If you have the whole working deck, I'd suggest putting a scope probe on the tape head while it is in record mode and see what the bias frequency and level is. Alternately why not just scavenge the record & playback circuits from the deck?
I have done this and the record signal was 50mV DC biased, and this is what works well-ish when I try. However I had read online that AC biasing works better and so thought I would try to use that instead, but perhaps this head just isn't designed for that. Also I'm using this as an analogue learning experience so I wanted to build it all from scratch. The record & playback circuits are all integrated into one purpose made chip which is a bit boring!
Also I'm curious, did you buy two of these to work with? Since cassettes normally use the same head for both record and playback I would assume your unit only has the one head (plus the erase head which probably won't work very well for either record or playback). And your tape delay is going to need simultaneous record and play.
Well, I am working to a budget so I actually got this for free. I also bought a rubbish old walkman (that stank of battery acid) on ebay but the head sounded very poor (and then a pin broke off it) so I'm trying to get another head for playback.
The erase head is actually a permanent magnet so definitely no good for playback or recording
Do you have any idea roughly what AC biasing levels to use? I'm a little scared to put too much current through a tape head.