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Loading an amplifier (transmitter)




 
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Author Topic: Loading an amplifier (transmitter)  (Read 15567 times)
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KX5JT
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John-O-Phonic


« on: May 05, 2009, 09:48:50 AM »

I have a newbie question.  I figure if I have this newbie question, I'm probably not the only newbie that has wondered about this subject.

For the most part, I understand basic electronics.  I know ohm's law.  I have a basic grip on reactance and resonance. 

What I am missing is a good understanding of "loading".   I run a Heathkit DX-60 which has a 6146 in Class C (screen grid modulated with controlled carrier), that feeds a cathode driven Class B linear amplifier, the Heathkit SB-200. 

Why do I load up the final to a certain value (goal is 150ma according to the manual) on the DX-60 and besides the obvious change in output power, what else is really going on?  Why would I choose to load it lighter or heavier?  What are the advantages and disadvantages to different load levels?  My DX-60 manual tells me to adjust the load in 1/4 turn steps, re-dipping the plate current, until the meter reads 150 ma.  Well, I can't make it reach 150ma.  The transmitter outputs more power when loaded lighter.  But, it seems to put out cleaner audio when loaded heavier.  Alas, it will not reach 150ma plate current indicated with 2.5 ma on the grid.  What then is this indicating?  Perhaps a tube is soft?  Even at 50 or 75 ma plate current indicated, she seems to work fine though. 

So any explanations with regards to "loading" will certainly help me and I suspect some other vintage equipment newbies.

John, KX5JT
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k4kyv
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Don
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« Reply #1 on: May 05, 2009, 12:04:04 PM »

There are several factors involved, but one is, with low level modulation, which can be screen grid modulation, control grid modulation or AM linear, you have to modulate the amplifier efficiency along with the plate current.  On the positive modulation peaks the instantaneous power output must be 4 times the resting carrier output, for linear 100% modulation.  Ignoring the controlled carrier aspect of the DX-60 for a moment, the plate current meter should not show any variations under modulation, indicating a steady average plate current.  The instantaneous plate current is varying from 0 to twice the average value, but the analogue meter is too sluggish to show those variations, so it just hangs steady at the average plate current.  That gives twice the output power at positive modulation peaks, but we need four times.  Since we have no external modulator to supply extra power to the plate on positive peaks, we must run the final at lower efficiency when running the unmodulated carrier, and gain that extra peak output power by increasing the instantaneous efficiency over the positive half of the audio cycle.

For correct modulation linearity and 100% modulation capability, the efficiency at the unmodulated carrier level must be set at half the peak efficiency of the final stage.  A class-C amplifier runs at about 70% peak efficiency.  So for grid modulation, we must reduce the efficiency to about 35%, while still allowing for the full 70% efficiency at the  crest of the positive peaks.  The way we do this is to overload the final amplifier, that is, load it up in a manner that the DC input is increased, but the power output drops off.  But with modulation, the grid voltage swing causes the efficiency of the amplifier to increase to twice the resting value.  So the plate current is doubled and the efficiency is doubled at the crest of the modulation peak, allowing the total  power output to quadruple over the unmodulated carrier level.

Therefore, as you tune up, the output will go down as you fully load the final, but with modulation, the audio will not be distorted.  If you load up to maximum power output without modulation, you have already used up all the power output capability of the amplifier for the carrier, and the efficiency is already close to 70%; you have nothing left over to accommodate the positive modulation peaks.  In other words, you have no headroom to allow the positive peaks to come through.  When you try to modulate, the audio will be distorted because you are shaving off the positive modulation peaks.  The average plate  current will actually decrease with modulation under those conditions.

It works exactly the same way with the AM linear, except that, instead of varying the grid voltage (control grid or screen grid) with modulation, you are varying the grid drive  to the final; the result is the same.

With high level plate modulation, the amplifier can be set to run at maximum efficiency at all times, since you are modulating the plate voltage along with the plate current, to achieve the quadrupling of power output.  But with linear amplification or grid modulation, the plate voltage remains unchanged, so the efficiency must be varied along with plate current to maintain modulation linearity.

The DX-60 is a special case; it uses controlled carrier screen grid modulation.  Additional circuitry causes the parameters to shift with the average percentage of modulation, so that instead of transmitting a steady carrier, the carrier output increases with modulation at a syllabic rate.  This allows more peak output power, by allowing the tube to run at lower power when no modulation is present and cool off between voice syllables. Another way of saying it is that the final runs at a lower duty cycle than it does at steady carrier output.  This complicates things slightly, but the modulation principles are still exactly the same as with steady carrier output.


Well, kids, that's to-day's lesson.  Your assignment for to-morrow is...

Damn.  I'm sure glad I'm not doing that any more!


