Front view of converter shows 12V output socket, terminals, fuses and switching transistor.

This project came about as the Model T is equipped with a six volt electrical system and I'd bought a small air compressor for tyre inflation that only worked on 12V. Having said that, the more heavy duty model of the same compressor does in fact work quite well on 6V. The higher the motor current of a 12V motor, the more likely it will work satisfactorily on 6V.
Of course this unit can be used to power other things; for example a Peltier effect car fridge, CB radio, mobile phone etc.

Commercially made Converters.
6 to 12 volt converters have been a commonly available commercially made product in the past. During the 1960's and 70's there were still many 6 volt cars in use, but most solid state car radios and tape players were 12v only and so there existed a need for these converters. Particularly common were VW Beetles up to 1966, and it's at VW swap meets where such converters will often be found. Usually they sell between $2 and $10. There's not a great demand for them these days, as unfortunately many vintage cars have been converted to 12V. New ones can still be purchased from some Model T parts, and other vintage suppliers, but they're considerably more expensive. And of course, 2nd hand ones appear on Ebay.
The design is much the same across the different brands. In Australia, the Japanese  "Bellsonic", once sold by Dick Smith was particularly common. They use a two transistor oscillator driving a step up transformer (often a ferrite toroid)  with the same transistors rectifying the stepped up voltage by means of their base emitter junctions. The load current flowing through the base emitter junction gives rise to a certain advantage. It is possible to arrange the converter to automatically start when the load is turned on, thus obviating the need for a switch on the converter. The Bellsonic unit is however, manually switched, because there is a bleed resistor and pilot light permanently across its output to assist with  regulation. It is designed for under dash installation where it's easily accessible.
Generally, the output is around 2A, although I have one from a Porsche which provides about 6A.
As my compressor draws much more than this, the commercially made units weren't really suitable for my needs, hence the design you see here.

Designing a 6 to 12V converter.
Initially, I tried duplicating a typical commercial unit by winding various transformers and experimenting with differing methods of driving the transistors, but didn't really improve on the commercial models. There was a 6 to 12V converter design published in Electronics Australia during 1974 using an iron cored Ferguson transformer which helped to understand the operation, but it was becoming clear that to be able to draw say 8A at 12V would mean an input current of at least 16A at 6V. That's a lot of current and when one takes into account things like transistor saturation voltage at that sort of current and the fact we have only 6V to start with, a different approach was called for. One important thing to consider was that the converter was only going to be used for short durations with the high current loads; a few minutes to pump up the tyres, or half an hour with the car fridge.
So why not use a small sealed lead acid battery to supply the heavy current, but charge it at a lesser current, which a simple converter can handle during its idle period? As it happened I had a pair of 12V 4.2Ah SLA batteries looking for a use.
The idea worked out very well, and the converter has been in operation since 2003.
The converter would have to be as small as possible given lack of space in the Model T. The logical place for it would be under the front seat. With not much space and the batteries in the same enclosure, the design would had to be simple. While the idea of one of my beloved vibrator power supplies and a home wound transformer were tempting, the unit would be too bulky. And a vibrator supply on its own had to be ruled out immediately; the switching contacts are only rated at around 4A, meaning less than 2A for a 12V output, with efficiency taken into account.

The Circuit and How it Works.

The circuit is essentially a simple switching converter to provide a voltage step up which provides 13.8V to charge a small battery. When the battery voltage reaches 13.8, the converter shuts off.

Step up circuit.
The incoming 6V supply is protected by an 8A fuse. This is necessary in case the switching transistor shorts out, which is the normal mode of failure with semiconductors. Filtering to reducing incoming spikes, and ripple going back into the 6V supply, is performed by a 10,000uF electrolytic condenser.
At the heart of the converter is the 555 oscillator. The frequency and duty cycle of the square wave output at pin 3 is set to provide the maximum current and voltage output from the converter. These parameters are largely determined by the characteristics of the 164uH choke. I used a ferrite toroid wound with 1mm copper wire for this.
When the BDX65A transistor is switched on, current flows through the choke and builds up a magnetic field. When the transistor turns off, the magnetic field collapses, creating a voltage somewhat higher than the supply. It's standard solid state voltage converter design. This higher voltage is rectified by a high speed power diode and filtered by 5,000uF of capacitance.
Capacitive drive is used for the BDX65A. As it's a Darlington type, not a huge amount of drive is required to ensure saturation. Using capacitive coupling from the 555 means that if the oscillator fails, the transistor cannot be permanently switched on. (Some commercially made switchmode supply designers could take note of that).
When the output of the 555 goes high, current flows through the 47uF and into the base. When the 555 output goes low, the 47uF discharges through the 1N914 base diode and ensures the transistor is cut off. This provides a clean switching waveform.
Unloaded, the output will rise to over 20V. The converter needs circuitry to prevent this as it would cause the battery to be overcharged, damage loads connected to the converter, and the converter would continuously run, always draining the car's 6V accumulator.

