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Wolverine
| | Posted on Tuesday, April 01, 2003 - 4:37 am: | 
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What are the drawbacks of a variable as opposed to a Hi/Lo switch? I'm told that variables are unstable (SWR's out of whack)especially with the sc2879, and sc 2290 pills. Also I'm told that a variable should only be used in an amp with a driver stage (Texas Star 500 ?) is this true or false?
2) Like the new Palomar 450, is the Boomer 400 a class AB-1 Amp?
3) Is it true that the only transistors that you
can ever "Volt", are the sc2879?, because the sd 1446, 454,455, etc. can't handle 18 or 20 volts?.
4) "Snowcaps Big Copper Coil antenna" are superior to aluminum Big Coil Antennas because of "Copper's" conductivity. True or False?
5) And finally, besides the AB-1, B, and C designations, I'm told that when "Biased", an amp requires less "Drive" to achieve the same watts. Example: An unbiased 4 pill amp, driven with just the radio (4 watts) will do 400 watts peak. A "Biased" 4 pill amp will do possibly 500 watts peak using the same 4 watt drive from the radio. True or False?? I talk to a lot of operators who tend to "Flap" their mouths with a "Know it all" attitude about given subjects such as these. I need expert answers, to separate "Truth from Fiction". Thanks in advance. |
Simon
| | Posted on Tuesday, April 01, 2003 - 10:47 am: |
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Lets start with #5 (easiest to answer)- All amps are biased this is what classifies them as A B AB C amps - the actual point or level they are biased at. Class A current flows for the full 360 degress of the input cycle thus minimum distortion occurs and makes them good for SSB signals. IN Class B currentflows for only half the cycle, AB is somewhere between these two (say 270 degrees). Class C current flows for lessthan half the input cycle.Now each of these classeshave a different efficiency and it is this that generates different ouputs for the same input. Class has the lest distortion butalso the least effeciencywith Class C being the most effecientbut also themost distorted output(cleaned up with filters in a radio). Most SSB radios use AB1 or AB2as a good compromise bewteen distortion and efficiency.
#1 Now having been in the trade for over 25years (including teaching apprentices)I have never seen instability produced by a variable power output circuit thatIS PROPERLY DESIGNED. All unreliable ones (that I have seen) have been badly designedor implemented modifications.just remember most amateur radios(HF ones)have a variable power output system thatworks correctly.The biggestdrawback would be the ease of forgetting to reduce power and over driving the amp. My suggestionis to have a T-pad attenuator permanentlyin line with the amps input thatwill drop your radios output power down to a level the amp can handle.
I cant comment on #3 or #4 as I dont have my spec books available and I have never played withthose antenna but letmeadd that antenna efficiencyis alot more involvede than what the coil is madeof. Such things as size of wire, ground efficiency, where mounted amongst others allplay there part.Iwould be surprised tosee muchdifference from the copper coiled antenna to the aluminium one if both coilsare madefrom the same size wire and are of the overall diameter and if the antennas weretested onthe same mount as near as ppssible to thesame conditions(at least on thesame day).
Now my answers are some what simplifiedand a more detailed answer could be got fromreading such books as ARRL Handbook or the RSGB equivalent amongst many others. |
Tech671
| | Posted on Wednesday, April 02, 2003 - 5:53 am: |
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1. It's not that instability factors in it's that with the 2897 transistor or in a quad or more of most others as you change input drive with the variable the input swr can fluctuate, sometimes to an undesirable level. This is why you don't see variables on amps using 2879s (usually).
2. The Boomer 400 is class C (usually haha)
3. Different transistors have different tolerences. What's more common is the design of the amplifier thay are in. Many production amps use 16v caps and some with biasing, not recommended to exceed 15.8v. ANY amplifier when voltage exceeds 16v the tolerance to reflection becomes much more critical, most common reason for "BOOM".
4. Better because of copper's conductivity... it will dissapate heat more quickly, wether it "works" better or not I haven't tried one. Copper is mighty heavy and mighty soft.
