Screen grid modulation questions?

[VE3GZB]

Hi Geo

Looking at the schematic I Attach.it looks complete to me,but i"ll attached another one

Gito

Thanks, this one is better, full parts list, coil listing too!

73s
geo

[Gito]

Globe Scout 680 schematic diagram ,still used 6146 as final transmitter tube and 6L6 as modulator tube

[The Slab Bacon]

the Glob Scrote makes a nice exciter for a leanyour amp.  The modulator circuit is full choke coupled Heising. Keep in mind, they only put out around 10-12w in phone mode.

[WD5JKO]

Geo,

     Mr. Bacon correctly pointed out that this is Heising modulation, a form of plate modulation. I find it interesting that the schematics that Gito posted show NO screen modulation to the 6146 except for at the higher audio frequencies where a capacitive voltage divider from the plate to the screen is present. The screen dropping resistor to the 6146 (12K'ish) might be high enough so that the screen self modulates like a big Tetrode that has a screen choke present in series with the screen supply. I find the Globe Scout circuitry interesting.

    Keep in mind that Heising modulation without "tweaks" is incapable of more than about 80% modulation. The big limitation is the downward swing of the class A modulator tube cannot go to zero volts, and will likely not go much lower than 150-200 volts. Depending on your B+, your modulation percentage limit may not exceed 50%. All that said, a casual look at AM operators on the bands today, 50% modulation or lower is not uncommon. So you will have good company. 

    The modulator tube is running class A with cathode bias. A little negative feedback to a prior stage will straighten out a lot of the asymmetry that will be seen at the modulator output as you approach maximum modulation.

     The biggest obstacle I've experienced with commercial versions of Heising is that the Heising choke is undersized with an air gap to raise the saturation current point. That choke must bear the 6146 plate current (abt 100ma), and the modulator plate current (abt 50 ma). With a gapped-core choke at 8H @ 150ma, the reactance of the choke is pretty low at audio frequencies below 500 hz, so the audio modulator must restrict the low frequencies quite a bit. If you don't restrict the lows, then that 8H choke will load the 6L6 such that severe distortion will be observed. You could replace that choke with a 15 or 20H smoothing choke, and things would behave MUCH better. That choke would be pretty big though, and very heavy.

    I figured a way around this Heising choke size limitation with my Gonset G50 (search my posts here on AM FORUM) where you can use a center tapped choke (or the primary of a push pull transformer). Here the B+ goes to the CT, and one end goes to the modulator plate, and the other end goes to the RF stage modulated B+.  Now something magic happens. The RF PA current subtracts from the modulator current such that the choke magnetizing currents buck each other. In my case I use two 6L6's in parallel in the modulator that pull 100ma from the B+. The 6146 also pulls 100ma, and 100-100=0. This means that there is NO net magnetizing current in the choke. This also means that the choke does not need to be gapped, will have much higher inductance, will be much smaller and lighter. When I made this change I could for the first time modulate cleanly down to 50hz.

   My write up, circuits, pictures, scope traces, etc. are posted. Just search for them here.

Good Luck,
Jim
WD5JKO

[VE3GZB]

Thanks! Sounds like a good approach to Heising!

[Gito]

Hi Geo

Yes it's Heising modulation,in the old books it's called Choke coupled modulator ,that says The audio output power of the modulator combined with the DC power in the plate circuit,is just as in the case of the transformer coupled modulator.how ever,there's considerably less freedom in adjustment,since no transformer is available for matching impedance(the transmitter tube and modulator tube).

 I attach the schematic drawing at it was.(Complete schematic diagram....covering the Globe Scout 65)

Yes Jim the choke must have large inductance, so when there's current  during positive going modulator excursions.the choke collapsing magnetic field pushes voltage above B+ rail.It is like a flyback transformer.
The Choke is the important part,it must have enough inductance,which must have the lowest flux leakage (no air gap).

In The ARRL handbook ,A complication is the fact that the plate voltage on the the modulator must be higher than the plate voltage on the RF amplifier,for 100% modulation.
There is an article  about a" modification  in the globe scout 65 in WRL word radio labratories constant current."
Just googling /searching in the internet  on   Heising Modulation.

In this article Heising modulation also known as Constant Current modulation.
In this article it used a resistor(450 Ohm in parallel  with a 10 ufd capasistor) drop the plate voltage of the RF amplifier(after the choke) /lower than plate voltage of the modulator tube ,moving the screen supply from the B+ to the plate voltage of the RF amplifier(that has been drop) ,so it's also modulated when the RF plate is modulated.
In this article the Author wrote ,he can get 18 to 20 watts carrier output in class C operation,and in the modified system allows 100% negative peak an nearly 100% positive peak

Gito

[VE3GZB]

I made this 1 watt rig, Heising modulated, some months ago. I made no contacts of course with only 1 watt AM.

But what I found was that I could keep the plate voltages the same, only I had to make sure the modulator tube (far right) could sink at least double the current of the RF tube (bottom).

In this example, the RF tube can sink about 25mA and the Modulator tube can sink about 45mA. I can get up to and over 100% modulation on this tiny rig (but still make no contacts).

