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RCI2990
| | Posted on Monday, May 20, 2002 - 10:42 pm: | 
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I know what class C means and class AB-1 but what the sam he// does class B mean? Are X force base amps suitable for SSB work? I am aiming towards the 60012 high drive for behind a AR 3500 100 watter but i want something that will
sound excellent on SSB... Please no "get a 3-500Z tube amp" or "get a Henry or a Ameritron or Heath" replies wanted. I just want to know what X force means by class B biasing thats all i want to know. TY! |
ss8541
| | Posted on Monday, May 20, 2002 - 11:53 pm: |
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class b means that the trans in the amp are on for -exactly- 50% or 180 degress of the input cycle from the exciter(radio). -anything- less is class c and -anything- more is ab. this is hard to juggle correctly. and if you can get it right, class b push-pull amps(all 11mtr amps are puhs-pull) will cause crossover distortion in ssb systems. that is why -no- true ham hf amps are class b. |
Bullseye
| | Posted on Tuesday, May 21, 2002 - 8:20 pm: |
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So what class of amps would be better for 11 meters? |
Tech671
| | Posted on Tuesday, May 21, 2002 - 8:47 pm: |
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Ok, here we go. Read carefully. The summary is that XForce class B biased amplifiers work and work well for SSB use as well as AM/FM. If youare skeptical then read the entire (long) post.
There are many amplifiers that are SUPPOSED to be class AB regulated, but
the regulation systems don't work properly because they are simply series
regulators.
There are literally DOZENS of brand names using that bias system with the
TO-220 case transistor in the right rear quarter of the amp., with a
potentiometer to adjust the bias voltage blocked by the TO-220 device making it
extremely hard to adjust. Those work fine until you drive them a little harder
than normal; then the bias shifts and actually hurts the amp more than it helps
it. But every one of those amplifiers was SUPPOSED to have class AB biasing in
it, and usually advertised that fact right on the faceplate of the amp.. In one
amp they had that biasing circuit in it, but actually shorted it out by
grounding the input transformer, so the bias circuitry was useless. If you
activated the bias circuitry, the amp ran very hot and became unstable; which
may be why it was shorted out in the first place.
Texas Star used a resistive voltage divider biasing scheme based on the
assumption that the supply voltage would never go above 14VDC, which is why the
Texas Star amps blow up when volted up. The 'fix' for that is to use a
3-terminal 12VDC regulator, like a 7812, on the BIAS supply feeder line switched
thru the relay contacts, then re-adjust the resistor values in the voltage
divider to get the bias correct with 12 volts applied.
In the biasing scheme used in the XFORCE amplifiers, the transistors are
turned on slightly, with about 1/2 amp (or less) per transistor of 'quiescent'
collector current. That means it is actually biased more towards class AB than
class B, but not close enough to class AB to be able to legitimately call it
that.
But in many cases it comes closer to true class AB than the 'so-called' class
AB biasing systems used in amplifiers today; especially when the drive levels
are increased.
RF amplifiers count on a NON-LINEAR transfer curve to accurately re-create the
RF envelope.
The transfer curve has to be perfectly LOGARITHMIC (square law) to have a
perfectly linear RF amplifier.
Take another look at all of the
grounded grid tube amplifiers that can run in class B and still do a fine job of
keeping the harmonic and intermod numbers down.
The 3-400Z, the 3-1000Z, and the 3CX1500A7/8877 Eimac triodes are perfect
examples of 'clean' linear amplification devices capable of running in the class
B mode and still having IMD numbers of -30DB or greater.
(That would be only 100 milliwatts of intermod for 100 watts output.)
(Less power than the FCC requires a license for on the CB band.)
Toshiba does it's factory IMD tests on the 2SC2879 with a bias voltage of .300 to .400 volts applied at the Base Emitter junction (which is shunted with a 10
ohm resistor). The bias voltage is adjusted to obtain a 100 milliampere quiescent collector current through the device.
With that bias voltage applied to a 2SC2879 running single ended, (one-pill),
the measured IMD is -38DB at 60 watts output.
38DB is a power ratio of 10 to the 3.8 power = a power ratio of 6309.57 to 1
That would put it's IMD power output at 60watts/6309.6 = 0.0095 watts = 9.5
milliwatts.
(With a push-pull pair (2-pill) that number would be lower yet.)
So you could theoretically have an 2-pill amplifier producing over 120 watts of
output with less than 15 milliwatts of IMD.
The MOST CRITICAL parametric is the bias voltage.
