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Amplifer Classes for AM




 
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kd6cub
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« on: October 26, 2017, 01:25:13 PM »

I'm relatively new to AM and have been playing with tube transmitters (still learning)

I have some questions about Amps, 1. what class would be best for AM service? AB is what I'm thinking so far but I want to make sure because I've seen posts on Class C, B, AB etc. 2. what would be a good tube to use for wanting to achieve around 250 watts out with 15-45 watt exciter.  

I currently have a few low power AM transmitters that run 6146's or 6DQ6-A's. so, I'm looking for advice, schematics, anything, etc. on which way to go because I'm wanting to home brew an amp so I can learn more. I figure I'll start by building a mono bander then expand from there.
eventually I may try to do the same but solid state instead of tubes.

thanks up front.

KD6CUB
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WD8BIL
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« Reply #1 on: October 26, 2017, 01:54:41 PM »

Hi Sean...... welcome.

Depends on where you will be doing the modulation. For a low-level modulated exciter you'll want the amp to be linear, AB1...AB2 .....

High level plate modulation of the final amp is usually done in Class C.

Other modulation schemes such as PDM etc. go into other classes such as D and E, (checkout the Class E forum!)

So, I'm going to assume you intend on using a lower power modulate exciter driving an amp. You'll want that amp, as stated earlier, linear. Most Commercial amps use AB1 or AB2.

I've been using that kind of setup since AM seduced me 20 years ago!

Buddly WD8BIL
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WD5JKO
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WD5JKO


« Reply #2 on: October 26, 2017, 04:46:05 PM »


This is a wide topic, and depending on the situation, all modes can be used.

Some of the solid state amplifiers run with NO bias on AM (class B), and do just fine. It all depends on drive carrier level, which if high enough diminishes the effect from crossover distortion. The same amplifier with bias class AB used for SSB might not be good for AM unless the bias source maintains regulation in the presence of the carrier excitation.

As far as tubes go, I did some work with what has been called a class BC amplifier. Here is a thread that discusses this topic on another forum.

https://analogforever.net/threads/the-class-bc-linear.76/

Jim
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w1vtp
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« Reply #3 on: October 26, 2017, 06:38:25 PM »

I believe you are talking linear amplifier service. My favorite is a pair of 3-500z's as is used in the Ameritron AL-82.

Al
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kd6cub
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« Reply #4 on: October 27, 2017, 12:04:44 PM »

thanks for all the help so far.

one of my transmitters is Plate modulated and the other is Grid.

so from what I'm reading so far I'm guessing Plate Mod I would want a Class C? and if Grid it would be Class AB. I still need to read up on the Class BC. I've been reading so much already that I have blood shooting out of my eyes. Information overload.

can't wait to retire so I have more time to just play radio.

any more help offered is greatly appreciated, I will start looking for schematics I guess. again, suggestions on a good tube that I can drive with as little as 15-20 watts without harming the tube. I heard that you can "under drive" a tube? and again, only looking for 200-300 watts out.

this will be for AM service only, no SSB at all
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Steve - K4HX
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« Reply #5 on: October 27, 2017, 05:17:27 PM »

Use Class C for grid modulation.
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« Reply #6 on: October 27, 2017, 11:44:25 PM »

I'm relatively new to AM and have been playing with tube transmitters (still learning)

I have some questions about Amps, 1. what class would be best for AM service? AB is what I'm thinking so far but I want to make sure because I've seen posts on Class C, B, AB etc. 2. what would be a good tube to use for wanting to achieve around 250 watts out with 15-45 watt exciter.  

I currently have a few low power AM transmitters that run 6146's or 6DQ6-A's. so, I'm looking for advice, schematics, anything, etc. on which way to go because I'm wanting to home brew an amp so I can learn more. I figure I'll start by building a mono bander then expand from there.
eventually I may try to do the same but solid state instead of tubes.

thanks up front.

KD6CUB

Most lower power transmitters are Screen Grid modulated, such as the DX-60, Knight T-60, Knight T-150, and the Hallicrafters HT-40 run the finals in Class C.

In most plate modulated transmitters, such as the DX-100, Heathkit Apache, and the Viking Series, the finals are always run in Class C as well.

Most Linear Amplifier tubes are Triodes with Grounded Grids. The cathodes are driven by RF from the exciter (low power transmitter or transceiver) and biased for Class AB2.

