to get those kind of effects on individual tracks & whole mixes.

https://en.wikipedia.org/wiki/Flanging#Origin

https://www.youtube.com/watch?v=Ko6rA57A-Xg

It could also be a flanger, but I'm not sure it was commerically available when they recorded that tune. Also it sounds more like phaser to me.

https://www.youtube.com/watch?v=wl2zvNW3rSg

Hope that helps!