The AM Forum

The discussion of Quadzilla and also the Slab bacon's PDF of the screen grid modulation setup posted for Gito got me thinking, probably a bad thing.

I drew a block schematic (left), not for use with GG amps, but for a regular plate modulated class C amp, that should raise the G2 voltage from an artificially low value (e.g. 250V) during silence, to the full value (500V or?) when the modulator is making 100%. In-between, the screen voltage varies with the peak level of modulation, having a short attack time and a long decay. So at the first peak, the screen voltage would be full, and stay there, and at silence, it would decay over a 1-second time period. In the diagram the screen modulation method is not shown, but choke modulation could be assumed.

That got me to thinking about another possibility - to modulate the G2 signal directly with the positive parts of audio waveform, and not so much with the negative. The came from the schematic (right side) described below:

I remember a GG amp schematic a guy showed me of an amplifier that rectified part of the RF drive ("detected" it to audio), and applied this 0-500V waveform (as he said) to the screen grid. This increased the upward modulation but not the downward, causing alot of carrier shift, but he was all about watching the other guy's meter swing. Dang CBers anyway. This was said to greatly compress the downward modulation and greatly increase the upward. The tube there was an 813 and a 6AX4 damper was the rectifier, and the transformer is a broadband RF transformer of 1:1:10 turns.

Any thoughts on these whacky ideas?

In the second paragraph, I am not sure anything, perceived "talk power", would really be gained. Maybe this is a bad idea because it would cause alot of distortion, even for voice.

In the first paragraph I was looking only to save the tube a bit in an existing transmitter. There is probably an easier way to do it but maybe not at the high level.

Patrick,

   Look at the following link:

http://amfone.net/AMPX/64.html

Then scroll down to the article on 'Enhanced Modulation'. This is similar to what you are contemplating except being done with a Piss Weak plate modulated rig.

I did this once with a Viking I where the modulator was a huge SS amplifier. The results were impressive! The enhanced carrier (no audio modulation) boosted the carrier up to 200 watts (whistle in Mic brought B+ > 1000v from about 650), and with audio turned on, the 100 watt resting carrier went to 200 watts (sine wave drive), and the PEP watt meter registered 800 watts PEP from a single 4D32..

I still have that rig.

Jim WD5JKO

Actually, ultramodulation is nothing but a form of controlled carrier combined with harmonic distortion of the audio waveform.  The positive carrier shift is achieved by the partial rectification of the audio output from the plate modulator.  The asymmetrical rectification results in even harmonic distortion of the audio waveform.

Not necessarily. Most male voice waveforms are asymmetrical. No harmonic distortion involved to achieve postive peak modulation well in excess of 100 percent.

While that's not distortion is IS the addition of fundamental and harmonics gives the asymmetry. Chop off the low end and the positive peaks go away also.

Ultramodulation, or attenuating the half of the waveform that lies below the zero-percentage baseline using diodes and resistive dividing networks, is not the same thing as achieving expanded positive modulation peaks by taking advantage of the natural asymmetry of the human voice.

For the latter, all you need is a modulator capable of delivering enough undistorted audio power to modulate in excess of 100% positive, and a microphone and speech amplifier capable of faithfully reproducing the audio waveform of your voice.  You just have to phase the audio in the right direction.

This kind of positive peak expansion results in zero carrier shift because the higher amplitude positive peaks are of shorter duration than the lower amplitude peaks on the other side of the base line.  No  rectification has taken place.  The area under the curve is the same, in both the positive and negative directions.

Likewise with ultramodulation, you need a modulator capable of delivering enough undistorted audio power to modulate in excess of 100% positive.  But Ultramodulation works by rectifying some of the audio on one half of the cycle to make one side of baseline higher in amplitude than the other.  This results in even harmonic distortion and positive carrier shift under modulation.

Sure, any non-sinusoidal wave can be decomposed into various arithmetic combinations of sine waves. 

With my voice, most of the asymmetry is present in the midrange. By boosting the 600-1 kHz range, I can increase the asymmetry.

No it doesn't, if the input is already asymmetric (most male voices).

Excellent!

Is there a reason the B+ generating moduator uses a half wave rectifier? Is this actually making a somewhat filtered half wave audio waveform or is it making DC for the 'carrier' portion? I am thinking it is making a sort of pulsating DC in proportion to the audio level at the syllable or lower rate. If so, could a bridge rectifier be used equally well?

^{Is there a reason the B+ generating moduator uses a half wave rectifier? Is this actually making a somewhat filtered half wave audio waveform or is it making DC for the 'carrier' portion? I am thinking it is making a sort of pulsating DC in proportion to the audio level at the syllable or lower rate. If so, could a bridge rectifier be used equally well?}

Patrick,

  In my case I used a variac + a step up transformer (60V -> 240v) for the A2 modulator (carrier booster). I then had a FW bridge and a 100 Mfd cap across the boosted B+. The time constant was pretty long (maybe 1/4 second or so, SWAG), so I also needed to provide progressive negative cycle attenuation (not loading) during the capacitor charge up time. Here is a low level version of what I did on a Central Electronics 20a:

http://pages.prodigy.net/jcandela/CE20AQRO/PNCL.jpg

    Take a variation of this with big fast response 5 KV diodes, and 20 watt power resistors, and that is what I had to do. The negative peak attenuation kicked in when the Mod B+ dropped below 80v, then 40v, then 20v, then 10v with progressively increasing attenuation ratio. The beauty of this is that just by reducing the audio level the circuitry was transparent. Even with hard progressive limiting the "perceived sound" was still good, but extremely loud compared to normal AM. Boosting the average carrier level was the key to keeping the receiver distortion down because most diode detectors don't like 200% upward modulation.

