> ... 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?
What do you mean by an 800-watt amplifier? At a minimum, you need a linear
amplifier with only 400 watts peak output capability to run 100 watts steady
carrier AM @ 100% modulation.
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.
Actually, linear amplifiers preceeded high level plate modulators for high
power AM broadcast transmitters. Originally, the class-B amplifier was used
in rf linear service. It wasn't until the late 1920's that someone figured
out that class-B could be used to amplify AUDIO signals as well, by simply
making the amplifier push-pull so that both sides of the audio waveform
would be produced. In a single-ended class-B linear, the missiing half of
the rf cycle is generated by the flywheel effect of the rf tank circuit.
Before the advent of the class-B audio amplifier, plate modulated
transmitters depended on class-A audio amplifiers for modulators, usually in
the "Heising modulation" configuration. These transmitters were even less
efficient than class-B rf linears, because the class A modulator ran
typically at less than 20% efficency. The total a.c. power input vs rf
power output of these transmitters was very very low, substantially less
than that of a class-B linear or a low-level grid modulated final.
Once suitable audio driver and output (modulation) transformers became
available in the early 1930's, high level class-B modulation systems became
popular because less expensive tubes having lower plate dissipation ratings
could be used. Like today, tubes sold at a premium during the depression
era. However, the total overall efficiency of a a high level class-B plate
modulated amplifier is not much different form that of a low level (grid
modulated or linear) amplifier.
Losses in low level modulated transmitters lie mainly in the power
dissipated by the final amplifier tube. In the high level plate modulated
rig, there is plate dissipation loss in the class-B audio amplifier
(remember it is also as linear amplifier) plus modulator tube filament
power, plus the power consumed by the class-B audio driver stage.
Continental Electronics, a long time manufacturer of broadcast transmitters,
never offered a plated modulated product. Their advertisements always
pointed out that from a BC station's vantage point, low level and high
level modulated (tube type) transmitters are about equally economical to
operate in terms of the electric bill for running the transmitter.
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.
Very true. And like AM, a properly operating SSB transmitter operates at
much lower average power than what it generates on occasional voice peaks.
That means that most of the time, the SSB linear is just as inefficient as
the AM linear. The main difference is that with SSB you don't have the
constant power dissipated in the plates of tubes as heat as a result of
delivering a steady carrier.
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.
Actually if you want to build a high power high level homebrew transmitter,
there is no need to spend megabucks to purchase a custom built mod
transformer. There are plenty of tube type broadcast transmitters sitting
around all over the country that have been removed from service. These can
be bought for very little, and often are offered for free if you are willing
to remove the entire transmitter from the premises. Abandoned AM BC
transmitters have become a presently available industrial surplus item.
Some hams prefer to make a few modifications and convert the BC transmitter
directly to ham use, but they contain a wealth of parts for someone who
prefers to homebrew from scratch. This involves completely redesigning the
layout using the same parts, and most likely building a new rack or
enclosure, which may or may not be constructed from materials salvaged from
the old, highly oversized and overweight broadcast transmitter cabinet. The
challenge is to find out about a discarded BC transmitter before it is
hauled to the landfill.
Don K4KYV
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