The power transformer often constitutes a major portion of the cost of home brewing an HF amplifier. This, however, needn't be the case. Relatively small power transformers of modest capability may be used for intermittent voice service (IVS) and CW service at a worthwhile saving in weight and cost.

The duty cycle (ratio of duration of maximum power output to total time power is applied to the primary windings) of a power supply is what determines the transformers required power capability. In SSB and CW service the duty cycle is much smaller that that of a supply used for FSK, SSTV and AM operation.. While the power supply must be capable of supplying peak power equal to the PEP input for a short duration, the average power demanded by SSB is generally only about one half or less the total PEP requirement. The syllabic rate, or interval between words, of SSB provide periods of low duty, similar to the way in which CW permits the power supply to rest during the non-keydown portions of a transmission. For this reason, the average power capability of a supply designed for IVS can be as low as 25 percent of the PEP level. CW, on the other hand, runs somewhat higher with the average at close to 50 percent of the peak level for short transmissions.

Despite the forgoing, most transformers are rated for commercial service. The IVS rating, on the other hand, must be determined by experiment. To that end, William Orr, W6SAI, in his exceptional text titled simply, Radio Handbook (see Reference Material), outlines his findings, below, based on the weight of the transformer (less case and mounting hardware). In my experience, I have found his results to be completely reliable and consequently feel entirely comfortable including them here, as well as elsewhere on this site.

So, haul that iron out of the junk box, put it on a scale and see what you come up with. You might be 'shocked' at what you come up with.