Spacing a coil from the chassis or shield

 This article is directed at shielding your coil but it also applies to placing it in proximity of metal objects. Transformers, other coils, capacitor, etc.I thought the rule of thumb is to space the coil two diameters away from objects which will effect its inductance. What does the article say?


 

variable air capacitor #2

I'm posting this for the brave at heart. I did one three plate using two sheets of plastic. Using square plastic I covered half the plastic with aluminum foil on one side and cut a plate from some sheet aluminum in a semicircle. stack the three pieces and drill a hole just above the foil. Solder the rotating plate to a rod that is the tuning shaft. Slide the other two pieces over the rod and put a dab of hot glue on the sides the hold everything together. You can use the blank disk that come in a box of cdrom.

The square coil

The coil inductance is determined by this formulas

 The factor are length of winding and radius of the windings. K is found from a chart.
 
The Diameter to Length ratio is calculated and then the K is found on the chart. The inductance to resistance ratio is the Q so a larger wire with less resistance or a shorter wire of smaller gauge will give different results. For the rule of thumb a 'square' coil will be the best compromise. There are to many variables to just go square though. For example a transmitter coil may be wound with 1/4" copper tubing.

This example shows the 'square' coil has more inductance with the same size and number of turns the wire resistance will be the same and the Q will be  higher for the 'square' coil. However if the 'square'coil was a receiver coil wound with 24 guage wire and the other coil was a transmitter coil wound with 1/4" copper tubing it could have a high Q so they could both serve their purpose. You could use this chart to find your values.
 
Then determine the Q you want and use this chart to see some posibilities.

The chart shows a larger diameter coil is higher Q.
For more information including spider wound coils check this link

Comparing different coil types

transistor tester

 
I have one of these that still works.  Mine uses a single 1-1/2 volt cell to light the neon lamp. The trannsistor has to oscillate to drive the transformer and light the bulb. The theory is that sometimes a simple ohm meter test will show a bad transistor to be good.

Am Tuner

 
 The capacitors in the base circuit illustrate filtering using capacitive reactance vs frequency responce. The RF is shunted to ground through the .00005 capacitor while the 5 MF capacitor passes the audio to the base. The capacitor opposes circuit voltage change by charging and discharging. The smaller capacitor charges quicker and the can no longer respond. We say they block DC and pass AC and they do. They block DC by charging to the DC level and then they can not pass it. They pass AC only as long as they can charge and discharge.

Transistor types (50 years ago)

 

Using one of those former to build a transformer from scrap

 
Ok you make a former and put some wire on it then what? Find some strip material. Banding is good. Next time your at the lumber yard and the guy cuts the bands from a bundle of lumber ask if you can have it. Then cut it in strips to fit your core.


That's half maybe three quarters of the job. It will function but not as efficiently as you would like. It needs some closure around it.
 
 The squares will surround the coil and the 'U' bracket will be the mount.
 
After installing the squares I pushed the 'U' bracket through the core. All that is left is to bend the brackets outward.

Here it is installed on my amp board. I made three. Two interstage and one output transformer.