Claravox really unstable pitch?

Ok, I guess I have to rephrase it:
Staring at the tuner and just trying to hit and hold that one note should not be the only/first thing you practice

Thierry is mentioning Korgs out of production DT-4, beside that most people seem to have settled with the current available guitar clip tuners and accept that they are a little bit "sluggish".

The D-Lev tuner is pretty impressive... 

because the cognitive instant it takes to translate mentally (e.g. D-G oh that's a fourth) is too distracting.

Yes, it is. Doesn't get better if the display is way to far away, so repurposing the Eurorack Module (Mordax Data, which is more of a scope anyway) isn't gonna work for me

First thought was building something myself but the whole electronic parts supply chain problems of the last years is annoying and I'm getting bored of tracking which Atmega I can maybe get again in 5 years of time ;-)  

In my first months of learning, with a Theremini I did regularly look at the tuner, but mostly not constantly. Mainly to check at the beginig of playing a melody line and after one or more times playing through it, to check and correct the tendecy to drift off pitch upward, which as I found out is a natural tendency that (a-capella/unaccompanied) singers have as well. Having a reference tone/drone or accompaniment playing is a way of coping with that. Practicing on keeping a reference pitch going in your inner ear is a additional more advanced approach. For me, as my hand gestures and fingerings got more precise, my hearing on/off-pitchness grew more accute and precise too.

I'm afraid your tuner also thinks G# and Ab are the same note :-)

Hahaha that's all too true. We born in the age of equal temperament are too lazy to get those fine nuances... until we stumble into them when playing an acoustic instrument. The easiest way to try this is tuning the dreaded B string on a guitar: if you do it with a tuner vs. using the other strings as a reference, the results will be noticeably different... and the "perfect" tuning of the tuner will sound out of tune!

Anyway, back to theremins...

Ok, I guess I have to rephrase it:Staring at the tuner and just trying to hit and hold that one note should not be the only/first thing you practice Thierry is mentioning Korgs out of production DT-4, 

Yes of course I'm not looking at the tuner ALL the time: normally at the start of an exercise and at the end, first to find the starting note and then to check how much my intonation drifted. Sometimes I'm pleased to see that I finish the piece at exactly the correct tuning... sometimes it drifted horribly.

Regarding "which tuner", I didn't know the DT-4, thanks for the info, it looks great!

I use a really cheap Thomann tuner, which however works great: fast, responsive and with a big display that gives tons of info: target note, big needle pointer, flat/sharp indicator LEDs AND cents counter. Furthermore, it has line IN/OUT jacks, it can give you a reference pitch AND also has a metronome... lots of things in this little box! Build quality is what it is (that is, not great  ), after a couple of years of use the display has lost so much brightness that it's hard to read. But for what it costs I cannot complain, I can just buy another and probably will.
Thomann CTM-700 Tuner-Metronome

I just saw that Korg also released a "Vocal Pitch Trainer" device. Given the many similarities between the theremin and the human voice, it may be a nice didactical tool. Anybody wants to try it out?
Korg VPT-1 Vocal Pitch Trainer

"I just saw that Korg also released a "Vocal Pitch Trainer" device, given the many similarities between the theremin and the human voice, it may be a nice didactical tool. Anybody wants to try it out?"  - Spider76

Interesting!  Seems like it might be more responsive than a guitar tuner?  The sharp / flat LEDs seem to have a range that you can set, but maybe that's not quite enough info?  I guess I'm not a fan of note / staff displays either as they are so key of C centric.  Too bad the video doesn't show an example of actual usage.  I have a much more nuanced and graphical pitch display app on my Amazon Fire tablet.