Over on the QRP-Tech Yahoo group, Ed Edwards, AE7TE, posted a link to an article (http://www.w7ekb.com/glowbugs/VFOs/vackar_vfo.html) by Kenneth Gordon, W7EKB, on Vackar-design VFOs designed in the 1950s for vacuum-tube applications. The Czech engineer Jiri Vackar introduced the design in 1949*. The schematic example Kenneth shows uses a 12AT7 dual triode in common-cathode mode with both plates at 255VDC. The driver is cathode-keyed. The output triode is arranged as a cathode follower. The VFO is series-tuned.
According to W7EKB, Vackar VFOs have three important design characteristics: they have a 2.5:1 tuning range, they have a constant signal-level output across that range, and they’re “rock solid” in their frequency output. It so happens I have a bag of 12AT7s just waiting for me to play with. I’ve been wanting to design and build a single-band tube transceiver just for the fun of watching the little buggers glow in the dark. Someday soon I hope to test this Vackar design as part of that longer-term project. I’ll let you know how it comes out. 73.
*For more on Vackar and his design, see https://cs.wikipedia.org/wiki/Ji%C5%99%C3%AD_Vack%C3%A1%C5%99 (copy and paste text into Google Translate) and https://en.wikipedia.org/wiki/Vack%C3%A1%C5%99_oscillator. For a solid-state version of the Vackar VFO, see Floyd Carter, K6BSU, “Meet the Remarkable but Little-Known Vackar VFO,” QST September 1978: 15.