screen modulation again...

[N2DTS]

50KW would be ok for 80 meters at night...

[KD6VXI]

50KW would be ok for 80 meters at night...

As a driver...

:-)

--Shane
KD6VXI

[N2DTS]

I received a chimney and got the 4x150 mod deck working again and use it on the 4x4x150 rf deck.
That is four modulated by two, it worked very well.
I think two by two would be a great setup, 1800 volts at 400 ma in, 720 watts input, about 600 watts out, with 600 watts of audio in AB1. About 120 watts plate dissipation out of the 500, run a slow fan.

2400 watts pep so you would need to turn the mic gain down....

With the right amount of air, four would do 350 to 400 watts screen modulated and sound very good.

[N2DTS]

I am working on version two, point to point wiring, higher voltage caps, and as a test, real variable resistors (pots) for adjustments.
A 1 meg pot would be too touchy to adjust and you do not want to be able to go to zero ohms, so I am trying a 100 or 250 K pot with fixed resistors to get in the ballpark and reduce the voltage and power on the pots.
The starting point for the first two pots are 260 k and 280 k, so a fixed resistor of 220K  with a 100k pot should put me in the ballpark as a start.
If the pots can not take the voltage, it will be back to trim pots mounted on a circuit board.

I am going to drop the input isolation transformer and just try a 100uf 450 volt cap.
I am also going to add a bypass switch so full screen voltage can go to the screens for tune up.
I will retain the phase reversal switch on the line level input.

It seems immune but I think I will add some RF bypassing.

Most of the mechanical assembly is done, just the meters, shunts and the  bypass and phase switches need to be done.

[N2DTS]

Second modulator so far:
http://n2dts.smugmug.com/Ham-radio/i-GCDfN4g/A

[AB2EZ]

Brett

Wow!

Very nice work!

Happy New Year!!!

Stu

[N2DTS]

Update:

[N2DTS]

Finished and tested, it works very well.
I had to change one resistor in series with the carrier level set pot to give a bit more output, but that was it.
I just defaulted the other pots to Johns starting point (260 and 280 K ) and did not adjust them, the modulator works great with them set that way.
I ran 600 to 800 volts into the modulator and nothing bad happened, nothing got hot.
I tested the modulator and rig up to 300 watts carrier and 1500 watts pep and it works great.

This deck has much higher voltage rated parts and all the adjustments on the front panel and no input isolation transformer.
I think it goes down to DC.

 

[N2DTS]

Modulator number one redone.
It also works well.

[N2DTS]

Found out something interesting about the screen modulator today.
I was running things up in voltage and had the effect of the screen voltage going high and staying high that I had partly solved by putting a 100K resistor across the output.
Well, I tried various things and found the problem went away if I increased the grid bias voltage.
I just increased the grid leak resistance.
The 4x150 type tubes must generate a lot of reverse screen current or something.

[Opcom]

Those tubes and the '250 type will make negative screen current and must have a well regulated supply. In linear amps, VR tubes are sometimes used as screen regulators on those, instead of just a dropping resistor. Much depends on loading, so it could be slightly different every time tuning is done unless the screen current meter is used for a loading adjustment.

I maybe missed the schematic of the modulator, is it the W2IMX one? - but a heavy swamping load on whatever you have, equal in current to the full screen current, might be useful on a single ended output stage.

Otherwise a totem pole type output stage as for an audio amplifier, like the attachment in some what, just as an example, ought able to sink or source plenty of current while holding a voltage, might be something to use to correct that and really regulate the screen voltage to the audio waveform.

Just going to throw some ideas out on the table as a thought experiment.

An advantage of that kind of totem pole amplifier stage is that the positive and negative sides should be able to have individually defined current limiting depending on resistance values, so the current can limited asymmetrically depending on if it is going into or out from the screen.

In audio amps it is used for the short circuit protection (or when dummys put a 2 Ohm load on the amp) and is always symmetrical. It works by placing a base emitter junction of a small transistor across an output stage emitter resistor, one on the positive side and one on the negative side. When the current through the resistor makes 1.2V the small transistor (Q3 in the circuit) turns on and shunts some of the drive current, limiting the stage to a certain current beyond which it can't go or it will clip.

In the schematic of the audio amp, Q3 and Q6 are the current limiting transistors They sense the voltage across R1 and R2. There are DC coupled circuits like this, and it need not be as elaborate as a hi-fi amp. This is one of those SWTPC "Tiger" class amps from the olden days. Nothing's in stone that a similar could not be run with a +/- supply, but with a sink for current any need for a (-) supply voltage should be smaller than the (+) side, right? I believe R15 and R22 set the output voltage operating point.

Others know more about audio amps than I, maybe someone else can comment on this wild idea.

This current limiting is never supposed to be reached in normal operation but it is a protection for the screen grids in case of incorrect loading which will burn up the screens of those tubes quickly.

