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Author Topic: class C linear amplifier  (Read 76653 times)
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K1JJ
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« Reply #50 on: January 13, 2013, 07:34:22 PM »


So Tom, Can you explain why in Class AB1 carrier and Class AB2 modulated uses an extra 100 MA of plate current?  I tuned this thing to perfection in both modes.. No matter what I do, linear mode, Uses another 100 MA at 2400 volts DC!!!!

C



Looks like everything is following as we expected, Clark.

Yes, the Ranger will need more power out as the amplifier stage is biased harder into class B.  For example, in pure class A, no power is needed at all, just voltage.  (Well maybe 1 watt to generate voltage across an L/C circuit)


The T-bolt: The reason for 450 ma plate current in class AB to achieve 1 KW output  vs:  350 ma in class "C" to get 1 KW out (actually class B)  is simply the effects of better tube stage efficiency.  ie, Less power input to give the same power out when using a class closer to class B.  This extra current is showing up as more heat and a higher temp reading in your tests.


What is quite appealing is that your temperature in the last test (carrier at 250w)  is the same at 1KW modulated in the second test.  This tells us that the efficiency is actually going up under modulation, just like a conventional linear in class B.

So the next step wud be to try various bias and RF drive settings and compare the input power (I X E) to the output power in the dummy load. Look for the best efficiency along with a reasonable RF drive level and still have good IMD.  There is usually a sweet spot to be found.

Good job, OM.

T
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« Reply #51 on: January 13, 2013, 07:58:53 PM »

Ok. I am going to read some of my old handbooks again to learn what I missed the first time.

This thing runs ice cold now and is really clean!

I think the EBS-1 with a selector pot (they mentiont that as an option) and or a simple switch for AM and SSB would really cool off alot of linears for the AM/CW mode.


Thanks for the lesson!

C
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Steve - K4HX
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« Reply #52 on: January 13, 2013, 08:55:15 PM »

Why would the current be any different under modulation?
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K1JJ
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« Reply #53 on: January 13, 2013, 09:13:43 PM »

Why would the current be any different under modulation?

He's seeing about 50 ma difference if I read the tests A vs: B  correctly.   Maybe carrier shift?


Clark, tell me about your EBS-1.   Since it triggers on 100 mW of RF, the AM carrier must be a problem. Did you use audio triggering like Shane suggested?

It goes in series with the fil CT of the cathode, right? 

What mods were made for AM use?   I was thinking of picking one up from Ameritron and trying it out on the homebrew 8877 amp - with some serious mods.

T
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« Reply #54 on: January 13, 2013, 09:19:43 PM »

There is absolutely no reason why the average plate current would increase with any level of modulation less than 100 percent.
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ke7trp
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« Reply #55 on: January 13, 2013, 09:24:44 PM »

I ran it fully modulated.  300 MA carrier and 350MA modulated.  Maybe the plate meter is off since I have 2400 volts vs 2000 listed in the manual.  It says to calculate the current if the B+ is higher on the meter.  

Does not matter though.  The delta between AB1/2 and ClassC/B is 100 MA.  At the same exact 1000 watts output.  Right on the bird and same Grid on scope.  

I ran the tests again with the tuned input in check. The numbers where the same.  Someone else needs to run tests and report in.

I was like you steve, I thought it would not matter modulated but it surely does.

I think its like Tom said.. In Class C mode when its modulated we are at Class B.  Vs linear mode which is AB1 and well into AB2 when modulated.  So under mod we compare Class B vs AB2 efficency. 

Right Tom?

C
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K1JJ
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« Reply #56 on: January 13, 2013, 09:33:44 PM »

I think its like Tom said.. In Class C mode when its modulated we are at Class B.  Vs linear mode which is AB1 and well into AB2 when modulated.  So under mod we compare Class B vs AB2 efficency.  

Right Tom?

C
I think once the class bias is set by that AM carrier, (and fixed bias combo)  it remains in that class.

I'm not sure if the class of operation itself is changing under modulation, but more the fact that the amp is acting more like an efficiency amplifer during modulation.  It's overall in to out efficiency goes up as the modulation increases.

 I'm still learning too... Grin

T
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« Reply #57 on: January 13, 2013, 09:34:23 PM »

In linear mode A/AB, the plate current should not change at anything less than 100% modulation with sine wave modulation.. Anything else indicates distortion or non-sinusoidal modulation. Plate voltage is irrelevant.
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« Reply #58 on: January 13, 2013, 09:36:10 PM »

It's not even clear to me how it could be more efficient. If the idling current is the same and the average plate current is the same, where is the extra efficiency coming from?
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« Reply #59 on: January 13, 2013, 09:41:31 PM »

??

We're talking about the change in efficiency when the amplifier is biased into a lower class (class C) and then pulled up into class B by the carrier bias effect.  Originally, it was in class AB.  Is this what you mean?

T
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« Reply #60 on: January 13, 2013, 09:48:39 PM »

In AB there should be no change in average plate current with sinusoidal modulation.

