This is the story of my quest to build an all-tube VFO for 26.0 to 26.5MHz, for use in a 21MHz SSB/CW superhet transceiver with 5.0688MHz intermediate frequency (IF). Construction efforts below were assisted by advice and commentary by G4OEP, K3IMW, K7HKL and VK6JY.

An oscillator needs to be reasonably stable - the frequency should not drift too severely as the temperature changes. This is more challenging with a valve VFO than a solid state oscillator. Even with solid state, it would be very difficult to build a stable enough LC VFO at 26MHz. The best approach is to build a lower frequency VFO and mix it with a stable crystal oscillator to get the desired output. Right from the start, my aim was to build an LC-VFO for 6.0-6.5MHz, mix it with a 20MHz crystal oscillator, and select the 26MHz sub with suitable filtering.

	1. Sub-mini valve VFO First attempt is to build a stable 6MHz VFO using a type CK512AX sub-miniature tube, originally used in hearing aids (idea from Andy G4OEP). This ultimately didn't work because the particular tube is unsuitable for this application. Read more...
	2. Mixer VFO mk1 The next attempt uses 12AT7 crystal oscillator and VFO; 6BE6 mixer, and EF91 buffer. It produces a 26.0-26.5MHz VFO that works, but the heat build-up in the box was very high and the drift was 50-70kHz per hour! Read more...
	3. Mixer VFO with insulated tank components The same circuit as 2, but now with the tank components insulated, and the tubes in the open for better ventilation. This greatly reduced the drift, and temperature compensation of the VFO tank components reduced the drift to 50Hz/hour after the initial warm-up. Read more...
	4. Mixer VFO using battery valves To reduce the warm-up drfit, this mixer VFO now uses low power battery valves - DF96 VFO, 1T4 crystal oscillator, DK91 mixer, DAF91 buffer. It works and the drift is within 50Hz/hour, but not really repeatable. Also shown here, voltage regulator tube experiments. Read more...

5. Another rebuild...

Now for the next rebuild. Maybe the final one, who knows. Now I decided to abandon the idea of the polystyrene insulation - insulation doesn't stop the temperature inside the VFO compartment from rising, all it does is slow that the rise down. Ultimately, when the VFO is used inside a transceiver and everything boxed up inside an enclosure, and after waiting for the temperature everywhere to reach its final value, eventually the VFO will end up at the same temperature, regardless of whether it is in a polystyrene-insulated box or not. The polystyrene just makes the whole thing take longer. It seems to me that it would be much better to minimise the temperature dependency by balancing the temperature coefficients, than to try to slow down the temperature rise.

This new version abandons the polystyrene insulation, the box contains just two DF96, one to be used as 6.0-6.5MHz VFO and the other as buffer. The rest of the oscillator (20MHz crystal oscillator, mixer, buffer) are less critical and can be outside on a separate subchassis.

More to come soon...