"Amplifier Power?"                    

Or, trying to decipher confusing manufacturer's claims ......

 

Once upon a time, in a CB world long past, there were those who sought to improve their signals through the use of a (albeit illegal) "linear" amplifier. Back in the mid 1970's, practically all base station amps, and most mobile amps were made using vacuum tubes.  Specifically, they were designs which contained at least one, two, or a bunch more (depending on power output) tubes which were originally designed for television horizontal sweep outputs. But these tubes also worked fairly well for R.F. amplification. A single stage design utilizing a pair of 6JU6 type tubes could boost your 4 watt signal to between 40 and 65 watts of power.  Expand to dual stage designs with 4, 8, 10, or 12 of these glowing marvels, and you could be touching 500 or more watts of carrier power, and up to 2 KW of SSB PEP power. Most people who ran these amps, found that they sounded good until you started to overdrive them. At that point they began to sound fuzzy, and the power meter would swing backward with modulation.  But at the same time, the people who were overdriving their amps (Usually by feeding them with a "peaked out" radio), were a bit surprised that the amp would often exceed the manufacturer's advertised ratings.  Back then, people were more interested in the "big number" which showed on the wattmeter, and some would sacrifice a little audio quality in order to make that "big number" as high as possible.

Then toward the end of the decade, and into the 1980's, solid state transistor amplifiers started to grow in popularity, mostly in mobile applications. Initial users of these smaller and less current hungry transistor designs would find, disappointingly, that they sounded really distorted at the rated power output level, along with nasty amounts of back swing. This led many to conclude that transistor amps were inherently junk, and many of them were simply called "distortion boxes".  But then later on, after a little experimentation, it was found that these solid state amps could be made to sound clean, but it meant that you had to cut the output by at least 50% to do so. Why was this? Is there some little inherent difference in the characteristics between how a tube amplifies over that of a transistor? Actually, the answer is even simpler than that.......

The answer, it seems, is that the manufacturers quietly changed how they rated the power output of their amplifiers. The early tube amps from the 70's were rated for dead key carrier power or RMS output.  Later transistor designs (And at least one tune amp maker as well) rated their amps in terms of peak power output. To understand the difference, and why this is significant, you have to recall how an AM signal is comprised. An AM signal consists of a carrier, which is varied (modulated) in amplitude (power) with your voice information. 100% modulation has an instantaneous peak power that is equal to 4 times the dead key power output. This means a 4 watt CB radio will have a peak power of 16 watts at 100% modulation (a 4:1 ratio).

Applying these facts to amplifier ratings means that an amp rated for 100 watts DEAD KEY, should also be able to produce 400 watts on 100% voice peaks.  On the other hand, an amp rated for 100 watts PEAK, means that this is its maximum power output rating that the amp can produce, and that it should be set for a dead key carrier output of no more than 25 watts in order to maintain the proper amount of headroom to properly amplify modulation. You can "fudge" this to a certain extent. You can compress the peak power a small amount in order to get the dead key number up. But the more you do that (usually as you drop below a 3:1 ratio) , the more fuzzy the signal becomes, the more back swing you will introduce, and the less "punchy" and more "pinched" your audio will sound.

So why all the confusion? Well, the manufacturers didn't help much (and most likely remained deliberately ambiguous about it).  First off, unless you read the fine print (if there was any), you wouldn't know which standard the amp was rated for.  One clue would be if the amp's SSB power rating is significantly higher (3 to 4 times) than the AM rating. That would indicate that the amp was rated for RMS power on AM.  Secondly, the early tube amps took the expected drive power into consideration and when driven by a typical CB radio's output, the amp's output was very close to the proper amount to maintain some headroom for modulation. A solid state amp, on the other hand, can be overdriven beyond the limits of linearity and well into compression, with a standard out-of-the-box 4 watt radio.  Most radios have to be detuned, or some sort of attenuator has to be put in line to drop the drive power down far enough to allow full modulation. But once this is done, many people will become disappointed. Their 100 watt amp, has now become a 25 or 30 watt amp, which can be more than a little downer.  This is where peak ratings cause the most confusion. A good rule of thumb to follow, when dealing with peak rated amps, is to take the maximum rated power and divide it by 4 (or 3 if you want to push it a little) for the true carrier power out.  So if you want an amp that delivers 100 watts of dead key power, you need to look for a 400 watt (or more) peak rated amp. Sometimes you might have to experiment.  Some peak rated amps will actually deliver 10 - 20% more peak power that the actual rating (The amp rating ends up being somewhere between dead key and peak, which is even more confusing). If you can drive the 100 watt amp to 120 watts, you can probably get away with a 35 to 40 watt dead key and still achieve acceptable results.

A good case in point is a typical Texas Star two stage 5 transistor (1 driving 4) amplifier. These amps are usually rated for around 500 to 700 watts out. In fact they will do close to 500 dead key watts out with 4 watts drive in the high power setting.  But at that point you are overdriving the amp and it doesn't sound all that good.  Maximum peak power, with a good power supply, is close to 750 or so watts.   The amp sounds the best on the "low" setting, and it still sounds acceptable on the 2nd stage. In fact, if you bypass the driver transistor altogether and feed the 4 output transistors directly with a 4 to 5 watt signal, the output power falls to around 160 to 180 watts of dead key power, which is just about exactly where you should run this amp for maximum clarity and acceptable linearity. But the manufacturer couldn't sell it for as much if the "big number" wasn't attained with a standard radio.

It's a shame the CB band isn't FM. Then the peak number and the dead key number would be one and the same.........

 

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