As far as Toshiba is concerned, the 'perfect' bias voltage for that device is
.300VDC to .400VDC across a base to emitter junction with a 10 ohm shunt resistance.
Toshiba picked the bias level that provided the lowest possible IMD numbers for
that particular device when they did their testing.
Any increase or decrease in bias voltage from the .300VDC to .400VDC level will only make the IMD numbers WORSE.
Measure the bias voltage used in the X-FORCE and see what it is..........
Measure it with no drive, then measure it with medium drive, and then measure
it again with high drive and record your three readings.
(Make sure you use an accurate digital voltmeter that is not affected by the
RF)
Now run the same tests with any other amplifier and see if it comes any closer
to maintaining the 'perfect' voltage under all input drive conditions than the
X-FORCE amplifier does. |
RCI2990
| | Posted on Tuesday, May 21, 2002 - 9:09 pm: |
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Hmmm ..
So i guess a X force base amp isnt for me then... Guess ill look for something esle like a heathkit or something. Problem is you see were i live i do not own the place i live in so hooking up a 220 volt cirkit is out of the question. And most of the big amps like the ameritrons, heaths and dentrons all use 220 volts. So either i find an amp that runs off of 120 that i can drive with my 3500 or i just run a straight station and forget the big amps... |
Tech671
| | Posted on Tuesday, May 21, 2002 - 10:49 pm: |
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2990, why isn't it (XForce) for you? |
Tech671
| | Posted on Wednesday, May 22, 2002 - 5:15 am: |
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>>with just carriers, the class b (or even class c for that matter) amp here will keep the rf rolling by what is known as the flywheel effect from the tuned circuits after the final. but when you add a complexed rf wave like 2tone ssb<<
Allow me to attempt to reiterate....
In the biasing scheme used in the XFORCE amplifiers, the transistors are turned on slightly, with about 1/2 amp (or less) per transistor of 'quiescent' collector current. That means it is actually biased more towards class AB than class B, but not close enough to class AB to be able to legitimately call it that.
This does not mean "Flywheel" or having the transistor "on" only due to carrier or under modulation.
I know from using and hearing the XForce product on the air, on SSB, the clarity is there with no crossover distortion.
So many people have discouraged prospective XForce customers due to telling them "class B doesn't work on SSB" when they have never used nor heard the product.
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Hook948
| | Posted on Wednesday, May 22, 2002 - 7:55 am: |
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Richard, I have a X-Force base amp I'll loan you. I'm interested in the test results also. |
RCI2990
| | Posted on Wednesday, May 22, 2002 - 2:57 pm: |
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This stuff is too far above my head for me to understand so ill leave it all at that and go on.....
Thanks anyways for confusing me guys....
All i wanted was a simple answer to a question if the X force amps were suitable to run behind my 3500 and if they would sound good over the air but i guess thats a longshot...
Bye.. |
Tech671
| | Posted on Wednesday, May 22, 2002 - 9:47 pm: |
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I began my first reply to this thead as follows..."The summary is that XForce class B biased amplifiers work and work well for SSB use as well as AM/FM. If youare skeptical then read the entire (long) post."
If you drive the 600-12 with 50-150w ssb, you should sound loud and clear on SSB. AM of course set your radio power so the amp has an AM carrier of 25-30% of what the amp will peak.
You WILL need a dedicated 30a circuit with 12/2 wire. |
RCI2990
| | Posted on Wednesday, May 22, 2002 - 11:21 pm: |
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Guess i didnt see it...... I make mistakes too you know im only human, 10-04????
I think for now ill stick with my trusty ol' palomar 300a 2 piece. I can drive it on low with my 3500 and get about 750 to 800 out ot it on SSB and the same on AM.... |
ss8541
| | Posted on Wednesday, May 22, 2002 - 11:25 pm: |
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well let me start with the fact that they may sound as tony tiger would say, GGRRRREEEAAATTTT!!!, over the air. -no one- will notice by ear if crossover distortion is present or not. the -only- way you will know it is there, is by observing the waveform on a scope. i have heard class c amps used on ssb that had great audio quality. but that doesn't mean that they worked as an ssb system should.
