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Author Topic: AM Transmitter with only one Power Supply  (Read 1917 times)
K1YTG
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« on: November 11, 2009, 12:24:55 PM »

I have an AM transmitter from the 40's.  It has a 4-250 in the PA.  It uses BC-610 coils for the tank circuit, inductive loading.  A pair of 100th tubes in the modulator.
They are both fed by the same power supply.  It works Ok but the voltage and current to the final dip with audio.  Also the RF output dips with audio.  It puts out over 400 watts and could be loaded even heavier.
This does not seem right to me.  If I remember correctly, the power output should go up with AM modulation.  I think the transmitter needs a separate power supply for the modulator.  
But being a 40's rig I hesitate to modify it .  It's kind of a museum piece in its 6 foot rack.
I wonder if this ever worked well as built.  It does work.  Should I leave it alone, or work to improve it.

Thanks, Norm
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WD8KDG
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« Reply #1 on: November 11, 2009, 12:51:40 PM »

Considering the time frame, it would not be unusual to see a common power supply for both the RF & Modulator.

As to seeing a power increase with modulation, just depends on how you are measuring power, average or peak?

Check the tube specs and voltages,(B+) and run the rig accordingly.

Above all, connect a scope and watch the waveform pattern, more info here than on the web.

73's
wd8kdg
Craig
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K4DPK
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« Reply #2 on: November 11, 2009, 05:05:39 PM »

What kind of modulation transformer does it use?

Many of us used old plate transformers in auto-transformer configuration for modulation transformers.  We had to make do in those days, and this was a way to get a 4:1 impedance ratio between the modulators and the final stage.  The only drawback was, you had to use a common power supply.

If that's what the builder did, you won't be able to use a separate supply for the modulators.

Phil C. Sr.
k4dpk
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K1YTG
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« Reply #3 on: November 11, 2009, 10:08:17 PM »

It has a Kenyon hefty Modulation Transformer.  Luckily the AM window has it listed and has a chart for the connections.  So I could use a separate power supply.
I will connect a monitor scope and see what it shows.
Thanks, Norm
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WD8KDG
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« Reply #4 on: November 11, 2009, 10:32:51 PM »

Also research the high voltage plate transformer. You will need to know the voltage output and amps it is rated for. From this it should be a little easier to guess the first starting point for loading the RF final, the 4-250A. You still need power for the modulator.

In other words, if you run the final at 300 watts input, the power supply still has to deliver enough power so the modulator will deliver 150 watts of audio for a 100% modulated carrier.

Hope this makes sense.

Craig,
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W8JI
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« Reply #5 on: November 12, 2009, 05:03:42 AM »

Norm,


You may have something other than HV sagging at work.


Plate modulated AM works like this:

The modulator average power adds to the carrier power. It adds at the same general efficiency as the PA. If you have a GOOD PA running 1000 watts input, it takes 500 watts of sine-wave to modulate 100%. If the PA efficiency is 65% you would have 650 watts RF output from dc power and an additional 325 watts from the modulator power. A true averaging power meter would show a 50% increase in power with linear sine wave modulation of 100%.

A peak reading power meter would show 2600 watts PEP.

A true RF ammeter would show a 22.5% increase in current with linear sinewave 100% modulation.

Now here is the important part for you....

A class C tetrode can NOT be linearly modulated with anode modulation alone. This is because the screen grid dominates the gain, so doubling the HV will NOT quadruple the instantaneous envelope power. A tetrode PA does NOT follow the square law required for undistorted AM. It will always have some distortion.

What has to be done in a plate modulated tetrode PA is:

1.) The tube has to be well into class C. This means high grid bias voltage and enough drive to switch the tube off and on hard in class C.

2.) Some modulation MUST be applied to the screen, cathode, control grid, or earlier stages. Normally this is done at the screen in a tetrode.

There are various methods of modulating the screen.

1.) The screen voltage can be dropped from the anode AFTER the modulation transformer. It cannot be regulated or bypassed for audio. This is the least critical method, but still requires some planning or experimenting.

2.) The screen can be allowed to float for audio, and as the anode voltage pulls high screen current is reduced. This allows the screen to go higher in voltage along with the anode. This method is VERY critical for antenna coupling, grid drive and bias,  and does not work well with all tubes.

So my question to you is more about the screen grid and control grid than anything else. To NOT have negative carrier shift, you MUST apply audio to the screen (or some other element) and not just the anode.

The PA stage MUST be well into class C.

How do they modulated the screen, or what is the screen circuit?

Tom
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W8JI
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« Reply #6 on: November 12, 2009, 05:09:58 AM »

http://www.w8ji.com/Heising%20modulation.htm

From my Globe Scout modulation web page above:


Modulation Problems with Tetrode Power Amplifier Stages
To 100% plate modulate any power amplifier RF output must follow the square of the anode voltage. If the resting anode voltage is 1000 volts and the carrier power is 100 watts, the net RF output power must reach 400 watts when the anode is at 2000 volts. Output must reach zero watts when the anode reaches the full negative peak. This only occurs when the plate current tracks the plate voltage change. Doubling the voltage doubles the current, and this quadruples the RF output power. The RF load impedance also stays uniform; voltage and current are changing in the same proportion.  

This condition of perfect AM, with only plate modulation, can only occur in deep class C low-mu triodes or FET's that are neutralized or that have insignificant driver feedthrough power. Such devices will have square law power output as supply voltage is modulated.

Anode current in tetrodes is largely controlled by the screen voltage. While anode voltage can be run up and down, anode current will not proportionally track the changes in anode voltage. A tetrode will not have linear, low distortion modulation unless we also properly modulate one or more of the grids. In other words we must use a combination of grid and plate modulation. (We could also correct the problem by modulating the driver stage along with the PA anode.)