I'll keep it assembled if only as an experimental plaything.

[Gito]

Hi Geo

A nice looking transmitter you've made

Back to Heising modulation.
1. The ARRL books wrote to have the same impedance of the Mod tube and the impedance of the RF tube
2.It wrote the voltage developed by the modulator cannot swing to zero without a great deal of distortion.
3,Geo wrote he can get 100% with his 1 watt transmitter.
4.Jim used a center tap power amplifier (choke) to minimize magnetizing current.

Reading it ,in my opinion when Jim used a center tap Choke it's not Heising modulation anymore but plate modulation using a 1 t0 1 ratio modulation transformer.

So thinking of that, Heizing is a plate modulated transmitter using 1 to 1 ratio ,the same winding is used as the impedance of the RF tube and also the impedance of the Modulator tube (2 in 1)

Why it has to have the same Impedance  of the RF tube and Mod. Tube.

 As a normal plate modulated transmitter using a Modulator transformer ,the impedance of the modulator and the impedance of the Rf tube and Mod tube is very important.

A mod amplifier has a 3 k ohm impedance modulating a 6 k ohm RF amplifier has a winding turn is the root of 6:3 that is about 1,5 turn ratio.
So too fully modulate this transmitter  we must have the right turn ratio of the Modulator transformer,and also enough wattage Modulator.

In my opinion this is also implemented in Hezing modulation.
A class A modulator must have a certain impedance load,  Say it's 3 K ohm
and like above example the load impedance of the transmitter is 6 k ohm.

The modulator can not 100% mod the transmitter since it need a "step up" Transformer and in the "modulation transformer"(choke)  it's 1 to 1 ratio.
Imagine it as 2 different winding in parallel (actually it is only one winding).
So the Modulator can not supply the peak voltage that is needed for 100% modulation.
So we must first know the impedance of our Class A modulator and has enough audio power to modulate the transmitter
Than find a Transmitter tube loaded to the impedance value of the modulator tube.
So we have 1 to 1 impedance ratio and with enough audio power,hopefully we can drive 100% modulation .

Maybe I'm wrong

Gito

[WD5JKO]

Gito,

  I like your analysis, and I think you are correct. With class A Heising, getting the largest undistorted swing out of the modulator is very important, and in most cases your limited to about 80% modulation.

  Some tweaks to Heising to get more modulation. Some can be stand alone or in combination:

1.) Drop the modulated B+ with a resistor, so the RF tube sees 10-20% less B+ than the modulator tube. Then bypass that resistor with a non polar cap so it is effectively bypassed for audio. The B+ is dropped, but the audio peak level is not dropped.

2.) Changed the modulator cathode bias to fixed bias. This puts the cathode at ground and increases the modulator plate to cathode voltage.

3.) Use a beam Power tube as the class A modulator tube. The plate can swing much lower than the screen voltage when compared to a pentode. A sweep tube like a 6DQ5 might be ideal here so long as we combine with item 3 below.

3.) As the plate swings low, compression becomes increasingly apparent, giving rise to 2nd HD. Use Negative Feedback to counter this.

4.) Use a low impedance audio driver so that the class A output can be run class A2 such that we can flow some G1 current to help get more plate swing in the downward direction.

5.) Return the modulator cathode to a low impedance - supply, such as -100 volts. This will increase the modulator plate to cathode voltage such that we can now swing more peak audio, and hit a higher modulation percentage. Getting the bias right will be more tricky though.

6.) Provide a sliding bias technique to shift the modulator bias point higher during high audio levels. Use this with items 3, 4 above.

7.) Use an oversized choke (20 H or more), or use the CT choke concept (oh, Gito says that is not Heising.. )

Make any sense?
Jim
WD5JKO
I know, some readers might say, "switch the darn thing over to P-P plate modulation". Yea but that is no fun.

Regards,
Jim
WD5JKO

[Gito]

 Hi Jim

Your analysis makes a lot of sense to me.
But Jim ,not all of Us has the Skill and knowledge like You  to Implant it on Our Transmitter.
I agree with all that You have wrote.
But one thing changing the bias ,does not it change the operating class? and changing the Impedance of Modulator Tube?Because in Heising "actually" is Plate modulation with a "modulator transformer" with 1 to 1 Ratio

Regards

Gito

[DMOD]

This is an old thread but I was looking at some of the theories put forth here on screen modulation.

Recently in Electric Radio there was an article on a dual final CW rig using 4D32s.

Sooo, I though maybe something like this might work, IE taking the ER article and modulating the screen grids.

This is only on paper so far and I haven't implement it. Some tweeking of component values may have to done.

In June 2014 issue Page 6, the screen grid voltage is regulated by a high voltage bipolar transistor.

What this circuit does is further solid state some circuits in order to modulate the screen grid.

NMOS M2 Modulates the voltage at M3's gate. M3's source provides the varying audio voltage to the screens. R13 limits the DC screen current to about 70mA max.

R8 sets the bias on M3's gate, and via DC and AC feedback from the source, tends to stabilize the bias and reduce distortion.

M1 replaces the 2E26 for screen grid protection in case of RF drive loss.