Jim
WD5JKO

My interpretation of Ultramodulation was negative cycle loading and  something to unbalance the carrier to achieve unlimited pos peaks. Any distortion was from the limits of the modulator. I could not find anything about even and uneven harmonics.
I copied a response that might give a little direction and refresh memory banks from the "Slab Bacon"

 He was describing the
> "upside-down tube" high level balanced modulator, which
> can be unbalanced for carrier output by applying DC
> along with the audio.  W3DUQ and I also used similar
> circuits way back when, and K1JJ tried one also.
>
>   Bacon, WA3WDR


Fred

   I am attaching some correspondence I had with Hoisy Hoisington, W4CJL, the founder of SPAM. The letter to Hoisy illustrates what I did to get 200% modulation and 800 watte PEP out of my Viking I transmitter. I later added the boosted B+ to in effect boost the carrier to 200 watts out when sustaining a high level of modulation at 800 watts PEP output.

   One big drawback of AM is that at 100% modulation, the each sideband power is 6 db down from the carrier, and for normal speech the average audio is way down from that. That is why here in Texas, I hear many loud carriers on 3880 from the Northeast, but it is common to only hear low frequency muffled audio coming through at a level equal or below my noise level. This is true sometimes when the carrier is S9 or higher. My attempt back in 1987 - 1996 with my Viking I was to get back some of the disadvantage of AM without sacrificing quality, frequency response, or intelligibility. To a large extent this attempt was successful.

  I still have that Viking I, and most of the modulator parts. It would be fun to give it a go once again. Maybe later on, but not right now. Too many other things on the 'to do' list.

Regards,
Jim
WD5JKO

Yes, I remember Hoisy experimenting with the upside down tube rig.

Back in 1972, Bill DUQ sent me a handdrawn circuit using 6DQ5's in upside down config. He sounded tremendous on it with his 200%+ modulation, so I built one up. It worked FB.  An upside down tube config is nothing more than a class C, plate modulated, TUBE high level balanced modulator.

I later built a 4-1000A  upside down tube(s)  modulated by a pair of 3CX-2500's.  Very nice. The problem was very few guys were set up to receive more than 130% modulation. Bill/DUQ had a tube sync detector and liked what he heard, but the rest complained of diode overload and intermod.  So I went back to conventional 130% max systems.

In this day and age, there's more sync detectors but still not the majority. 

I STILL have the capability to do 200% and more with my FT-102 or FT-1000D using a solid state balanced modulator.  I would save all the trouble and simply use a SS balanced modulator (NE-602) with a big linear for those special occassions you want to go over 130% positive.  Again, when doing 150-200%, most of the time guys will say you sound like double sideband and tough to copy cuz of their diode detectors.

It's also easy to add CONTROLLED CARRIER to the exisiting NE-602 balanced modulator circuit.  (The original question in this thread)

Later -

T

Tom ... befo the Yaz gets yer feet wet, did you prototype the ne602 stuff as discussed on the quadzilla thread ? ...I am  continuing my tests with much modified Viking I/II  ... got the tx making rf but having to rework loading circuit ... the control carrier stuff has passed initial tests and am hoping to fire up the modulator externals soon and see if the system will do as expected ...will let you know ...73 ...John

Hi John,

No, I didn't give it a try yet.  Frank / GFZ sent me a modification to the NE-602 in the FT-102.  HuzMan said he was gonna try it a few weeks ago, but I haven't heard anything yet from him.

I'm not sure I'm gonna bother at this point, cuz the standard fixed carrier with 130% modulation using the balanced modulator works very well as-is.  If my linear amps couldn't take it that would be different, but they can.. :D

I'll let ya know if Huz comes up with anything or I try it myself.

Let me know how your own modified Viking works out with controlled carrier.

T

error - error Wil Robinson!

Sounds like Bill Eitner's (And to an extent, Billy D Williams) SCAM and NPC, to a lessor extent.

I developed a different method of doing the same thing that allows for large shifts positive, but will not hit 0 volts, effectively giving negative peak compression (to use Eitner's phrase).

On the :modulator: amplifier, I did the same thing on the base of a transistor...  It's remeniscent of a G2DAF design, to an extent.  Makes the other guys needle go nuts on RX, increases IMD, but sounds REALLY loud.  Over quite a large bit of spectrum, too!

Hope that helps....  Don't trust any of it, I'm probably wrong.  BUT, my pidgeon meter is uh swangan

--Shane

For the circuit using two audio inputs (http://amfone.net/AMPX/64.html), what is the radio of voltage between the modulator (audio 1) to modulated DC supply (audio 2)? Maybe 1:1?

It looks like the residual voltage on the capacitor provided by the audio 2 input would serve to buck the negative modulation coming from audio 1 input so that the HV can't go below zero plus the 12 volts DC carrier level.

Can we dispense with the second transformer as so:

revisiting this thread and all the thoughtworthy ideas, found this while looking up transformers today:

kind of a brute force method. I like it.