In fact, since many people refuse (or just don't know how) to click on a
link, allow me to paste some of that here, that is germane to the
overall discussion of power levels, ratings, and class of operation of a
tube.
-Geoff/W5OMR
=====================================================
Amplitude Modulation ("AM")
using natural asymmetrical voice
a joint effort by
John, WA5BXO
Bob, WA3WDR (Bacon)
Tim, W5TOB
Don, K4KYV
When I (John) first tried AM, I had a Knight Kit T-60 transmitter, which
used controlled-carrier modulation. Controlled-carrier modulation is a
method of modulation that maintains a low carrier output until
modulation is applied, and then the carrier will increase with the
average level of modulation. This is done to keep the power consumption
and heat low when you are not modulating. It was often used in rigs that
were inexpensive and had components that were not sturdy enough to
maintain a higher level of carrier and modulation. Modulation was
accomplished by applying audio voltage to the screen grid bias voltage
of the RF output tube. Controlled-carrier operation resulted from
deliberate rectification of the audio waveform in sections of the
modulator that were DC-coupled to the modulator output, and this was
arranged to cause the average screen bias voltage to shift upward when
audio was present, thereby increasing the carrier level when modulation
was present. If this system was not overdriven, the resulting signal was
readable, although not pleasing to the listener. The diode detector type
receiver's AGC voltage would bump up and down with the carrier shift.
This caused a very annoying rise and fall of background noise. Also,
with loud speech, the rectifying section of the modulator could easily
overperform its function. The resulting distortion was so severe that
most of the audio was actually eliminated, just when it should have been
the loudest! It was very difficult to avoid overdriving the modulator,
without the modulation being too low to hear well. The group of hams
that I wanted to join on 3850 KC just couldn’t hear me, or complained
endlessly about the awful sound from my rig’s controlled carrier
modulation.
Their complaints were constructive, and they convinced me that I needed
to upgrade my modulation technique if I was going to join in the AM fun.
My solution was to build a plate modulation system. The modulating audio
voltage was derived from an external audio amplifier that could deliver
the proper audio voltage to the plate supply circuit of the final RF
amplifier. The improvement this made in signal output and audio quality
was remarkable. The group could hear me, and hear me clearly. Over time,
I learned more, but the knowledge came slowly. It was almost 10 years
before I really understood the circuit, and the math behind it. A lot of
this understanding is due to my association with Don, K4KYV.
The T-60 was typical of many relatively low-cost transmitters available
to newcomers to the hobby in the years from 1960-65, and it is an
example of how the manufactures were trying to sell equipment. The
advertisements would say something like “Here is a transmitter that will
run near the legal limit for a novice on CW and has the capability of
running AM when the novice upgrades to general class.” The Knight Kit
T-60 rig described above was purchased for about $70. The cost of adding
the AM capability to the transmitters design was probably about $5. The
external modulator that I constructed was built from scrap and
hand-me-down parts, but had the parts been purchased, they would have
cost more than the Knight Kit T-60. The original $5 modulator that was
put into the Knight Kit T-60 was a bungled attempt to add AM capability
to a low-cost transmitter, but it sold a lot of transmitters.
Why was screen modulation used? Because it was inexpensive and simple.
It did not require any transformers, and only small, low power tubes
were needed in the modulator.
Why was controlled-carrier modulation used? Mostly to reduce RF
amplifier plate dissipation. Efficiency is low in a screen-modulated AM
transmitter. Typical carrier efficiency is only about 35%. The typical
6146B could only produce about 15 watts of carrier power, and at 35%
efficiency the plate dissipation was about 28 watts. The thought was to
reduce plate dissipation when no audio was present, by reducing the
carrier output. Efficiency was lower at lower output levels, but
dissipation was lower, too. The idea was that average plate dissipation
would be lower, so more carrier power could be produced when audio was
present, without overheating the tube. However, the transmitter designs
really did not produce much more usable carrier power during modulation,
and distortion was so bad that this power seemed higher, but it really
did little good.