In "so called" C, where is the extra efficiency coming from?
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« Reply #61 on: January 13, 2013, 09:53:01 PM »

It does work. I used to bias my Gnational NCL-2000 to give less idling current than I could get away with on ssb. Based on a couple years usage and plenty of on air reports from intelligent folk with panadaptors, I'd give it the green light.

Of course the concept can't be taken too far....

That's great news! 8122 are expensive. What is the idling current on yours?
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« Reply #62 on: January 13, 2013, 09:55:52 PM »

In AB there should be no change in average plate current with sinusoidal modulation.

In "so called" C, where is the extra efficiency coming from?


Heat now being converted into Bird watts - as measured by Clark's class AB vs: C/B   temperature tests?
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« Reply #63 on: January 13, 2013, 10:04:47 PM »


Some are trying to take this 5 steps further referencing CB amplifiers. That is a far cry from what works on AM on the ham bands.

As far as I'm concerned this is not much more than a curiosity.



Not true at all.  I only pointed out that the claims made by some of the mfgs of those amps are always poopoo'ed as impossible, but then on the amateur bands they call the technology something else, and it's WOW, LOOK AT THIS!!!!!

You may think of this as a curiosity, but other people might actually want to learn what, if anything different, is going on.

I think Tom hits it on the head pretty well later, where we may be using our amplifiers in class A during carrier periods, resulting in the poor efficiency.

I've done some experiments with audio derived bias in both transistor and tube circuits.  It DOES work.  It also works with EBS designed to change the class of operation (although you need to set the EBS to act upon modulation peaks, rather than a carrier).  You run pure class C during the carrier, GREATLY increasing efficiency, and during any modulation (as detected at the microphone level, not RF in my circuit), the bias slides up to AB / B.

Saves old unobtanium based tubes a lot of Pdiss averaged over time.

Clark, about 15 years ago.  1998 maybe?  It was near Y2K, I remember that much.  Man, that is close to 20.  Getting old? Smiley


--Shane
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This cutoff bias scheme has been done a lot by experimenter on tube CB amplifiers. A bias control knob was added and the result was talked about as less carrier and more 'swing'. Same PEP maybe, but generally they don't measure that and the meters in use are questionable. On CB it is taken too far e.g. the 4 watt carrier and 120W "swing", but there is less concern for a non-interfering signal among those users.
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K1JJ
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« Reply #64 on: January 13, 2013, 10:12:55 PM »

Just to be clear...

There's actually been two threads going here.  One is about the AM carrier bias effect that supposedly pulls the linear stage up one class, essentially creating a standard linear amplifer, that has been biased to class C  to operate in class B on AM.  Or, a class B stage pulled up into class AB, etc., due to the AM carrier.


The other thread is about EBS - (electronic bias system)   This is a circuit that is triggered by RF or audio and quickly adjusts the amplifier's bias up or down to cause it to idle with less heat when not talking, while returning the bias to normal when talking.

They are two different ideas, but can be closely related.

T
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« Reply #65 on: January 14, 2013, 08:33:01 AM »

It does work. I used to bias my Gnational NCL-2000 to give less idling current than I could get away with on ssb. Based on a couple years usage and plenty of on air reports from intelligent folk with panadaptors, I'd give it the green light.

Of course the concept can't be taken too far....

That's great news! 8122 are expensive. What is the idling current on yours?

Its been 10 years since I unloaded that amp so I'm not sure. Maybe 190 mills. Increase the bias on ssb until it starts to get ratty then switch back to AM Grin
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« Reply #66 on: January 14, 2013, 10:37:51 AM »

Tom, I dont think that its in Class B on Carrier. The reason is that I simply following the Class C tune up procedure for CW mode and johnson clearly states thats class C.

Also.  My 2x4 is biased solely by my FT450 transciever. It has no protective bias.  That is certainly Class C.

I guess the only question is what happens when its modulated? Well, that should be Class B as Robert said.  It makes sense to me.  The grid curent is increased during modulation.

The only thing missing to me is the Tbolts lack of Grid voltage.  If I could tap a choke leading up to my volt meter, I could see the actual - voltage and based on that look up the 4-400A tubes and confirm I am in Class C.

My point is that until you speak and modulate the grids, Its just Class C CW mode


C
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K1JJ
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« Reply #67 on: January 14, 2013, 12:30:45 PM »

Clark,

That would be a desirable thing if it rested with carrier in class C and then shifted to B under modulation. That wud be ideal.  But I'm not sure.

I think what's happening is whatever class the amplifer starts in, it gets shifted upwards a class by the constant carrier. This puts the amplifier into the linear operating point simply by the audio now able to swing around the carrier without cutoff. And by adding bias, we can shift the amplifier back down a class where it started and belongs for best efficiency.

But we can speculate all we want...  Grin  What we need is more guys testing and posting their results. I'd like to see several with 3-500Z GG amps trying a string of diodes in the cathode fil CT lead and running the temprature tests as you did.  It's so easy to try and the returns are great if successful.  