5. True the biased A, AB, B require less drive than that of the C class to excite the transistor unless the amplifier is specifically designed to suppress the input drive. |
Wolverine
| | Posted on Wednesday, April 02, 2003 - 2:42 am: |
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Whoa, slow down simon. I am no tech, by on means. But I do know what the designations class A, B, AB, and C denote modulation efficiencies for Am, and SSb. Even though you so eloquently explained it technically, I think you missed my question. I meant biasing for added power/watts. Now if anyone out their on the forum can answer these difficult questions for me (and maybe simple for you), please answer them in layman's terms that a technical "Neophyte" can understand. Scratch question #4! I'm really interested in the questions( order by preference), #2, #5, #1, and #3. Alsworld, Bruce, where are you when I need you!!! (LOL). |
ChillyDog
| | Posted on Wednesday, April 02, 2003 - 10:34 am: |
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Wolverine,
Actually, Simon provided a beautifully concise and accurate respone to your question. You did ask for "expert answers, to separate "Truth from Fiction"." An expert answer may require a little thinking, along with a willingness to give up preconceptions, on your part.
I don't know what you mean by modulation efficiencies. The terms A, AB1, etc., describe what is called amplifier classes of operation, and, as Simon says, every amplifier is biased into a particular class. When refering to common emitter transistor power amplifiers, the CB crowd misuses the term "biased" to refer to operation above class C. This is because a DC component (a bias) is applied to the base of the transistors. Still, all amps are biased. Part of Simon's answer was an attempt to educate you beyond the "Flap" you've heard.
The differing efficiencies of the classes refer more to power output vs. DC input than to power output related to drive. Further, as you said, the higher classes, A, AB1, require less drive to achieve the same output power. Still, the drive differences are not significant, and you will not see a 25% increase in output as a result of changing amp class.
Unfortunately, the way amps are used (misused) in CB means that during typical CB operation more total power, including spurious and harmonic signals, is generated by class C than other classes. This trash power does not help communication and can cause problems in adjacent channels and out of band, but it looks good on certain meters.
For best overall operation, only use class AB or better. Don't worry about the CB mythology that permeates the airways. Read Simon's answer a few more times; it may take some work to understand it, but it's worth the effort.
Regards,
Bob |
Alsworld
| | Posted on Wednesday, April 02, 2003 - 11:37 am: |
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Wolverine, hey dude this is where I shutup and learn from those much more knowledgeable than me. If I could add something worthwhile I would, but instead I'm learning here on this subject as well.
Good answers to good questions.
Alsworld |
Wolverine
| | Posted on Thursday, April 03, 2003 - 3:30 am: |
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You're right ChillyDog. I got a case of "Brainlock", while reading Simon's dissertation, or should I say presentation of the facts (Too much/Too fast for my brain to assimilate), so I stand corrected. I'm slow, but very, very sure. So, hats off to Simon, and the rest of the forum members who are patiently trying to bring a "Tech" rookie up to speed, using no "Flap", just expert answers. Thanks. |
Wolverine
| | Posted on Thursday, April 03, 2003 - 4:19 am: |
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By the way, I know that X-force are class C amps for maximum power, whereas Magna-force are Class B. Any Recommendations for a good "Name Brand" class AB1 amps? I believe some of "Palomar's" amps might be AB1, but now I'm not quite certain. I'm in the market for one now, and maybe this will solve my "Bleedover" problems (Harmonics) that I'm having with my neighbors. If I can gain a 25% increase in power, so much the better!! |
Tech671
| | Posted on Thursday, April 03, 2003 - 6:08 am: |
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Sorry guys, I can't put it any simpler than I did. |
Kiwikid
| | Posted on Thursday, April 03, 2003 - 7:06 am: |
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Hello Team,
Hey Wolverine,
One answer to question 4:
Identicaly manufactured antennas made out of different metals such as aluminium,copper and stainless steel will,in theory, give different efficiency results as the generated rf is mostly on the surface area of the antenna so the copper antenna will perform the best followed by the aluminium and the stainless one being least effective.
Regards Kiwi Kid |
Triplecguy
| | Posted on Thursday, April 03, 2003 - 11:56 am: |
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Wolverine, FYI, I purchased a Palomar 250 gold about a year ago and it was listed on the website as class C, however when it arrived at my house, it had class AB1 labeled on it. I don't know for sure what it is but it works great on SSB and decent on AM except for hi power, but I think that is because I am over driving it, which makes no sense to me why they would build a linear that takes less than the legal amount of power to drive it correctly. Heck, I don't get a lot of this stuff, but I am trying. With the exception of this website, don't believe anything anybody says about their products unless you have had past experience with that exact product. It is probably best to order from Copper or someone they refer you too. There are many people who found this out the hard way. Good luck and let us know what you find!!!
