| Author |
Message |
    
Moonraker
Intermediate Member
Username: Moonraker
Post Number: 144
Registered: 3-2005
| | Posted on Thursday, February 02, 2006 - 8:54 pm: | 
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I have read alot of things about amplifiers being biased AB, AB 1, B, C. What does this mean? What makes these "classes" different? If you have an amp re-biased (if possible) what is changed? And finally, how can using a class C amp on SSB damage it? |
Rldrake
Junior Member
Username: Rldrake
Post Number: 45
Registered: 1-2006
| | Posted on Thursday, February 02, 2006 - 11:46 pm: |
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Running ssb on a class C amplifier won't "damage" anything. The signal will just splatter and sound like crap, as is the norm with the "cb" amps. Class C amplifiers are non-linear and are ONLY suited for FM and CW transmissions. They are not suited for any mode that is amplitude modulated. The other "class" designations primarily deal with the effeciency of the amplifier. These other classes are generally suited for all transmission modes since they are designed to be linear with a rising and falling modulated (varying) carrier.
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Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 1140
Registered: 9-2004
| | Posted on Thursday, February 02, 2006 - 11:58 pm: |
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im by no means an authority on this, but here is what i have learned.
in class C operation, the amplifier transistors are turned off until you key the mic. then they are on, until you unkey.
thats on AM.
on SSB, everytime you speak into the mic, the transistors are turning on and off with your voice peaks.
they were not designed to switch back and forth so fast, and eventually, people will tell you that you sound like you are "clipping" because the transistors are on their way south.
in class AB1, the transistors are always "on" and just have to amplify the signal to varying degrees.
i believe there is a way to mod a class C amp for class AB1, but it takes a bit more than just a cap.
thats what i know,
matt |
Rldrake
Junior Member
Username: Rldrake
Post Number: 47
Registered: 1-2006
| | Posted on Friday, February 03, 2006 - 2:18 am: |
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The "class" of amp has nothing to do with transistors "turning on and off". Virtually all legal limit amplifiers are "tube type" as are most quality, medium to high power (linear)amplifiers. "Modding" a non-linear class C amp wouldn't be very practical. Such would amount to removing just about every component and replacing with proper components...the cost would be more than just buying a suitable linear amplifier...even if the case of the "c" amp had the room in it for total rework. |
Chad
Advanced Member
Username: Chad
Post Number: 676
Registered: 11-2004
| | Posted on Friday, February 03, 2006 - 8:54 am: |
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A class "C" amp can be biased to class "AB" It will become less efficient but at least it will be useful.
As for hearing a transistor going south. Not likely. Unless you start losing transistors in a bank a single transistor does not have a declining failure mode. It works or it don't. Tubes on the other hand can and will decrease performance with age. But barring heat saturation a transistor will have predictable results time after time until it releases it's magic smoke. If that happens you have done something wrong. Even with heat saturation, let it cool down and it will work normal. Anyone who said that transistors need to be changed out from time to time was selling transistors for a living 
Chad |
Wolverine
Advanced Member
Username: Wolverine
Post Number: 599
Registered: 2-2003
| | Posted on Friday, February 03, 2006 - 9:37 am: |
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Fascinating.... just when I thought, that I knew about everything about amp classes, I learn something new. My brain is still working. |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 1143
Registered: 9-2004
| | Posted on Friday, February 03, 2006 - 5:39 pm: |
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i may be showing my ignorance here, but i believe i have seen with my own two eyes; an amplifying transistor work intermittantly.
i have definitely seen it with a radio final.
maybe they are not "supposed" to break down intermittantly, but i have replaced a transitstor or two that worked half the time, and the replacement fixed the problem.
just my own findings; flame away,
matt |
Chad
Advanced Member
Username: Chad
Post Number: 677
Registered: 11-2004
| | Posted on Friday, February 03, 2006 - 7:53 pm: |
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Aw no flames, ANYTHING is possible, I have replaced thousands of transistors, I have never had an intermittent one or one actually loose Hfe gradually. I HAVE seen a few get thermally unstable, right before it blows up. I will admit to having not much expierience in RF amps but AF it either shorts or (rarely) open.
But then again I thought I'd never see a resistor failure result in a DECREASED resistance Once I have, Anyone else? Device and failure in next report.
Chad |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 1145
Registered: 9-2004
| | Posted on Friday, February 03, 2006 - 9:04 pm: |
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i think thats what i have experienced chad; thermal instability. and im sure if i hadnt replaced the part, it would have completely gone very soon.
now my question is: what are some common causes of thermal instability?
could using a transistor biased for class C on SSB cause this?
matt |
Moonraker
Intermediate Member
Username: Moonraker
Post Number: 145
Registered: 3-2005
| | Posted on Friday, February 03, 2006 - 9:21 pm: |
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After reading this and other info on the net, I have a much better understanding of the subject. Thanks. |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 1148
Registered: 9-2004
| | Posted on Friday, February 03, 2006 - 11:51 pm: |
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guess i need to do some searchin' on the net too!
matt |
Wildrat
Advanced Member
Username: Wildrat
Post Number: 755
Registered: 12-2003
| | Posted on Saturday, February 04, 2006 - 9:27 am: |
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When I was working as an Aircraft electrician I learned one thing early on : With electricity anything is possible even if theory tells you otherwise, I found many things on those planes, and seen the same thing on things that did not leave the ground that was not suppose to happen. What I taught myself was to be open to anything odd, and when your telling yourself that a certain component or system cannot cause the problem, Check it anyway!
Wildrat |