A good beginner Linear Amp could start with two, 572B's in parallel, with 2250 volts on the plate and biased for -2 volts. AM input power from the transmitter/transceiver can be up to 15 Watts. Power gain is about 9.5 dB.

https://frank.pocnet.net/sheets/164/5/572B.pdf


A good tube for higher power Linear Operation in Class AB2 is two, 3-500Z's in parallel with 3500 volts on the plate and the cathode biased with a 12V 50W zener. AM input power from the transmitter/transceiver can be up to 25 Watts. Power gain is about 13 dB.


https://frank.pocnet.net/sheets/140/3/3-500Z.pdf


http://www.sengpielaudio.com/calculator-amplification.htm


The power supply for AM Linear operation should be well regulated, since it is a continuous carrier situation with both sidebands being transmitted.

Phil - AC0OB
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« Reply #7 on: October 30, 2017, 10:54:53 AM »

Here is a Lew McCoy design using a Tetrode 813 as a Linear Ampifier:

All grids are AC grounded but the Control Grid is the grid DC biased for AB2.

A Fairly simple Linear design and 813s are tough tubes.

Phil

* 813 Linear Amp.pdf (832.07 KB - downloaded 69 times.)
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kd6cub
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« Reply #8 on: October 30, 2017, 11:00:55 AM »

PHil,

wow, great info, thank you very much. I'm finishing up some work on one of my transmitters then I'll start the Amp project.

thank you again.

thanks to everyone who's helped point me in the right direction here.

Sean
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KU8L
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« Reply #9 on: October 31, 2017, 12:40:50 PM »

Seems to me that if you are using an exciter to create the AM RF envelope then the amp stage must be run in a linear class.  THe various classes referred to earlier were refering to the stage that is modulated.  ie, if the exciter is plate modulated it can run class C.  But from that point on, the stages need to be linear.  I too run a pair of 3-500s but have some 813 stuff here also.  the 813 is a very rugged tube and has been around our circles for ever.

Curt
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KD6VXI
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« Reply #10 on: October 31, 2017, 08:34:08 PM »

No, you don't need a 'linear class' of amplification with a carrier.

There is a caveat to this statement tho.

You MUST watch the signal output with a scope.

The carrier will cause a level of self bias.

This is not to say you can use a tube cut off 90 pct of the drive waveform and have this work. BUT, class b amplification, and class BC amplification work just fine to increase efficiency and amplify an am signal.

Where it me, I'd use an adjustable bias.  Better yet, switchable.  Then the amp would be useful for nearly any mode of operation.  CW, ssb, etc.

Pure modulated class c has nothing to do it what's being proposed here.  This is an amp.  Not a transmitter here.

--Shane
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KU8L
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« Reply #11 on: November 01, 2017, 10:08:21 AM »

This is interesting.  I am a student of the Shrader manual and am familiar with the BC - class "Linear".  I suspect not understanding how to monitor these effects is one reason why I hear so many really crappy AM signals on the air--Not necessarily splattering, just real crap audio.

I will maintain that, regardless of what class one uses, the amplifier must be essentially linear in performance.  THe idea of an amp distorting a signal in a way that it improves the output/efficiency without splatter is still distorted.  So it becomes a question of degree---how far can you go toward the edge and still make it sound good.  Clearly (!) being able to properly monitor and interpret samples is a key to this.

Thanks for reminding me of some of that AM practice I had forgotten about. 

Curt
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K1JJ
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« Reply #12 on: November 01, 2017, 11:57:43 AM »

re: Heavily biased into "Class C" amplifiers running linearly on AM...

Some years back I experimented with class C biasing using a pair of GG 4-1000As in linear service.   Simply put, I found that by increasing the bias, thus pushing the operating point of the transfer curve down below class B well into class C, the amplifier could still be made to operate linearly on AM - but with better efficiency, maybe 5-10% or so better IIRC.  This is a lot when compared to running conventional inefficient A, AB or B AM linear. The carrier acts like the biasing and enables the amplifier to operate back into the class B area.  There's a few threads about this effect on this BB. I think Carl / KM1H tried it as well as a few others and reported back similar results.

