rabruner at aol.com
rabruner at aol.com
Tue Feb 16 22:54:23 CST 2010
Ham rigs with clamp tube modulators, such as the DX-60 and others are not capable of 100% modulation, and attempting to "talk them up" just produces distortion and splatter. The varrying carrier level also contributes some to distortion as well. Listening to selective fading on AM shortwave stations demonstrates what that can sound like taken to an extreme. These transmitters can sound acceptable when correctly adjusted and operated with restraint.
We shouldn't confuse screen modulated AM broadcast transmitters with these 'controlled carrier' transmitters. The broadcast transmitters achieve full modulation, good carrier stability and excellent audio fidelity. Not the same beast at all.
Grid modulation takes a lot less power than plate modulation, but without
applying techniques like Doherty, Terman-Woodyard etc, the efficiency is about
the same as straight linear, about 30% carrier for AM. Screen modulation was
(and still is, in some circles) popular, because only the screen voltage needs
to be modulated (usually half or less of the plate voltage), and the audio power
required is relatively small. A modulation transformer can be used, but it is
usually not necessary, and transformerless coupling of the transmit audio
permits high quality sound from a simple transmitter. But despite that, the
sound was usually pretty raunchy! There were a number of screen modulated AM
transmitters on the market back in the day.
Controlled carrier' was popular in many screen modulated designs. The transmit
signal was reduced when no audio was present. It made the meters kick up when
the user would speak (a prized characteristic, it seemed to mean you were loud),
and the reduced duty cycle was a little easier on the transmit tubes. But it
would have sounded better if the carrier had instead been raised during periods
of low or no transmit audio, to maintain approximately constant peak level,
because then the receiver agc would have created a noise reduction effect. In
fact this is seen in amateur AM linear operation where the linear amplifier has
fast alc, and it can actually sound good. But instead we got lots of noise
between syllables with most of the old controlled-carrier screen modulated rigs,
not to mention the atrocious distortion that came from very poor design, so the
received signal sounded a lot noisier than it really was.
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