Paul, WA3GFZ, wrote: > ... If one needs an 800 watt amplifier to run > 100 watts AM, then how does a Viking with 3 > small 6146s run 125 watts AM?
It's due to the difference in efficiency between a non-linear, plate-modulated, Class-C amplifier and a linear, Class-AB2 amplifier. A Class-C amplifier can have up to 80% efficiency, so making a 125-watt output would only require 156 watts of plate power input and the final tubes would dissipate 31 watts. Things are not that neat, so a typical 125-watt final runs a pair of 25-watt-plate-dissipation 6146's. (The Valiant is actually capable of 150+ watts output). However, Class-C amplifiers are NOT LINEAR. Driving one with a modulated RF signal results in a severely distorted output signal that is useless for communications. The extreme nonlinearity of a Class-C amplifier is actually the reason it is able to achieve its high efficiency. The reason the modulation is not distorted in a modulated Class-C amplifier is that it is applied in series with the plate supply and actually varies the gain of the Class-C stage in a linear manner without relying on the amplification of the stage itself. Amplifying an RF signal already modulated by the driver (in the present case, the Ranger) requires a linear amplifier. Linear amplifiers have low efficiencies at low signal levels and only achieve reasonable efficiency on signal peaks. Unfortunately, for AM, the carrier level must be held at 1/4 the peak output power, so it is always in the low-efficiency part of the linear-amplifier characteristic. If the carrier amplitude is raised so that it is in the high-efficiency part of the linear-amplifier characteristic, positive modulation peaks will cause the amplifier to saturate and extreme distortion will result. Using linear amplifiers to amplify low-level-modulated signals is a viable mode of operation for radio services that transmit on an intermittent basis at relatively low power levels (as amateur radio does). However, in radio services such as broadcasting where the transmitter is on the air continuously (at high power levels), the electric power wastage becomes a significant expense factor. And the roughly 4X increase in power supplies and tubes required presents an unjustifiable capital expense, even for amateur stations. The result is the almost universal adoption of high-level modulated transmitters by both commercial and amateur operators. The emergence of the requirement of linear amplification for SSB over the past half century has resulted in the creation of relatively cheap linear amplifiers for amateur radio use - so much so, in fact, that the economics have now swung back in favor of low-level modulation, or at least placed it on par with high-level methods. Price a modulation transformer from Peter Dahl, or look at the nearly $4000 tag on the resurrected Globe King 500D. That Ranger driving a funky old Heath SB-220 begins to look pretty good. Jim Bromley, K7JEB Glendale, AZ