For 100% linear modulation in a tetrode stage, the screen must follow anode modulation in the correct proportion. This can be accomplished by operating the screen from the same modulated source feeding the anode.
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K4DPK
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« Reply #7 on: November 12, 2009, 07:04:23 AM »

Follow the screen back to the power source and see if there is a screen dropping resistor on the way to the modulated HV.  Also, see if there is a filter choke in the screen lead.

Phil C. Sr.
k4dpk
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OLDSWAB
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« Reply #8 on: November 14, 2009, 07:51:09 AM »

Just remember that it might dead key 400 w., but by backing down the power and watching the modulated peak it might also peak at 400 w.For an old rig like that. I don't think that large output was a problem.
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K1YTG
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« Reply #9 on: January 07, 2010, 10:57:07 PM »

I thank all of the contributors for their replies.  I have looked over the old transmitter and it seems to have a big dropping resistor to the screen.  It gets fed from the HV supply, not from the modulation transformer.  I have looked for a choke and do not see one.  There is a clamper circuit with 2 tubes to short the screen voltage to ground if there is no drive.  So a choke could be in there that I missed.  But so far I have not seen a choke.  But I'm not sure I know what kind of choke to look for.  Not an RF choke, or a power supply choke.  What shape would it be?  SO the screen seems to get its voltage from the wrong place.  It makes me wonder if this old transmitter was ever a working unit.   Another telling sign is that the shielded audio wires to the audio gain on the front panel went by a power transformer and it picked up a bad hum there until I re-routed the cable.  That made me wonder how it could have been operated with all that hum.
When I got the unit I thought that I was bringing a good transmitter back to life.  Now I think I have a poorly designed one that needs re-engineering.
I have used a monitor to listen to the signal on AM.  It sounds a little distorted with the highs cut off slightly.  But the audio is very understandable.  Looking at it with a monitor scope the envelope looks pretty good up to about 50% modulation.  After that it shows distortion.  The curve goes up to a peak OK but has a flat line on the downside for 1/3 of the way down.  And the same on the bottom it is a mirror image of the upper curve.  The null at the center Shows a heavy line in the shape of <  . This is just to the right of the node, just where the next positive rise begins.  I do not know how to interpret these lines.
One thing I have been thinking about is The loading of the transmitter. I am running it at about 2200 volts and 290 ma.  I think this is set up should be loaded heavier but I don't want to strain the power supply further.  So perhaps I should change the taps on the modulation transformer to adjust to this load.  Being homebrew I do not know what the design intended or if the parts ever worked together well.  
I thank you for all the ideas, there was a lot of good information in the replies above.
Norm
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W8JI
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« Reply #10 on: January 08, 2010, 11:00:12 AM »

I thank all of the contributors for their replies.  I have looked over the old transmitter and it seems to have a big dropping resistor to the screen.  It gets fed from the HV supply, not from the modulation transformer.  I have looked for a choke and do not see one.  There is a clamper circuit with 2 tubes to short the screen voltage to ground if there is no drive.  So a choke could be in there that I missed.  But so far I have not seen a choke.  But I'm not sure I know what kind of choke to look for.  Not an RF choke, or a power supply choke.  What shape would it be?

In your system you really don't need a choke specifically. A resistance will work fine if the resistance is high enough and if the screen is not bypassed to ground for audio. What you really need on the screen is a high impedance back to the HV, most likely 5,000 ohms or more would be enough, so the screen can "wiggle around" at an audio rate.

A choke is required when the screen dropping resistance can't be made large enough. The choke would need a reactance of several thousand ohms at the lowest audio frequencies.

It would be much better if the screen was fed after the modulation transformer, so the modulation transformer modulates the screen supply voltage along with the plate. There really is a very specific percentage of modulation that must be applied to the screen and plate to make the modulation linear.

Look at what my Globe Scout has and what I did:

http://www.w8ji.com/Heising%20modulation.htm


Quote
SO the screen seems to get its voltage from the wrong place.  It makes me wonder if this old transmitter was ever a working unit.   Another telling sign is that the shielded audio wires to the audio gain on the front panel went by a power transformer and it picked up a bad hum there until I re-routed the cable.  That made me wonder how it could have been operated with all that hum.
When I got the unit I thought that I was bringing a good transmitter back to life.  Now I think I have a poorly designed one that needs re-engineering.

Most old gear is that way. Even some commercial rigs are full of small technical problems.

Quote
I have used a monitor to listen to the signal on AM.  It sounds a little distorted with the highs cut off slightly.  But the audio is very understandable.
 

That's all most people cared about.

Quote
Looking at it with a monitor scope the envelope looks pretty good up to about 50% modulation.  After that it shows distortion.  The curve goes up to a peak OK but has a flat line on the downside for 1/3 of the way down.  And the same on the bottom it is a mirror image of the upper curve.  The null at the center Shows a heavy line in the shape of <  . This is just to the right of the node, just where the next positive rise begins.  I do not know how to interpret these lines.

Too bad you can't post pictures. Even if you could though, there probably is not enough info to tell what the problem is.

Quote
One thing I have been thinking about is The loading of the transmitter. I am running it at about 2200 volts and 290 ma.  I think this is set up should be loaded heavier but I don't want to strain the power supply further.  So perhaps I should change the taps on the modulation transformer to adjust to this load.  Being homebrew I do not know what the design intended or if the parts ever worked together well. 


They almost always modulate BETTER at lower power, not worse. I am starting an AM modulation specific web page. Eventually there will be enough on it to help, but it might be a year to complete.    http://www.w8ji.com/amplitude_modulation.htm

Tom
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