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Don, K4KYV                                       AMI#5
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KX5JT
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John-O-Phonic


« Reply #2 on: May 05, 2009, 01:56:46 PM »

Thank you Don.  This is exactly the discourse I was looking for.  Kudos.

John
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k4kyv
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Don
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« Reply #3 on: May 05, 2009, 04:08:09 PM »

It took me years to figure this out, because none of the handbooks and technical publications I had read, ever gave a full explanation that made sense.
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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KX5JT
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John-O-Phonic


« Reply #4 on: May 05, 2009, 04:41:15 PM »

It took me years to figure this out, because none of the handbooks and technical publications I had read, ever gave a full explanation that made sense.

I certainly looked for a  good explanation before presenting the question.  Well I am very glad I asked.
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« Reply #5 on: May 05, 2009, 05:54:02 PM »

Yes, thanks Don, I too found this helpful.

Rich
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« Reply #6 on: May 05, 2009, 06:13:19 PM »

As per usual, Don, right on target. I keep learning from you on every post. Until now my only consideration for modulation was high-level. Excellent discourse on the details of what happens using low-level mod.

Walt, W2DU
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K1JJ
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« Reply #7 on: May 05, 2009, 07:28:35 PM »

Yep, good summary and concise, Don.

The mark of someone's specific knowledge is knowing the DIFFERENCE between this and that.  That is often the hardest thing to answer... Wink


We should put your page in the technical handbook sticky area.

T
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« Reply #8 on: May 05, 2009, 08:05:50 PM »

Keep those newbie questions coming.

A fellow newbie.

Dave

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KD6VXI
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Making AM GREAT Again!


« Reply #9 on: May 06, 2009, 01:31:34 PM »

Don (and others),

Would you also get darn near the same results by tuning the transmitter for max PEP under modulation?

Seems that using an injected tone (or the one that was discussed months ago here, made on a computer with a specific peak to avg ratio) to load to the max modulated value would produce the best efficiency, at least, fully modulated.

Your explanation made perfect sense, I've always adjusted to max PEP into the load, while ensuring not going over any indicated values...  And then increasing the loading about 5 to 10 percent.

--Shane
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ke7trp
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« Reply #10 on: June 18, 2009, 04:31:25 PM »

Here is the difference for my Globe champion 300.

By the book they tell you 330 MA of loading.

at 330 MA this produces 220 watts of AM Carrier on the Bird 43.  PEP output power is 600 watts at steady tone.

If I retune it for max PEP I can easily get 700 watts pep out. If I turn the Grid drive up to the max of 15 (normaly run 12 book states 10 to 15 is ok) this will make 750 watts PEP.  AM Carrier is 200 watts. at 280 MA.

So:

330 by the book loading =  220 watts carrier and 600 pep.  Backswing to 280 MA when modulated.

Tuning for max PEP power = 200 watts carrier and 750 watts PEP.  280 MA with slight wiggle when modulated.

Interesting.  The only reason I never tried this is that I have been told many times to always load these AM transmitters to what the book says or you could damage the modulation transformer as the voltage will be much higher on voice peaks. 

The Scope looks much nicer and cleaner in the PEP tuning mode.  Which is harder on the tubes and iron?

Clark
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KD6VXI
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« Reply #11 on: June 18, 2009, 07:24:53 PM »


The Scope looks much nicer and cleaner in the PEP tuning mode.  Which is harder on the tubes and iron?

Clark

I'd say (And Don might be able to chime in and point me wrong) that whichever one gives the least stress on the parts is the one to use...

IE, you can calculate which one is dissipating more by looking at input power - output power.  I'd use the one that gave me the least amount of dissipation in the transmitter.  I'd rather my antenna put the RF to work, rather than my tank network & tubes, thank you! Smiley

I've always ballparked a new (to me) transmitter by the book....  Then you can pulse tune it with low duty cycle and see lots more PEP, and as you noted, it usually looks cleaner.

Am I killing the xformer(s) this way?

--Shane
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ke7trp
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« Reply #12 on: June 18, 2009, 07:56:46 PM »

I am going to bet don will say the higher PEP and lower carrier.  IN the end.. The Bird shows more power and the plate current is lower.

Clark
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Steve - WB3HUZ
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« Reply #13 on: June 18, 2009, 09:46:40 PM »

If you are worried about saving the iron, use the tuning method that produces more PEP and then just back off on the audio a little. You may also want to take a look at a scope when trying different tunings. More PEP isn't necessarily better. Clean audio is the name of the game.
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ke7trp
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« Reply #14 on: June 19, 2009, 01:08:04 AM »

Yeah. I am using a scope.  I dont run it up to 100 or past. Just nice and clean.

Clark
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