also, xforce manufactures class c amps for am use. class c is only acceptable for high level modulation(audio added to collectors). only radios are high level modulated, since the mod occurs -inside- the radio . and not all radios are high level. the lincoln/2510 is low level, just like -all- external amps. they run class ab for am and ssb. any low level amp(all external) biased class c are only acceptable for fm or cw work. if it is used for am with input from a radio that has 95-100% neg mod, the neg peaks will actually flatline out of the amp. so here your neg mod is actually at 100% for a definite amount of time(flatlining; overmodulated). hello, splatter city. the only way to fix this is to increase bias voltage(right on in to ab). once again, it may sound good, and show good swing compared to an ab amp, but that isn't how it is supposed to work.
what i am saying is that if xforce is so concerned about imd's and proper operation, then all their amps(for am only or am/ssb) should come standard with what they call class b biasing( or if they have no crossover distortion, what would really be class ab).since they don't, a term called 'marketing ploy', comes to mind. you know, offer something no one else has. like truck maker x has the king cab, and truck maker z has the extended cab. when you narrow it all down, they are the same. here, it looks like b=ab and ab=b.
when i'm setup with the spec an, i'll get with hook(take this as me taking you up on your texas hospitality), and do my tests with a digital cam, for scope, and spec an results for am and ssb.
also the test results for toshiba, may be for 2tone ssb from a single txer, but for most tests like this, it is 2 seperate txers on different freqs (28.000 and 28.001 for this example) run through a combiner(return-loss bridge) into an amp, and then on to the spec an. this will give a better imd value than the single txer with an output of 2 freqs(2 tone ssb). |
ss8541
| | Posted on Wednesday, May 22, 2002 - 11:53 pm: |
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--------------
quote from tehc 671..
..In the biasing scheme used in the XFORCE amplifiers, the transistors are turned on slightly, with about 1/2 amp (or less) per transistor of 'quiescent' collector current. That means it is actually biased more towards class AB than class B, but not close enough to class AB to be able to legitimately call it that.
This does not mean "Flywheel" or having the transistor "on" only due to carrier or under modulation.
-------------------
your right for a push-pull amp, but for a single ended amp or dual parallel amps, even the ab has the flywheel effect(like any single or dual final galaxy or rci radio). the difference is that the ab only has to make up 20% of the waveform compared to 45-50% for class b and even more for class c(which is why they are only truly acceptable for fm or cw since carrier level -always- stays the same, or for high level modulated circuits).
in a push-pull amp, each trans only does half or 180 degrees of the work. since class ab trans are on for more than 50%(more like 75-80%)of the input they will work fine for a push-pull amp. a class b does 50% each. since the trans are not ready to go 'above and beyond the call of duty', you end up with the problems i have mentioned previously.
i hope you haven't take any of this the wrong way 671. i was the 1st batter to take a swing at this, so please don't think i'm bashing 671. throuhout the whole post, i have never said that the xforce will not 'sound' good for him. my problem is if they are really class b, or ab. they may work fine, but if so, then that leads back to your quote about being more toward class ab. in electronics there is no, its kind of this more than that. it either is or it isn't. |
RCI2990
| | Posted on Thursday, May 23, 2002 - 1:59 pm: |
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Ill just hold off on getting one until i can either talk to someone that has one in thier possesion on someone that has dealt with one in actual use on air. Until then ill stick with my trusty 300A! No offense to any of you good guys out there trying to help out but all this tech talk really has me confused more than i wanted to be.... ;-) |
Tech671
| | Posted on Thursday, May 23, 2002 - 9:47 pm: |
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Hook, you said "until i can either talk to someone that has one in thier possesion on someone that has dealt with one in actual use on air". I have used heard and anything else with the class B. I have yet to have any complaints of distortion or lack of clarity.
SS, XForce was originally class C only. The reason for designing a bias system at all was for the SSB users that wanted a product of higher quality than the current crop of amps available. The objective was to keep the highest level of performance AND clean transmit for SSB. The result was the class B biasing.
No one is claiming that XForce class B amps rival the Henry, Ameritron, etc. mobile solid state amps, but that's not to say that they don't.
They do give the consumer an amp that is high performance and clear SSB transmission.
The original post was "I want something that will sound excellent on SSB" and that the class B XForce will do.
