amplifier power claims -bogus!

This is a short story about the claims made by those who offer linear amplifiers and the lying meters and tricks they often use, whether they know it or not. Some reasons why amplifiers die will also be discussed. If I take heat off purveyors of such gear, so be it. At least I know how to measure power and how to keep the equipment in one piece. By the way, I just noticed amateurlinearamplifiers.com, a seller of linears I assume, has had its site suspended. Pity, it was a good reference with lots of amplifier pictures.

Anywhere you go, whatever CB linear you see, the seller will often claim that the unit puts out about twice what is really does. It is because of the way the power is measured and the meters used to measure it. A measurement of the undistorted output of the amplifier in question, using a decent wattmeter and a dummy load, will show the truth they would rather not let you see.

Several factors contribute to the outlandish claims. The true power rating of an amplifier must be measured with undistorted RF power output at the peak of the voice signal in AM operation (PEP Output).

Almost every time, a "CB type" wattmeter like a Dosy or Workman will read 1.5 to 2 times higher than the power shown on a ham, military, or Bird meter. A quality meter designed for accuracy, connected to a proper load, is the only way to honestly measure watts.

Another error with "CB wattmeters" with regard to AM operation is that most of them "swing" a lot under modulation even when the average power has not increased much.. Whistle into the mike, and see the power shoot up to the end of the scale! Reality is not so. A good wattmeter used with a properly adjusted (not chopped up) CB will swing the needle very little on modulation if the amplifier is linear. You can put Dosy and Bird wattmeters both inline with the load or antenna and see this.

A good ham radio wattmeter costs a little more than a CB wattmeter and is much more accurate. No need for a Bird unless you are building or repairing. A ham radio meter has to be accurate because hams need to carefully observe a 1500W peak power limit. What is sometimes done in practice may be another story, but the various ham gear manufacturers offer accurate meters for this purpose. It's considered a piece of necessary test gear. A Bird is more costly but only gives 2-4% better accuracy.

CB Linear manufacturers have also tended to give amplifiers model numbers that suggest a certain power level, and not specify whether it is input or output power (power used by the amplifier, or power put out to the antenna by the amplifier, two very different things!).

Whether it is a tube or solid state amplifier, this practice prevails. That's why a "TX-800" is often a 400 watt amplifier, not an 800 watt amplifier as implied. The "TX-800" has four 2SC2879 transistors which are rated for 100 watts output each. Perhaps the TX-800 consumes 800 watts -a far more likely scenario.
The old HEC-300 tube linear does make 300 watts inside its high voltage power supply. Clean RF Output from the unit is about 165 watts.
The Sweet Sixteen is an 8-transistor, 800 watt amplifier. Not 1200W.

Do they lie or trick (overdrive)? Not so, they say! "It does make 800 Bird Watts!" OK: Overdriving it till it's about to fail is one way to get almost 800 watts out of the "TX-800" 400 watt amplifier, even using a good meter. Do they know the truth? I bet most of them do.

Just as with a guitar amplifier that has an undistorted RMS output of 100 watts RMS, but can be severely over-driven increasing the wattage to 200W RMS, an RF amplifier can be overdriven with the same result. It's not good for it and the extra CB power is not heard by the receiver at the other end.

The point is not to be fooled by the CB-shop demonstration of an overdriven amplifier, even with a Bird meter.

Look up the transistors and see what the ratings are. Those are the facts. Any other claims are incorrect.

Now, about driving a transistor amp hard: How many times do we see an amp with big power claims and "1 driving 2, or 1 driving 4"? The problem that comes into play here is that the driver transistor may be able to make 60-100 watts, but the 4 output transistors it is driving only need about 6 watts each for them to reach full rated output. Look it up in the data sheets.

The way RF power transistors are made is that instead of just one 'stack' of silicon 1/8" to 3/16" square, there is usually the 1/8" piece on the bottom (the "collector" element), then a rectangular grid of a hundred or more tiny almost microscopic little stacks or bars of P and N silicon which make up the base and emitter elements of the transistor. It's done that way to improve performance.

So there is a 60 watt amp (driver stage) driving a load (four output transistors) that can only take 24 watts. One by one, the tiny little bars of material crack, melt, or are just burned up from overloading and overheating. The amount of clean power the individual transistor can make diminishes over time due to this abuse. MOSFET amplifiers can suffer a similar fate when constantly over-driven but those devices fail in a different way. They still die and the money spent on the amp is gone.

Why are the amps designed poorly to fail like this? Profit. CB users often don't know better, they just look at the watts.

Your goal, Mr. CB linear designer, is to wring every last penny's worth of power out of as few amplifying devices as possible!

What to do about this mess? Back to the beginning: find out what transistor types are in the output stage of the amplifier, take the wattage from the data sheet (online), and reduce the power from the radio until their combined wattage is never exceeded on voice peaks. The amp will last decades.

There are very precise ways to do this but an easy way to get close is to set the radio power so that with the dead key, the amp is making 1/4 of whatever rating you have determined is the max for the amp in question.

One of the attachments has pictures of the inside of a typical RF transistor showing the structures described. It might be interesting to read a little about it. Scare the !##* out of the next tech you run into.

 

I think I have one of those old 200 watt per channels amps with the built-in EQ on the front. 1" tall x 6' wide.. What did they do, maybe 12-15 real watts per channel to 4 Ohms?

Been a long time since the car stereo days but I used to install them.
Do you remember the Sanyo PA-6050 and PA-6100? The first really powerful car stereo amps in the 1980's to have honest ratings IIRC. I saved for months to buy one and blew up a few 6x9's with it. Speakers also had lying ratings!

 

Good point on the minimum necessary power, Gadfly. I didn't bring it up because it was not bearing on the power meter subject but some idiot hams don't get it.

Hey RubberFrog, like he says the license is very easy to get these days. Lots of rules though, if you can deal with that you will have only friends on the air. "Practice tests" at qrz.com, pick the "technician" to start with. Ham radio has an extra bit of fun inside because you can build your own stuff if you want. You don't have to though, there's plenty in the stores. A good operator who can stay cool under stress and difficult environmental conditions is as important as the one who can repair a radio in the field and keep a station on the air. All talents are important.

Note that individual hams are 100% not cool sometimes. There are a lot of hammy hambones who for some twisted reason want to be on the air and spread their idiocy. It's those rotten operators that curse and play music on the air, jam other stations, run excess power, and usually could not scatter manure with ten rakes tied behind them and the bull chasing after (much less operate calmly in an emergency or fix their own radio or build something, anything..). The FCC eventually gets them due to complaints and recordings. Haha maybe they came from broken homes or drink too much!

I don't know everything and if there are mistakes in what I post, they should be pointed out.

The only reason I posted about this topic is that there is so much b-s about CB linears that I figured if I could try to clear up some of it, operators might make better choices and their gear would work better and last longer. It's mostly an article I had already written a long time ago. I'm not going to be one of the holier than thou or stuck up hams and judge the CB operator with the linear.

Most hams like newer stuff but I like the old-time classics and have been working on restoring these three old transmitters. The brown one is a 1947 RCA 250W broadcast AM band transmitter I dragged off a mountain in Kentucky to convert to ham use and the blue one is a 1957 home-built 1000 watt ham unit found in a scrap yard in Dallas. The gray one is an 1948 Link 250W FM Police base station free from an estate. The guy was going to part it out so I felt obliged to take it as it was complete.