[AMRadio] power ratings


Geoff w5omr at satx.rr.com
Tue Jul 12 19:47:10 EDT 2005


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






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