D23 insures he screen voltage never tops 350 volts.


Anyway, something to think about for next winter's projects.

Phil - AC0OB

[DMOD]

This is an old thread but I was looking at some of the theories put forth here on screen modulation.

Recently in Electric Radio there was an article on a dual final CW rig using 4D32s.

Sooo, I though maybe something like this might work, IE taking the ER article and modulating the screen grids.

This is only on paper so far and I haven't implement it. Some tweeking of component values may have to done.

In June 2014 issue Page 6, the screen grid voltage is regulated by a high voltage bipolar transistor.

What this circuit does is further solid state some circuits in order to modulate the screen grid.

NMOS M2 Modulates the voltage at M3's gate. M3's source provides the varying audio voltage to the screens. R13 limits the DC screen current to about 70mA max.

R8 sets the bias on M3's gate, and via DC and AC feedback from the source, tends to stabilize the bias and reduce distortion.

M1 replaces the 2E26 for screen grid protection in case of RF drive loss.

D23 insures the screen voltage never tops 350 volts.


Anyway, something to think about for next winter's projects.

Phil - AC0OB

[AB2EZ]

Phil

High voltage FETs often have a very high gate-to-source capacitance (around 3000pF), and therefore a fairly low gate-to-source impedance at audio frequencies of a few kHz.

The source impedance of the circuit driving the gate of M3 (47k ohms) may be too high to produce a flat screen modulation v. frequency response.

Stu

This is an old thread but I was looking at some of the theories put forth here on screen modulation.

Recently in Electric Radio there was an article on a dual final CW rig using 4D32s.

Sooo, I though maybe something like this might work, IE taking the ER article and modulating the screen grids.

This is only on paper so far and I haven't implement it. Some tweeking of component values may have to done.

In June 2014 issue Page 6, the screen grid voltage is regulated by a high voltage bipolar transistor.

What this circuit does is further solid state some circuits in order to modulate the screen grid.

NMOS M2 Modulates the voltage at M3's gate. M3's source provides the varying audio voltage to the screens. R13 limits the DC screen current to about 70mA max.

R8 sets the bias on M3's gate, and via DC and AC feedback from the source, tends to stabilize the bias and reduce distortion.

M1 replaces the 2E26 for screen grid protection in case of RF drive loss.

D23 insures he screen voltage never tops 350 volts.


Anyway, something to think about for next winter's projects.

Phil - AC0OB

[WD5JKO]

Phil

High voltage FETs often have a very high gate-to-source capacitance (around 3000pF), and therefore a fairly low gate-to-source impedance at audio frequencies of a few kHz.

The source impedance of the circuit driving the gate of M3 (47k ohms) may be too high to produce a flat screen modulation v. frequency response.

Stu

   It appears to me that m3 is in a source follower or common drain configuration. Wouldn't the ciss input capacitance of that FET be negated as the gain of the fet approaches 1?

   Looking at the circuit makes me want to build it. I wonder if D1 or D3 might clip on audio peaks depending on audio drive level and pot R8 setting?

Good Stuff.
Jim
Wd5JKO

[AB2EZ]

Jim

You are correct.

Since the AC part of the gate-to-source voltage will be only a tiny fraction of the AC part of the gate-to-ground voltage... the gate-to-source capacitance will not load down (i.e. draw excessive current from) the circuit that is driving the gate of the source follower.

Thanks for pointing this out.

Stu

Phil

High voltage FETs often have a very high gate-to-source capacitance (around 3000pF), and therefore a fairly low gate-to-source impedance at audio frequencies of a few kHz.

The source impedance of the circuit driving the gate of M3 (47k ohms) may be too high to produce a flat screen modulation v. frequency response.

Stu

   It appears to me that m3 is in a source follower or common drain configuration. Wouldn't the ciss input capacitance of that FET be negated as the gain of the fet approaches 1?

   Looking at the circuit makes me want to build it. I wonder if D1 or D3 might clip on audio peaks depending on audio drive level and pot R8 setting?

Good Stuff.
Jim
Wd5JKO

[DMOD]

It appears to me that m3 is in a source follower or common drain configuration. Wouldn't the ciss input capacitance of that FET be negated as the gain of the fet approaches 1?

Looking at the circuit makes me want to build it. I wonder if D1 or D3 might clip on audio peaks depending on audio drive level and pot R8 setting?

Good Stuff.
Jim
Wd5JKO

The M3 fet is a source follower configuration. We are not trying to saturate it with a fast rising square wave, but rather allow it to operate in it's linear region with a relatively slow rising and falling complex audio wave. For fet gates, this voltage represents a voltage range of approx. 1.0V to 4V gate-to-source voltage. I have tested these high voltage fets and found that to be the case.

D1 keeps the M2 fet from saturating, and D2,3 keeps the screen voltage from rising to severe levels.

I would expect the bias to be set so the quiescent voltage level on the screen is about 250 volts or thereabouts. Of course, R8 also sets the power level.

I think I will add an audio level control to allow the input audio level to be attenuated.

Again, a work in progress, but I appreciate the comments.

Phil - AC0OB