I may be able to help you fellows with mains frequency conversion.
If you know the top end amperage required, then the following idea can be scaled to suit the throughput.
The basis, is something like the Jeanna's Light or a recent circuit by Sergdo on OU.com.
The power comes in, can come from anywhere - solar, batteries, whatever we choose and the output is frequency dependent on a variable pot. For mains current abilities, the circuit could be driven from a car battery.
You may have heard large toroids singing away in some peoples videos....using the pot, you can vary that frequency, from upwards of 20kHz, to as low as a few Hz. That singing is the inverter frequency.
In common with anything, size dictates the frequency, it being much easier to have a very large toroid tune down to the lower Hz while listening to it. The frequency becomes easily audible when inverting to 120V ranges with using, say, the toroid from the neck of a CRT computer monitor.
The circuit is very simple and very effective AND might just offer frequency tuning abilities for these experiments with the Davey boilers

Best thing, to me anyway, is that a 1.5V AA battery can run a normally mains powered 2W LED lightbulb !!!
So we're looking straight away at perhaps avoiding the need for a plug in the wall.
If one bell input is the mains frequency and from a traditional plug, then the inverter circuit can supply whatever frequency we wish to use for the other bell.
If both bells ran via 2 inverter circuits, then both become tunable, both become able to be powered by much smaller sources of power like a car battery.
Who knows, maybe at resonance and with the correct inverter frequencies, we may be able to boil water with a flashlight battery

With now seeing that a size such as 0.9" toroid can deliver at least the voltage, if not the amps due to wire gauge, we may imagine a larger toroid fitting under a traditional household kettle. In such a way, the kettle becomes a full Davey boiler, but looks to the mainstream market just like a traditional kettle.
Here's a pic of one of my inverters running, the whole circuitry fits around the screw terminal of the unaltered household bulb:

Davey Sonic Boilers -- and Controls. Experimental results and discussion.
Here''s a video demo, just to see how simple the circuit is and to see it in operation for potential Davey boiler uses:
http://www.youtube.com/watch?v=vmgerjLztkc&feature=youtu.beOn my previous video, which shows the audible 'benefits' for tuning the frequency, one viewer just posted in answer to another viewer and it's related:
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@spicer41282
You can build a JT circuit to resonate at various frequencies, depending upon your coil and capacitors you add. You can intentionally keep the resonant frequency real low ... for a nice pleasing HUM ... or high ... say 30khz or greater to be above human hearing. It might bother your dog though.
morpher44 2 hours ago
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Here's that video, shows quite clearly how the audible tuning would need no speaker, external circuitry etc:
http://www.youtube.com/watch?v=afilkouuiJo&lc=v_eEVzpNnfRkC2NJSr3so12yQkcUFM3KhFQaZtqbS5AI hope the idea is of merit and use.