DAN |
Wolverine
| | Posted on Friday, April 04, 2003 - 3:49 am: |
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Tech 671, your post answered all of my questions to the letter, so don't sweat it (Check out, the post dates, and times). I was refering to Simon's post, it just caught me off guard with the "rush" of fresh info. KiwiKid, that was an interesting post for sure. But as tech 671 stated, copper is mighty heavy, and soft at the same time. Therefore, I won't be cancelling my order for an aluminum Baby 55 antenna (if they ever get them in stock) from the east coast. I've narrowed my choices down to either a magnaforce 600 (depending on amp considerations) or a Texas Star 500 ( I'm worried about the 4 sc2879's and the variable knob). These are my class AB1 choices. When my income tax check arrives, and if I have enough left, I might get an X-Force 200HD (Class c) for back up, since that will probably do as much or more watts peak, than my Boomer 400. |
Tech671
| | Posted on Friday, April 04, 2003 - 5:24 am: |
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The Magna Force amps are class B just under the threshold of AB cold. As they warm slightly the bias voltage creeps up just enough to bring them into AB-1.
I believe Copper has access to what is known as the "Cobra" line of amps. They make a 350 and 450, both having 5 pole filtering added to the design and run in regulated AB biasing. You can call and ask for them. |
ChillyDog
| | Posted on Friday, April 04, 2003 - 12:26 pm: |
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Wolverine,
Tech671's mention of the Cobra 450 reminded me; I was very impressed with one that passed my bench.
The XL 450, as Tech671 noted, are _regulated_ AB1 biased. That means the bias is not just generated by a diode and resistor (as with about every other CB class AB amp) but by circuitry that controls the bias voltage to compensate for temperature and other variations. Very desireable feature!
The filtering Tech671 mentioned is a low pass output filter included within the case of the amp. It helps to reduce TVI and other out of band interference. The fact that the Cobra XL 450 has this filter not only means it will be less likely to cause problems with other communications, it also indicates the extra care put into the design of this amp.
The amp came clean and well assembled. According to my notes, input SWR was 1.3 across the CB frequencies, and the output impedance was good, too. The model I saw had four 2SC2879s and was nicely driven by an older stock 148. The 148 output was: 3.5W carrier, 15W peak in AM; 15W PEP in sideband using two-tone. With that drive the amp put out 68W "dead key" with 270W peak. I didn't try driving it any harder; with four '2879s you could push more watts out, but it was running cool and sounding good as it was.
I am not in favor of using power in CB operation, but I will on occasion help a friend with a power amp install. (If it's going to be done, I'd rather see it done right!) This particular instance I was pleased to be involved, because of the quality of the amp. If all CB amps were this good I might be more accepting of using power on CB!
Regards,
Bob |
Bigbob
| | Posted on Friday, April 04, 2003 - 7:51 pm: |
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I must have one of those rare boomer 400s biased ab1,on ssb my dx2517 drives it well with mike gain at 3:00 peaks 575,avgs.400 and crystal clear from 5mi.to 90mi.and beyond,I'm not power mad,if it smokes,I'll try another one,but so far after talking for a half hour it just get's a little warm,but most of the time 2mins. on 5mins. off is how I use it,just lucky I guess. |
ryan
| | Posted on Saturday, April 05, 2003 - 5:50 pm: |
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also note that variable power has nothing to do with SWR. i wonder who the crackpot was that told you that?
trying to stop the usage of amplifiers is as pointless as trying to stop people from smoking pot. its not gonna happen |
Wolverine
| | Posted on Saturday, April 05, 2003 - 7:06 pm: |
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Well, I finally decided on the new amp that I'm going to get. It's a Palomar 450 AB1 HD. I should be getting an honest 500 watts peak out of the amp, especially since it's rated AB1 + it's a high-drive. This is the last amp I will purchase until the year 2006, since the U.S auto dealerships will convert the electrical systems of most or all vehicles to 48 volt system, or is it 46. Anyway, that may just be another rumor. I've finally quenched my "Mo Power Syndrome" without going through adding batteries, caps, or alternators. When my Baby 55 antenna arrives I'll be in "5th Heaven" Thanks all, for your valuble imput. "Life is Good". |
Sparkomatic
| | Posted on Sunday, December 21, 2003 - 10:03 am: |
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Ryan.......I had a gray 300 that had 4 stages (1)LOW-70 watts (2)med-100 watts (3)High-180 watts and (4)Max-230 watts
On the low and med stage the swr was way high..somewhere around 8-1 but on the high and max the swr was 1.2. On the cheaper mass produced amps this will happen and I've seen it........when the amp is low/swr high ...amp is high/swr low. |
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