Updated March 5th, 2026: This page is very incomplete….and it’s mostly about analog circuits.
Scroll down for links to many articles about VFO’s and some designs. The subject is endless and I had to stop somewhere…
Stabilizing VFO’s and eliminating keying “chirp” is sometimes difficult to achieve, especially in tube-based VFO’s where the heat generated by the VFO tube can cause expansion changes in the VFO components, and thus the frequency.
Many methods of stabilizing have been advertised, including having the tubes installed outside the VFO compartment so that the oscillator’s tank circuit is not subjected to the tube’s heat.
The 6AG7 tube has been used as the oscillator in many designs but that is a very hot tube that can cause drift because of its 650 MA heater current (4.1 watts). If a big bottle is desired, the old 6J5 triode’s filament burns only 1.9 watts (300 MA) and the winner of the heat reduction race is the miniature 6C4 triode with its 150 MA (0.95 Watt) filament.
Note that Hammarlund used the 6C4 in the old SP-600 VFO circuits; probably to help with stability.
The 6AG7 tube’s elements can be used to include a buffer stage, so the 6J5 and 6C4, being triodes, will require a second tube (6C4, 6AH6, 6CL6, 6AG7, whatever) as the buffer. But these are cheap, and can be installed outside of the VFO cage, keeping their heat away from it.
A recommended tube layout is: 6C4 Oscillator ->6C4 Buffer ->6AG7 Amplifier/Multiplier/Driver, with the oscillator tube and its frequency-determining components being isolated as much as possible from the heat-producing buffer and amplifier. We can do the same thing with this lineup: 1/2 12AT7 Osc -> 1/2 12AT7 cathode follower buffer -> 6AG7 amplifier/multiplier/driver.
Substituting the Vackar VFO for the 6C4 design, followed by a buffer stage, and then an Amplifier/Multiplier/Driver, as with the 6C4 design, is also an excellent alternative.
All oscillator tubes need to have their voltage stabilized by one means or another; commonly by a voltage regulator tube or a solid state equivalent.
Also, the VFO’s plate voltage should be as low as feasible, to further reduce heating. The 6C4 is the winner here, as mentioned in the first compendium article, below.
Mechanical stability cannot be over-emphasized. Use the heaviest wire available for tank coils and even the wiring itself. A shield between the tube and the tank coil, or mounting the tank coil below the chassis, can reduce thermal effects.
Use the heaviest chassis and panel available, to prevent vibration or flexing. Be sure the tuning capacitor is of good quality and firmly mounted.
Consider having the power supply remote from the VFO, on rubber mounts, or on a separate chassis, to avoid heat transfer and possible vibration from the 50/60 cycle transformer windings.
Don’t run the oscillator tube at its maximum voltage rating. For example, running an oscillating 6AG7 with 225 volts on the plate is better than with 300 volts on the plate. With the lower-heat tubes, such as the 6C4, this is not so much of a problem.
Of course, tubeless, solid state VFO’s are much less subject to heat problems.
With that as background, a compendium of articles about VFO’s, beginning with a very stable design that deserves a lot of credit, are listed below.
But first, for those needing a tutorial about VFO’s, read the document found at this link.
— VFO DESIGNS —
A High-Stability Oscillator Circuit (QST, May, 1948) This very simple and highly stable Clapp-designed VFO is very interesting (it’s actually a series-tuned Colpitts circuit). Follow the VFO tube with buffering and amplifying, as described in the 12AT7 Vackar design below.
Put the regulated power supply and buffer tube on a separate part of the chassis (or at least with a heat shield between the oscillator tube/coil/cap, and all other components, and it will be extremely stable after initial warm-up.
The 12AT7 High Stability Vackar VFO (pronounced “vazkar”. ) This stable design, with its wide tuning range, is excellent. To improve stability even more, don’t key the oscillator’s cathode. Ground it, and a second tube, such as a 6AH6 pentode, or 6C4 triode could be used as a buffer, fed from the cathode follower output of the second triode of the 12AT7. Keying could then be done in a following amplifier/multiplier/driver stage using a tube like the 6AG7. This further isolates the oscillator from any keying influences.
Here is some interesting discussion about the Vackar, from VA3DIW.
A solid-state MPF-102 Vackar VFO
The Remarkable but Little-Known Vackar VFO (QST, Solid State version)
Another Vackar VFO Design: 2N3904 – how simple can it be? (WORD .doc)
A 1950 VFO Exciter (from QST)Note how the frequency-determining components are in a box, isolating them from the heat-producing parts.
A Cathode-Coupled VFO from VK3AT
Lewis McCoy’s (W1ICP, SK) “Easy to Build” VFO; which has been copied and built many times.
The Franklin Oscillator – High Stability!
An Improved VFO Driver Amp for Tube Rigs Using a modern synthesized VFO with a tube-type “boat anchor” transmitter might require increasing the output from the solid-state VFO to drive the amp. Here’s a solution.
AA8V’s 6AG7 amplifier design takes an input from a low-level tube VFO and produces enough drive for common final amplifier tubes such as the 6146B, 807, etc.
Using the super stable BC-221 or LM series heterodyne frequency meter as a VFO for a transmitter, Page 1, by K4CHE. Here’s Page 2 of the same website and article. W4NPN’s LM-8 will remain spot on frequency for days, once warmed up. So will his Hammarlund HQ-140XA receiver.
Into the VFO Rabbit Hole: A sampling of solid state designs.
Wide Frequency Range VFO (WORD .doc)
~ VFO Stabilization Articles and Suggestions ~
Weebly VFO Stabilizer Links (Lots of Links!)
VFO Stability – W4NPN’s Comments:
1) Use heavy, thick well supported wire in construction.
2) Ensure the coil is firmly supported, not right next to a heat source, can’t vibrate and is wound on a stable core such as ceramic.
3) Use voltage stabilization for the oscillator B+ and for the following stage.
4) Use several fixed capacitors in the tank circuit rather than a single one. This spreads any heat over several and reduces expansion/contraction and thus drift.
5) If a tube circuit, locate the tube where it will be well ventilated.
6) Use a cathode follower or solid state equivalent after the oscillator. Key the follower and later stages and let the stabilized oscillator run freely.
7) Sometimes power transformer laminations can vibrate and the vibration can make its way to the oscillator and cause problems; turning the oscillator into an FM transmitter. Mount the transformer on rubber mounts or on a separate chassis.
—————————————————————————————…more later, as I find interesting and simple designs.