Notes on Transformers

Transformers are frequently the focus of much attention at the Alpha booth at hamfests. People want to see and then lift a transformer. We often get comments like "Wow! That's heavier than my entire amplifier". For all amplifiers the transformer is a key component, and I thought I would provide some more information on our transformers, especially since there will be some minor changes in the near future.

Transformer action has been known for close to 150 years. They became widespread when alternating current became the dominant source of electricity distribution, and early AC development owes much to the genius of Nikola Tesla. Commercial transformers have been under continuous development now for more than a century, and their performance, both in terms of reliability and efficiency is quite remarkable. In simplest terms, a transformer consists of iron, copper and paper. Specialized versions of all three of these have been developed with properties specifically suited to transformer construction. The biggest improvements have been in the transformer steel. Originally, transformers were made from stamped iron pieces which were shaped into thin "E" and "I" shapes which were then stacked together to build up the magnetic circuit of the transformer. The reason for making the core out of thin laminations, rather than solid, is to prevent the flow of "eddy currents" which arise from the changing magnetic field.  These currents flow around the magnetic flux, rather than along it- so the magnetic flux can flow along the arms of the E or I, and the eddy currents cannot pass from one lamination to the next. The biggest improvement was to find steel alloys which increased the resistance of the steel without reducing its magnetic properties which further reduced eddy currents. Silicon is the principle alloying material, and transformer steels can contain as much as 6% silicon. A further refinement of silicon steel is to roll (stretch) it so that the individual crystals (or grains) that make up the material become oriented in the same direction. This results in a thin sheet material with very high permeability and low loss. This material has been given the name Hipersil (High performance silicon-steel).

In 1942, Westinghouse developed a method of winding ribbons of Hipersil on formers to build up transformer cores, much like the stacking of E and I laminations. By continuously winding the material onto a core, the direction of the material always coincides with the optimum direction for the magnetic flux. Through the years Hipersil cores have become a mainstay of many transformer, motor and other magnetic components. They allow the designer to shrink the size or obtain better performance from a component. The transformer components are generally wound on an oval former, and then cut into two U-shaped halves. These were then inserted into pre-wound coil bobbins and strapped closed again to complete the magnetic circuit.

Early Alphas, such as the 70 and some members of the families using 8874 tubes used E-I stamped lamination transformers. Starting with some of the 76 and 78's, and certainly all amps from the 86 onward have been shipped with U-shaped tape-wound Hipersil cores, as the cost of manufacturing such transformers came down.

This is a picture of a current production Hipersil transformer using U-cores. The strapping material for clamping these shut is visible on the ends, and the two pre-wound coil bobbins can be seen. Various windings are split between the two coils to make them come out to be the same diameter- this allows the largest diameter wire to be used, resulting in lowest losses. It is fitted with a lifting handle to assist with getting it into and out of the amplifier chassis. We ship our transformers in a separate box from the main amplifier chassis, due to their extreme weight, and the danger that shipping loads could damage the chassis.

Ideally a transformer would be wound on a tape core that has not been cut into two U's. This would cut out a fairly expensive manufacturing step. However, that would mean the coil would have to be wound through the center of the core, which is difficult to do by machine.  

Recently machines that wind directly onto toroidal cores have become fairly widespread. Working with our transformer manufacturer we have come up with a new design that uses two tape-wound toroids and is a functional replacement for the older U cores. This will allow us to hold in check some of the cost growth that we have been seeing on some of our major components.

This is a picture of one of the new toroidal units. The toroids are packaged side-by-side along their axes, much like two doughnuts. A bolt through the middle of the "doughnut holes" secures them to the mounting plate.

The new unit is electrically identical to the old one, but it is a little bulkier. This has meant that some components have had to be moved around a little bit in order to accommodate it. This is shown in the two CAD renderings of the AP8406 6-meter amplifier.

On the left is an amplifier with the older style transformer, and on the right the current production version. You can see that it has been necessary to move the auxiliary muffin fan to the outside of the chassis, in the same way that we do on the 9500. Also, the primary Molex connector bracket has had to be changed to the same one we use on the 9500.

Other than the physical aspects, you will not notice any change in performance. For those amplifiers which shipped with the older transformer style, we will continue to have a supply available in the unlikely event that you need a new transformer. Within a few months, all new amplifiers will be shipping with the new transformer.

73, Gordon, W0RUN 

The staff here has taught me that as the seasons change, there is corresponding ham radio and amplifier activity. I am learning to gauge that as I plan for our operations and repair activities for the fall and winter. We have worked hard on reducing the sales backlog for our current products and are very excited about the prospect of our new antenna tuner.

Last month I introduced our technical team. This month, I would like to talk about my 'go-to' girl, Carol McSherry. Carol has been with RF Concepts for a couple of years and has made herself invaluable to the company.   She does any number of activities which are critical to our shipping new amps and repairing older ones. She puts together some of the assemblies in the new amplifiers; she pulls kits for monthly amplifier builds; she manages communications with our contract assembly staff; and she finds that last little part that I need and can't seem to locate.

Just when I think I have seen every model of Alpha amplifier, a different one shows up for repair and takes me by surprise.   That happened today.  I guess it won't be the last time.

73, Kathy, KB2HDT