Regulation.
An LM393 comparator is used to sense when the DC output reaches 13.8V. The output voltage is sampled and compared with the voltage reference, which is a red led. Leds make good voltage references because they are stable at around 2V and don't require much current. If the voltage at pin 2 of the LM393 rises above 2V, pin 1 will go low and shut off the 555. The converter then stops and will not restart until the voltage has fallen again. This rapid turning on and off results in an average voltage of 13.8 across the battery.
A later improvement as shown was to introduce a degree of hysterisis with the comparator. This was because the converter never really shuts off and was putting a small but constant drain on the 6V supply. A small amount of positive feedback was provided by the 390K and 22K. The 13.8V now has to fall to 13.4V before the converter switches on.
What happens now is there is a short burst of about a second every few minutes where the battery is kept topped up.
Of course, as the 12V output is drained the converter runs for a longer period, and with high loads runs continuously.

Charge Indicator.
To indicate the operation of the converter, a red led is driven by the other half of the LM393. The negative input is fed from the base of the BDX65A via a simple isolating resistor
and capacitor filter. When the transistor is being driven, there exists a small negative voltage at the base due to the 1N914. Thus the LM393 turns on, driving the led.
However, it was found that the led wasn't always switching off when it should, and jacking up the positive input by 600mV solved this. This meant there really had to be a definite negative voltage at pin 6 to activate the charge led.

Inside the unit. The circuit was built on a piece of Veroboard. The choke is mounted on the panel under the three electrolytic condensers.

Output.
In view of the battery load, the 5,000uF filter capacitance might seem superfluous, but it serves two purposes. First, to keep ripple down as the battery ages, and secondly for the setup adjustment it's easier to run the converter without the battery to set the voltage trimpot, so something is needed to filter the output.
On its own without the battery, continuous output is around 2A at 12V. However, it shouldn't be used like this because of the constantly changing voltage output. Unlike other switchmode supplies which vary the duty cycle of the switching waveform, this one simply drives at full output until the maximum required voltage is reached, then switches off until it falls beyond a slightly lower point. This was done for simplicity. While the duty cycle of a 555 can be varied by controlling the voltage on pin 5, this also varies the frequency and reduces efficiency in the choke circuit.
The battery is fused before it reaches any other circuitry or the outside world. As with the input fuse, led indicators show blown fuses. I thought this was a good idea as when out in the car and something goes wrong it means one can narrow it down at a glance rather than having to get out a test lamp or meter and start pulling things apart.

Construction.
I built up an aluminium box to fit under the front seat. This also houses the two 12V 4.2A SLA batteries. On the front are all the led indicators, fuses, and terminals. I provided a two pin polarised socket of the type used in the house for the home lighting plant, to which the 12V appliances can be plugged in. The unit is permanently connected to the 6V accumulator in the car and runs even when the battery isolation switch is off. There is another 10A fuse in an inline holder to protect the always live supply to the converter.
As I've pointed out elsewhere, it is essential not to let lead acid batteries discharge as they will be permanently ruined. Apart from natural self discharge, there is a 500uA drain from the voltage sensing circuit. While I always have the 6V accumulator connected to a maintainer when at home, there is no problem with the converter discharging it over several days away from home.

Operation.
This is completely automatic; one simply uses the 12V load as required. However, due to the actual converter circuit only being able to provide about  2A, one can see if more than this is drawn, the SLA battery will discharge. With a 4A load one could, in theory, run the converter for about four hours before its internal battery is flat.
The other thing to keep in mind is the charge rate from the car generator. I've set this to 5A, but at full output the 6 to 12V converter pulls 7.5A from the car battery. With 12V loads under about 2A, one can drive all day without anything discharging.
In practice, these limitations have not caused any problems. The tyre compressor is only run for a few minutes, and a trip to the shops and back with the 4A car fridge is less than an hour.
Has anyone spotted the potential advantage of incorporating the SLA battery? Well, it means that if the 6V car accumulator was completely flat or faulty and one was away from home, then a jumper wire can be run from the 12V terminal to the coil box and one could continue driving. The CB radio or mobile phone is also not put out of action.

6 to 12V converter mounted under the front seat next to the radio.

While the converter worked well, it did require replacement of the SLA batteries after a few years. Given the cost of these, I eventually decided to replace the converter with a commercially made unit which can provide 6.3A at 12V. It appears to have been made in Germany and intended to be used in a Porsche. It can run the tyre compressor under load.