Some amateurs have made simple improvements to the screen modulator
circuitry of the T-60 and similar rigs. Transmit audio quality can be
quite good with circuits almost as simple as the one that was originally
used. However, the old problem remains: efficiency is low, and power
output is relatively low.
Of course, you can get full class C efficiency with high level plate
modulation, and the same 6146B can give you about 49 watts of AM carrier
output in this mode. But the problem was, you needed a powerful audio
amplifier with a modulation transformer, all of which was more
complicated and expensive. So, some amateurs had another idea: use a
linear amplifier. You could generate AM at a very low power level, using
either plate or screen modulation, and amplify the modulated signal up
to high power with a simple linear amplifier.
Transmit audio quality can be excellent, and the modulator does not
usually require a modulation transformer, but there is a problem. The
efficiency of a class B linear amplifier at full output is about 66%.
However, when amplifying an AM signal, the carrier output level has to
be far below the maximum output level of the linear amplifier, to allow
headroom for the positive modulation peaks. At carrier level, the
efficiency of a class B linear amplifier is only about 33%. At 33%
efficiency, that 6146B can only give us about 14 watts carrier with
about 28 watts of plate dissipation. So we were back to the same problem
we had with screen modulation: low efficiency, and relatively low power
output.
There were some very complicated and exotic modulation techniques
available in those days that could produce quality AM at high efficiency
with no high-level modulator, but these were so complicated and
difficult to tune that they were impractical for amateurs to use.
High-level plate modulation was the way to go. Many high-level plate
modulated amateur transmitters were sold in that era. AM operators
quickly associated strong signals and intelligible audio with high-level
plate modulation of a Class C RF final.
The FCC regulations at the time stated that the general class license
holders could run no more than 1000 watts DC input power to the plate
circuit of the RF final amplifier. This was easy to measure with DC
meters. The plate supply voltage was multiplied times the plate supply
current, and the product was the DC input in watts.
RF amplifier efficiency determined how much output power could be
achieved. The efficiency of a screen modulated RF output stage or a
class B RF linear amplifier at carrier level is about 33% to 35%, giving
you about 330 to 350 watts of carrier output on AM for the maximum legal
input power of 1000 watts. On the other hand, a plate modulated class C
amplifier has about 75 percent efficiency, giving you about 750 watts of
AM carrier output for that 1000 watts input. And the output tube in the
final RF amplifier runs a lot cooler in class C than in class B AM
linear operation, so smaller tubes can be used. But the only practical
way for hams to get high RF amplifier efficiency with AM was to apply
the modulating audio voltage to the plate supply of the RF final, and
the audio circuitry required to do this must be capable of at least 500
watts of audio. So, to get the extra power output within the legal
definitions, most of the big-gun operators opted for the high-level
plate modulation method.
The high-level modulation method is the application of the modulating
voltage to the plate circuit of the class C final, causing the output
amplitude to vary in accordance with the applied modulation. One hundred
percent (100%) modulation was generally defined as the point where the
maximum modulating voltage, during its negative half cycle, opposed the
DC supply voltage sufficiently to reduce it to zero. If this voltage
dipped below zero, over-modulation and splatter were the result.
Most people agreed that the peak of the positive half cycle of the
modulating audio voltage, added to the DC supply, could go as high as
necessary to faithfully reproduce the audio as an image of the
microphone output. Even if the positive peak was more than two times the
amplitude of the negative peak, the modulation was not considered
illegal unless it contained distortion products that caused splatter
over an excessive bandwidth. Over-modulation was only considered to
occur at the point where modulation characteristic became non-linear,
producing distortion and splatter.
The audio voltage from a microphone is often not symmetrical, unlike a
sine wave from a signal generator. This asymmetry is a natural quality
of speech and other sounds. This article discusses the use of voice
waveform asymmetry in AM systems.
========================================================================
The rest of the site deals contains graphs, graphics some links and lots
more math.
Very well worth the read.
http://www.qsl.net/wa5bxo/asyam/aam3.html
73 = Best Regards,
-Geoff/W5OMR