You want solid state performance for AM or SSB? You won't go wrong with XForce. |
RCI2990
| | Posted on Thursday, May 23, 2002 - 10:55 pm: |
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Tech671. Those quotes were from my earlier post. I used to own a XF 40012 low drive base amp and i used it sparingly on SSB more on AM but i NEVER once had a bad signal report. I just want a good amp that i can run and not have to hire a electrician to install a new 220 outlet to run an amp. LOL TY for clearing a few things up for me.. |
Hook948
| | Posted on Friday, May 24, 2002 - 12:37 am: |
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671?? I think you have me confused with someone else on the quote I never checked opinions on the X-Force before I bought mine. I'm using it right now with the FT-990 set at 7-8 watts pep SSB. I -DO- like the ease of a solid state amp and a solid state CAN be as clean if not cleaner than ANY tube type amp depending on how it's built. I think the X Force gives alot of power for the $$$,I just don't like the complaints I get sometimes about being heard a couple channels away. I'm interested in seeing what it looks like from a "Third Party",Someone who dosent build and sell there own amps. This way I'll know for sure whats really going on. I am no tech by any means and feel at best I might rank as high as the big dummie class,But I'm thinking the complaints I get may be due to the class B biasing and the unregulated power supply. I could very well be thinking wrong (highly possible)and maybe it's just some folks have nothing better to do than to give me a hard time on the air.(again highly possible)This is -THE- reason I offered to send him my amp. If it checks out fine than GREAT!! If not, Maybe he can give some advice to me about what would make it better and I'll check into those options if any needed. |
Tech671
| | Posted on Friday, May 24, 2002 - 8:11 pm: |
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My mistake Hook, I meant to direct that to 2990. |
Hook948
| | Posted on Saturday, May 25, 2002 - 1:01 am: |
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Hey No problem 671 I also wanted to say that it's a very good chance that the folks that were doing the complaining may have had a really poor/wide reciever also. |
ChillyDog
| | Posted on Saturday, May 25, 2002 - 10:11 am: |
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A couple of quick comments:
Using two RF signals at 28.000MHz and 28.001MHz _is_ two tone SSB, and is the prefered way of testing an amp. Feeding two audio tones into a SSB radio to drive an amp will result in exactly that type RF output. For example, with a radio set at 27.995MHz USB and audio tones of 500Hz and 1500Hz, the RF radio output feeding the amp will be 28.000MHz and 28.001MHz. The difference is the radio will also generate it's own intermod products, which will also be introduced into the amp and affect it's perceived output. Using two RF tones as amp input will eliminate radio induced intermod.
My second point is in support of Hook's concern. It doesn't matter what sort of biasing used when getting over-the-air reports _on_ channel. Just about any class bias amp _can_ sound good on frequency; it's in the amount of splatter generated where the differences in biasing come into play. Bottom line is this: the more linear the amp, the less off channel interference. Class A is most linear, class AB is a reasonable second when the output is filtered, and I won't go to B or less regardless of my mode of operation.
Best wishes to all!
Bob |
RCI2990
| | Posted on Saturday, May 25, 2002 - 11:52 am: |
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Those guys probably have a RXer that is as wide as a barn door!! Well i found a heathkit SB 220 for a very reasonable price near were i live that a guy wants to sell. He told me he has gone throuygh the amp 100% to make sure it all works ( ive also known the guy personally for about 7 years and can take his word) so more than likely ill go that route since i cant afford a 800.00 amp rignt at this point. Im sure someone will pop on there and say some thing bad about the 220 but hey to each his own! |
RCI2990
| | Posted on Saturday, May 25, 2002 - 2:03 pm: |
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Bummer. found out the 220 does NOT come with the cover... :-( Guess that was not meant to be! LOL |
ss8541
| | Posted on Monday, May 27, 2002 - 9:36 pm: |
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hey chillidog, i found some time to pull out the text books and re-check what you said. as always, you are 100% correct. as a young tech, my mind hasn't yet 'formatted' all the info it has 'absorbed'. but just give me a few more yrs. between my work in cb and lmr, my knowledge grows at a rapid rate. i did remember that 2 txers were used due to no self induced imd's. that is why i made the statement about 2 txers cleaner than one ssb txer with 2 tones.
anyway, thanks for stepping in and correcting my small but yet large mistake. i have always respected your knowledge and yrs of experience. i don't want or intend to mislead anyone. atleast you (rf engineer) agree with me on the use of class b for ssb, or am for that matter. that mistake may have made some other factors change but the whole fact of the post did not change. |
ChillyDog
| | Posted on Wednesday, May 29, 2002 - 2:57 pm: |
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ss8541,
Thanks for your kind and generous comments.
And don't underestimate your knowledge; you are very far advanced along the right track and doing a lot to help people. Have fun with your new spectrum analyzer; that is the best tool for RF work.
Take care, my friend,
Bob |
Northstar
| | Posted on Thursday, May 30, 2002 - 2:38 pm: |
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You will find that the Xforce Class B amps will show a spike on the 2 meter band. |
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