Wow... Blast from the past! I haven't seen a tape echo since the 80s. Cool project. 👍

Great work! I build one with an older tape-deck: 

https://www.youtube.com/watch?v=OYjC2h2sVaE&t=10s

Now that looks like a very promising project. I had great joy with my Polish made "Sochor" tape echo, unfortunately something on the pcb broke. I managed to add pwm motor control, it worked in a certain range, but when the pulse was too thin the motor just died down. It sounded really great, especially with the speed changes and stopping. A simple clave sound became warbling organic water sound. It used bigger width studio tape, I am a bit afraid of wow and flutter on original cassette tape. I also had a "melos" tape echo but it's very unreliable and the tapes (similar to playtape, thinner then normal tape) are hard to get in good shape. Since some years I am trying to convert an old commodore 1541 drive (some versions have DC motors!) to a magnetic disc echo using 3M DD disks. I was always struggling to find the bias circuitry and everything you commonly need for a tape echo, not wanting to slay my Denon 3 head deck. If you please could share some bits of your circuit it could help me a lot, maybe I can even test your circuit for my application to have a universal delay pcb? But I must say tape heads are still a riddle to me, how much ohm is which head and how to utilize... 

I'll be sharing the schematics and PCB soon for sure, I just need to finalise a few things before it's ready to share.

I didn't worry too much about all the technical details of head inductance/resistance and what not; The knowledge on this stuff is out there but it's quite hard to find and put together in a complete way. So I just threw stuff together and hacked at it until it worked. I was lucky to find people that had already built similar machines and so had some practical experience I could draw on.

When i saw the picture my first idea was WOW i will build a model of my old revox B77 :-)                                                                   

That's a nice looking machine.

Really cool. I had once a Mitsumi Tape Deck with 3 Heads and Speed Control, that could make echo effcts. But with yours one can make longer echo times becaus of the long way of the tape between the heads.

You could make the direction reversible for shorter echo times.

I've wanted to do this forever! Keep it up, I'd love to build one!  Let me know when you are ready for others to prototype it, I'm in!

That diystompboxes site used to be my home, and now I don't have out there nearly enough.

Sure thing. It'll take a bit of time to get the build instructions together but will be posting it all here at some point soon.

Yea it's a great resource - the DSP section is quite good, too. I've been working on a digital glitcher and the people over there have been a big help.

That is so cool! I love those cassette reels!

I owned a Maestro Echoplex and gave it away when the digital revolution started. Kick my self now. I'm definitely looking around for parts to try something like you did. It would be cool if you could make the play head movable to adjust the delay time like the echoplex did.

Yea I really wanted some nice reels for it - they do nothing useful in the design other than look nice but gotta have them.

In this design I'd make the write head the moveable one as the read head is fixed in place on the cassette player and would be a pain to move. Nice idea though for sure. Another idea might just be multiple write heads in different positions that you can switch between for a few options.