I already have diodes with a switch ready to go in all my amps as well as a digital air exit gauge built in, so I shud run the tests myself.   I already instinctively add more bias for linear AM - just a habit from the past.

If anyhing, we may get a group who now can run their amplifiers cooler with the same cleanliness with longer tube life - or run a little more power with the same temperature.   Or we may get some who say it didn't do anything at all...  whatever it takes.  I'm just a bloke looking for answers...  :-)

T
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« Reply #68 on: January 14, 2013, 12:39:36 PM »

Agreed. We need more people testing and posting results.

I think the carrier is class C.  There is no difference from Class C/ CW mode with ranger and Tbolt when I am not modulating the rig.  When I modulate, the Grid goes positive.

Its simply Class C/CW tuneup... until you talk.   The johnson manual states its class C in CW mode and the amp has super efficiency in that mode.

More tests from others will show us alot!
C
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K1JJ
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« Reply #69 on: January 14, 2013, 12:54:36 PM »

Well, while we're awaiting more test results...

I shud review this, but it might be fun to get some more input first. I understand that a standard linear amplifier is really an "efficiency" amplifier. The bigger the input signal, the higher the amp efficiency and higher output. A linear amplifier's efficiency may shift from 30% at dead carrier to 65% under full modulation.

Now the question that applies to what we are doing here:  Assuming what I said is true, does the amplifier class shift along with the modulation to achieve this changing efficiency or does the class remain constant?  If not, what mechanism is causing the efficiency change in the linear amplifer?


T
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« Reply #70 on: January 14, 2013, 04:27:01 PM »

Running 3-500s in GG for these tests?  Don't forget that input power is passed through and added to power derived from Ep*Ip.
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« Reply #71 on: January 14, 2013, 05:33:00 PM »

Running 3-500s in GG for these tests?  Don't forget that input power is passed through and added to power derived from Ep*Ip.

Yep, dis is true.

Though when the two tests are done (class AB and B/C comparisons)  the same feedthru power will be present both times, so will not be a factor in temperature measurements or in/out power comparisons ...   Though it will certainly be a factor in calculating overall raw, stand-alone efficiency numbers.

Do you have a GG amp you cud try, Rick?


Once I finish up on this 4CX-350FJ project, I will be posting my 8877 results too.


** second thought.... Since the amp will be harder to drive in class B/C, then more feedthru power will show up during this side of the test - and shud be accounted for.   Maybe that's what you were warning about, yes?


T
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« Reply #72 on: January 14, 2013, 06:34:57 PM »

This sounds similar to the bias shift circuits that were around in the late 40's and 50's. Time to dig out the old CQ's, etc. I remember one in particular using a 304TL.

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« Reply #73 on: January 14, 2013, 10:30:07 PM »

Carl, I posted about the 304TL amp and the consensus was not do build it.  But I see now what the designer was thinking.

Tom,  I am worried that on a GG Tride amp, the drive power will have to go way up.  If it acts like this Tbolt, its gonna need more then the 100 watt rig most have.

I cant wait to see your results.
C
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« Reply #74 on: January 14, 2013, 11:31:15 PM »

Tom,  I am worried that on a GG Tride amp, the drive power will have to go way up.  If it acts like this Tbolt, its gonna need more then the 100 watt rig most have.

I cant wait to see your results.
C


Yeah, good point, Clark.   Most riceboxes are at their limit when driving a pair of 3-500Z's anyway. And from what I am learning about building and tuning a clean amp system, it's a good practice to run each amp in the chain at 1/2 power or less to get super IMD results.   My new system chain uses amps that are running 1/4 of rated power, so it will be interesting to see what it does soon.

As far as being hard to drive an amp that has been biased harder for AM use, we can simply add some bias until it starts to hit our driver limits.  If it works, then even SOME additional  bias will be better than none.   Though, if we then need to run the exciter to the limit, I wud opt to keep things as they are with no additional bias so the overall chain runs cleaner.    

That's the advantage of using amplifers in the chain that are overrated for each job. They can exhibit that better IMD without stressing out.

Quick summary of what I've been doing lately:  I just bought an ElectroCraft two-tone generator kit board today. Plan to do some serious IMD testing and will post some pics and IMD results in another thread when the new amp system is complete.  Getting close. I will be using the 3 mW clean output of the FT-1000D driver stage into a lab ZLH-3A amp (100mW from 1 watt amp)  into the homebrew 4CX-350FJ (25 w from a 100w amp)  into the homebrew 8877 (500 w from a 2kW amp), etc.  I'm hoping for -45db 3rd IMD or even prossibly -50dB 3rd results.  The average ricebox is about -31dB third, so I'm shooting for at least a -15 to -20dB cleaner signal. (less splatter)   I've been trying to accomplish this goal for over 20 years now with rather fair to dismal results so far. It's not easy to break thru the -35dB IMD barrier while QRO.   Oughta be sporting... Wink  

T




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