> some kind of Control Voltage like V/Hz
No. That's why the Good Lord and Shockley gave us transistors and $2 chips. Tubes make terrible volt-controlled oscillators.
> a really nice smooth sound.
{sigh} Yes, that's why we have Wein Bridge Oscillators. The simple volt-controlled multivibrator does not make SINE waves, and the several "sine converters" all suck. The Moog-users' trick is to run the VCO into a VCF, both fed the same control voltage, to filter-up the triangled-sine wave to something that does not scrape the eardrum. (An advanced technique is to use the VCF with positive feedback and an amplitude limiter, but I don't recall ever having enough modules on our ARP to do that properly.)
That Heath is a very fine test-bench oscillator. If you don't test-bench with ACVM and oscilloscope, trade it to someone who does. I have four oscillators already, and am about 5,000 miles over the ocean, but someone in your land needs it.
> using varactor diodes.
Huh. I see they even make hyper-abrupt varactors with almost enough pFd. But the signal levels here are higher than a radio, I suspect it would change frequency throughout an audio cycle, which amounts to gross distortion. And as you say, the control-law would be screwy.
Two 6SN7. Amazingly cheaper than the H-P 200AB, yet about 99% as useful on a typical audio test bench. Someone should mass-clone it (but where do you get that tuning cap today?). The alternative seems to be the Global 2001, a nice but expensive box with kinked "sines".
> An all tube synth would be awesome.
Once upon a time, when Goldilocks was still a young girl, I maintained an ARP modular (and solid-state) synth. It was roughly a half-time job just keeping up with the tarnish on the many thousands of contacts. An all-tube synth would be like that early computer: techs with shopping-carts full of tubes walking up and down the aisles, getting about 15 minutes between failures.