What is meant by this asymmetric current limiting is that if you want to limit positive screen current to 50mA and so make R1 a 24 Ohm emitter resistor on the positive output transistor is appropriate, but you wish to limit negative screen current to 15mA, then (depending on the amplifier stage) then making R2 an 80 Ohm emitter resistance would be appropriate on the lower output transistor. - IF I understand this right.

I'm not saying redesign the thing or even do it, just thinking about the screens of 4X150/250/350 tubes and their unusual negative current issues and delicate nature.

[N2DTS]

Very interesting.
But part of the reason I like the circuit John designed is its simple with very few parts.
I am sure there are many circuits that could be made that work even better.

As it is, all I really needed to do was to increase the bias voltage if I was to run things at high voltages, and the 100K resistor on the output is likely a good thing as it equalizes the no load/load aspects of driving a screen which is not a fixed load.

And I know the 4x150 type tubes are not supposed to have rugged screens and grids, but I have never had one fail.
With the screen modulator in use, screen current is VERY low, and the voltage is about 200 volts.
I think the resting current is 10 or 20 ma for the 4 tubes.

I started off with a 0-500 ma meter which did not move, and I changed it to a 150ma meter and it still barely moves at 300 watts carrier output.
The grid is a different story, 15ma or even less on 4 tubes works ok if the grid leak is high enough.
I noticed that after I increased the bias, the rig sounded even cleaner and I have no problem running the plate voltage up to 2000 volts and the modulator input voltage to 900 volts.
That was about 450 watts carrier output.

Doing this with tubes at high power has the issue of running the cathode/filament voltage very high to get the negative swing needed on the screens.

[N2DTS]

What really amazes me about screen modulation is the amount of peak modulation you can get out of a rig running it.
No phase shift I guess, and no sort of power limitation, its easy to get 5x the carrier power out on peaks, say 1000 watts with a 200 watt carrier at 90% negative modulation.
On some rigs I get 2x the carrier power (ten tec Argonaut 5), 3 times (3x4d32 rig with shared supply), and 4x on the big rigs with separate modulator and RF supplies.

How much this matters on the far end I am not sure, but there is no lack of peak power output if you want it.

I think it breaks down like this on 40 meters:

50% run under 75% modulation.
25% run over 100% in both directions.
25% get it about right.

[N2DTS]

The modulator at 30 Hz, quad of 4x150's, 300 watts carrier, homebrew rx IF output:

[N2DTS]

Spectrum at 300 watts carrier, 90% modulation, 400 Hz

[Steve - K4HX]

Looks to be about -35 dBc. That's pretty good!

[N2DTS]

I think its 25, 5db per division.
Looks good on the scope and sounds very clean tho.

[K1JJ]

Hi Brett -

Congrats on getting the screen modulator working! And a nice looking build.

Do you have a triangle you can put through?  See how low you can get it and still look good. Anything below 50 Hz is great.

-25dB 3rd is OK.  It's a good start, OM.  I'll bet you can do better by trying various loading and making sure all the voltages are stiff. Play around with drive levels too.  Look at grid drive and current, screen current and audio form.  Look closely at the audio power coming into the screen. Be sure it is staying true.

Determine what is causing the slight left bending on the peaks of the sinewave. (pictured)  Probably some kind of saturation somewhere. Push it to 120%+ positive modulation and it should be easier to see and troubleshoot.

As Jeff / NBC told me...  refine, refine, refine.  

T

[N2DTS]

I think I got rid of the bending later on, voltage input to the modulator changed that I think, or maybe it was plate voltage.
I have a waveform program in my laptop I can use for a triangle.
I did adjust everything and about 28db was the best I could do at full power, at much lower power levels it gets to 42 db, (at about 100 watts). Might be the way the screen modulates the carrier for this type of tube.

The waveform gets funky looking at 20 Hz, nothing but caps in the modulator, so I wonder what is hosing that up.
Not that I talk at 20 Hz....

[Steve - K4HX]

I'm seeing the carrier at -75 and the 2nd harmonic of the modulation at -107. That's -32 dBc. -25 dBc would not be very good.

I think its 25, 5db per division.
Looks good on the scope and sounds very clean tho.

[N2DTS]

I thought it was the separation between the tone (sideband) and the harmonic, not the carrier.

I'm seeing the carrier at -75 and the 2nd harmonic of the modulation at -107. That's -32 dBc. -25 dBc would not be very good.

I think its 25, 5db per division.
Looks good on the scope and sounds very clean tho.

[Steve - K4HX]

It can be whatever you want it to be. I stated -35 (actually -32) dBc, so it's relative to the carrier. Yours would be relative to the sideband or dBs or something like that. As long as you do comparisons with like values, it's all good.

[N2DTS]

Triangle wave solid state modulator, into quad of 4x150's 80 meters full power output:

[N2DTS]

Saw tooth, then square wave:

[N2DTS]

The qix/dx60 modulator into a pair of 4x150's, 70 watts carrier out on 40 meters:
Triangle and saw tooth.