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Class "C" Amps for AM, SSB?

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Old 07-27-2009, 08:53 AM
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Default Class "C" Amps for AM, SSB?


I'm not an electronics engineer, but I can read schematics well enough (part to part) so by looking at the schematics of linear amplifiers, I am still unable to determine what class the amp is in.

In theory, a Class "C" amp will not work properly (or at all, depending on the theorum) with AM and SSB, but yet, virtually all 10-11 meter "CB" amplifiers are, supposedly, Class "C". Many of them sound very good on AM and SSB which, in theory, shouldn't be the case(excluding the IMD and all that stuff that may go with them).

What gives here? Are these amps "pure" Class "C" or are they a "hybrid" of sorts?

73's,

Robbie


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Old 07-27-2009, 09:18 AM
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Just looking at a schematic isn't going to tell you much, if anything, about the biasing of a particular amplifier. You can see the various components, but without actually putting the amplifier on the bench and putting it through a few minutes of instrumented testing, you aren't going to know anything about the dynamic characteristics of the amplifier.

Another problem, particularly with "CB amps", is that they might start out at Class AB, but after a few minutes of being run hard, the temperature rise in the circuitry will slide it from AB, through B, and well into C.

Regardless of how well an amplifier "sounds", the only real way to determine whether it is distorting for whatever reason is to use a spectrum analyzer to actually take a look at the output. A spectrum analyzer is NOT an oscilloscope. A good used o'scope might cost $3-400. The Spec An will run twice that and more.
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Old 07-27-2009, 09:23 AM
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"In theory, a Class "C" amp will not work properly (or at all, depending on the theorum) with AM and SSB, ..."

I would suggest you re-evaluate that 'theorum', it obviously isn't complete as indicated. It's true that class 'c' isn't exactly the best for most 'AM' voice modes, but it is 'do-able'. It's also 'over do-able', like any other class of amplification.
- 'Doc

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Old 07-27-2009, 11:43 AM
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A quick and easy way to tell if the CB amp is biased class C is to look at the input transformers on the schematic (or in person). They usually ground the input transformer through a ferrite coil, usually wound 3 times or so. If you actually see a bias circuit on the schematic, then that would be a clue that there is something going on there.

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