Another way to do it is to playback the recorded vocal track with the tape machine in "Sync" mode, record the output from the sync head to another track on the same tape, vary the tape speed up and down a little while recording. After this is done mix the song with all tracks in "Sync" mode with the exception of the "dubbed" vocal track in "Repro" (or "Replay" on some machines) mode.
Most of the modern-day multi-effects processors(Lexicon, Eventide, Yamaha, Roland, Digitech, Boss, etc.) have the ADT program.
Lawton,
The most convenient way to get ADT is to add a mono chorus effect (in case your FX processor doesn't have an ADT program) to your vocal:
Delay time-----------20-50 ms.
Modulation range/depth-----------less than medium
Modulation rate/speed-------------slower than medium
Feedback/Regeneration------------none
In a 2002 MIX magazine interview, Geoff Emerick also talked about using the ADT. Here's the bit:
----------------------
MIX: I'm confused about how you were doing ADT — Automatic Double-Tracking — back then.
GE: It's funny you should say that, because not long ago, when I saw Jack Douglas, who had worked with John, he said to me, in a humorous kind of way, “How did you do that ADT? I could never get the copy tape to go fast enough to actually lie on top of the original voice like it was double-tracked.”
From his question, I got the feeling the tape was going so fast it was about to go up in smoke; John must have told him you could do it while you were recording, but actually you could only do it when you were remixing. You have to take the signal of the vocal from the sync head. So, you're mixing off the replay head. The sync is in advance, and you put it into another ¼-inch tape machine, then you put that on frequency control and you slow it down. You're not trying to advance it; it's already advanced. You're just slowing it down. The trick is taking it off the sync head.
---------------------
Here's the interview should anyone be interested:
http://mixonline.com/recording/interviews/audio_geoff_emerick/index.html
---------------------------
"I want the ADT, Geoff!"

