> head arrangement hopefully is erase head, record head then playback. ... I wonder if that unit actually had erase heads...
Probably; but it surely has a combined record/play head, and one single amplifier switched between the two functions.
> I do have another tube tape machine of this type
I've never seen ONE of that model, you got TWO?
And you know it is good: it says "professional" next to the 2-cent RCA jack.
It says "Armour Institute" inside.
OK, lucky bastid. Side by side. Full reel on left spindle left machine. Empty (dummy) reel on takeup spindle left machine. Takeup reel on takeup spindle right machine.
Thread tape from far left, over left heads, NOT through pinch roller, guide through dummy reel. Now to right machine, over heads, through the pinch, and take-up.
Left machine in REC. Right machine in PLAY.
I bet there is no tape-break sensor; if it knows the tape is mis-threaded, you have to gimmick the safety features. And you should shove a stick in the dummy reel; it will work spinning or stationary, but stationary is better.
Is that reely a steel reel?
It may be 15ips, or 7.5ips. (I bet the capstan comes off and the innard is half-size.) At 15ips you get just about 1 second delay between side-by-side machines, a short riff. At 7.5ips and 30 feet between heads you get 48 seconds (I have done this live on stage before an audience), a whole verse.
Just like that, you get one repeat and you better be done before the reel runs out.
Take a mike stand, jam a 1/4" rod, slide a 3" reel on it, adjust same height as the decks. You may need more guides on the decks, be clever (but non-magnetic). Cut the desired length/time of tape, splice the ends (the best Scotch Tape is excellent), thread around the path, move the mike stand to carry the loop. Jam all spinning spindles. Now you can play forever (well, the tape can last longer than the drugs).
If you un-tack one wire on the erase head, the tape-loop is not erased on every pass, and prior sounds will come around again, but weak because the REC head has some erase effect.
For re-re-re-peat, take the PLAY output and +mix+ it back to the REC deck. This will apparently need an external mixer. I guess the $49 2-in Bee-ringer toy is plenty good for this.
You ask: why skip one pinch-roller? The two machines don't run the same speed. The tape either breaks or spills. One tape, any number of heads, but only one pinch.
Gah. I'm going to smell like tape-dust for a week.