The more bias, the more carrier drive is needed to bring it back into the area where the cross over point is still intact and conducting. For example, listen to blower noise on an AM carrier and you will see it is heard when the class C operating point is pushed back up into class B with more carrier drive. However, this will NOT occur on ssb and the amplifier idling point (minimum signal / crossover on ssb) will disappear into cut off. This will create severe distortion on ssb. The amplifier needs idle conduction on ssb whereas on AM, the carrier pushes it back into conduction to achieve the same result.

This is one of the few "free lunches" I've ever seen in amplifier work. I run all of my linear amps this way on AM - biased  far negative using a "reasonable" level of extra carrier drive.  I have run careful IMD / spectrum analyzer SDR tests to see if there is any deterioration at all and I can see absolutely no IMD difference from conventional class B biasing to using this class C biasing technique. It just takes more drive to pull off. The efficiency does truly increase by biasing the amp into class C and then pulling it back up to class B using the carrier. The modulated audio then swings cleanly around this new operating point . I suppose the downside is we need more carrier drive than normal to pull it off, so there is the trade-off.

Like always, the DC bias in linear operation MUST be stable with diodes/zeners or whatever - grid leak bias will not work in linear service.

The bottom line is when seeing the same Bird wattmeter carrier output, the tube plates glow darker in the "AM class C" mode.

This technique has been written and talked about for many years before I discovered it on my own by experimenting. I simply switched in the bias I needed using diodes in the CT of the GG amp and then played with the drive to arrive at the same conclusions of the hams before me.

BTW, Don't be fooled by the extra "feedthrough" drive power that appears at the output of the GG linear when increasing drive -  thus making the efficiency look slightly better than it really is.  This needs to be accounted for.

T

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« Reply #13 on: November 01, 2017, 02:32:11 PM »

http://amfone.net/Amforum/index.php?topic=32635.0

Great archive here for Class C vs AB
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« Reply #14 on: November 01, 2017, 03:01:19 PM »

The tube is still seeing class B or better.  the bias is just coming from a combination of the static Bias and the dynamic bias developed by the carrier self-bias?. 
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« Reply #15 on: November 01, 2017, 04:24:41 PM »

While the B/C amp is doable, I would NOT suggest this circuit for the novice Amp builder.

Stick to class AB2.


Quote
No, you don't need a 'linear class' of amplification with a carrier.

...Pure modulated class c has nothing to do it what's being proposed here.  This is an amp.  Not a transmitter here.

Modulated RF from a low power class C final needs Linear amplification if power is to be increased and to keep the RF envelop clean.


Phil - AC0OB
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« Reply #16 on: November 01, 2017, 06:47:42 PM »

http://amfone.net/Amforum/index.php?topic=32635.0

Thanks for finding the old class C thread, Jeff.  Looks like we did go thru it before and a few guys tried it and agreed it seemed to work.

The best way to test this is to make the bias variable by adding a string of switchable diodes to the CT of the GG linear final's fil transformer. Then load the linear up on AM using normally recommended class AB biasing. Throw a carrier for a few minutes and note the plate color.  Then increase the bias so that the amp does not idle - IE, no plate current with no signal drive and is now below class B  into class C.  Then increase drive until the output power is back to the previous level. Look at the final's plate color and see if it is darker. Run some IMD tests to be sure the cleanliness if still intact.

So what is really happening here?  We know for sure the amp is biased into class C because of the bias increase and seeing the idling current with no carrier signal keyed drops below zero.   When we begin to add (carrier only) drive, the tube is still in class C because the output stays at zero out. The tube is spending a part of its time in class C, whereas normally it is forced to stay in class B because of the lesser bias.

As we add drive, the tube comes out of class C due to the carrier bias effect and enters the class B 180 degree/ 0ma idling point. We then fine tune the drive and bias so that the faintest sound can be heard, like a low blower noise. This tells us we are now in the class AB mode for clean audio.

So, is the answer that we are spending more time in class C with the RF carrier cycle swing creating the increased tube efficiency there?

Does the carrier operate like a switch in class C using the pi-network to fill in the cycle - whereas the audio still needs class B operation since it does not have a tuned circuit?  (even though the audio is riding on the carrier)   I'm all ears to why there appears to be more efficiency here while maintaining cleanliness...

BTW, I'll bet the SDR "Pure Signal" guys could have some real fun biasing their linears even heavier on AM and then using Pure signal to make them pristine. Rob / AEX, have you done any more with your linear since your last posting about this subject in the 2012 thread above?

T




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