screen modulation again...

[N2DTS]

Had fun on 40 today, running the 2x 4x150 transmitter with the QIX/dx60 modulator at 200 watts carrier and the 3x 4-400 rig with the solid state modulator designed by W2IMX at 300 watts carrier, working W8GPA (Rupe) who was running an AF67 into a big amp, 2400 volts and 400 ma, 960 watts in giving 300 watts carrier out.
I was running 3000 volts at 300 ma, 900 watts in for 300 watts out, about the same.

The 6EW7 did fine in the DX60 modulator.
Also worked WB3FAU (Russ) and WA1HLR.

[N2DTS]

Some progress.

[N2DTS]

Got it done today and tested.




It seems to work fine, looks good on the scope with a sine wave input.
I had to add another zener diode to the string of three in the WA1QIX design as the negative modulation was only going to 70 or 80 percent.

Some fine tuning might be needed, it does not sound quite as clean as the normal QIX design.
Likely the operating point of the 6B4 is slightly off.

[N2DTS]

My wife knocked the modulator over while it was out of the rack and broke a 6B4 tube.
After I replaced it the modulator was all distorted, so I replaced all the tubes with tested good ones and the distortion went away.
I hooked it up to the 3x 4-400 rig and it works fine, and nothing gets hot, the 6B4 takes the current just fine, but the voltage swing out is not enough for full power.
I can only get 1000 watts peak out no matter how much voltage or audio I put into it.
Modulates the pair of 4x150's fine.

It runs best with 700 to 800 volts in, but it also seems to limit the negative modulation to 60%.
The negative peak limiter needs work or removal.
Removal would just be removing the three 12 volt zeners I suppose.

[DMOD]

My wife knocked the modulator over while it was out of the rack and broke a 6B4 tube.
After I replaced it the modulator was all distorted, so I replaced all the tubes with tested good ones and the distortion went away.
I hooked it up to the 3x 4-400 rig and it works fine, and nothing gets hot, the 6B4 takes the current just fine, but the voltage swing out is not enough for full power.
I can only get 1000 watts peak out no matter how much voltage or audio I put into it.
Modulates the pair of 4x150's fine.

It runs best with 700 to 800 volts in, but it also seems to limit the negative modulation to 60%.
The negative peak limiter needs work or removal.
Removal would just be removing the three 12 volt zeners I suppose.

What does your "Present" screen modulated circuit look like and what are the voltages at each stage?

Does your System look something like this?

[N2DTS]

Here is the circuit:
http://amfone.net/Amforum/index.php?topic=26603.0

I just put one stage of a 12au7 in as the 1st stage of the 6de7 and a 6B4 in as the last stage (2nd stage of the 6de7).

That modulator modulates the screens of three 4-400 tubes. No tuner or amplifier.

[DMOD]

It runs best with 700 to 800 volts in, but it also seems to limit the negative modulation to 60%.
The negative peak limiter needs work or removal.
Removal would just be removing the three 12 volt zeners I suppose.

What runs best on 700 or 800V, the modulator or the 4-400's?

You need to be clearer in your responses and we can't follow or help you until you clarify, so help us out here.

Since you don't seem to have circuit drawing or scanning capabilities, take a look at the following two pages of the pdf file.

Does this look like the present circuit you now have???

If not, tell me what is different, especially for page 2.

What are the voltages on the grids, cathodes, etc.

And you are running 100 mA of current (required for 3, 4-400 grids) through a 6B4 tube that is only capable of 60 mA?  

[N2DTS]

The 4-400's are running with 3000 volts on the plates.
The circuit is as you posted, 800 volts into the modulator.
I tried some experiments, added 10K in series with the 39k ohm to ground, no real change, added another two 12 volt zeners to the single one that is supposed to be the negative peak limiter, no change, only 60% negative, 100% positive, about 1000 watts pep with a 300 watt carrier.

I can not really measure voltages with it in use.

The 6B4 does not need to handle 60 ma, since the voltage is only about 150 volts with screen modulation, the current is very light. Might go higher on peaks, but average current is about 10ma I think.

The solid state modulator will modulate the 4-400's very well, well past 100% in both directions.
I just can not get the swing out of the 6B4.

[w4bfs]

if I understand you have 1/2 12au7 driving a 6B4 as a cathode follower which drives the screens of the pa

also if I understand correctly you have provided 800 volts to the 6B4 plate and the 12au7 thru a 100k res

check to see if 1/2 of the 800V is on the plate of the 12au7 when operating and no audio ...this allows max audio swing to be developed.... the 12au7 is a fairly low u tube .... if that voltage is low try a 12at7 .... these voltages are definitely large for all these tubes ... I would consider using something else

[DMOD]

DTS,

It sounds as if the circuit is "flattopping" somewhere, or you may be running out of emission current with the 6B4.

In screen grid modulation, the quiescent (non-modulated DC) voltage on the screen grids of the 4-400's needs to be about 0.4 times 500 volts = 200 volts. The minimum supply current for the 3 screen grids should be 75 to 100 mA.

Also keep in mind that the P-P AC audio voltage on the Cathode of the 6B4 will only be about 0.65 X the  P-P voltage of the 12AU7's plate, since it is a cathode follower.

To find the problem, you need to take voltage measurements and scope those areas mentioned.

[N2DTS]

I will see if I can find a way to measure the voltages.
Its really only the negative modulation that is a problem, the 60% max value.
I was thinking it has something to do with the negative peak limiter built into the modulator but maybe not?

The modulator works ok on the 4x150 rig at about the same voltages, so maybe its a current problem like you guys said.

If I am thinking right, when the 6B4 draws a lot of current, the cathode goes more negative, which is the negative modulation voltage to the screen. When it draws little current, the cathode goes more positive.

I can always sub the 6B4 with a 6dq5 or a 6dq6, good for more current...

[DMOD]

The modulator works ok on the 4x150 rig at about the same voltages, so maybe its a current problem like you guys said.

If I am thinking right, when the 6B4 draws a lot of current, the cathode goes more negative, which is the negative modulation voltage to the screen. When it draws little current, the cathode goes more positive.

Like BFS said, you need to monitor the voltage at the plate of the 12AU7.

But how did it work right on the 4X150 rig if you didn't know what the operating  voltages were?

A cathode follower's cathode simply follows the grid voltage. It does not invert the waveform on the grid.

You set a quiescent DC operating voltage on the 6B4 grid pin 5, which the 250k resistor does. This causes a positive voltage to appear on the cathode, at the transformer center tap via pins 7 and 2.

You then couple a high voltage P-P audio signal to the grid through a DC blocking cap (0.1uF). The cathode voltage then bounces above and below the quiescent DC operating voltage, providing an audio signal to the screen grid.

The P-P audio on the grid is 1.35 X the P-P audio seen on the cathode. Stating it another way, the P-P audio seen on the cathode is 0.65 X the P-P audio seen on the grid.

What WA1QIX's, K4TAX, N1UVI, WA1HLR, W0CAB, KT4LJ, WC3K, and other circuit modifications try to do is make sure the modulated screen voltage goes down to zero volts or negative, so the modulation waveform goes down to M = 0% and - 99% (negative) modulation.

I would recommend you read and study this book before going any further:

https://www.ermag.com/index.cfm?v_link=product_detail&v_key=632

Phil - AC0OB

[N2DTS]

Well, its a working design that I use in another modulator (that works very well).
I replaced the standard 6de7 with a 6ew7 in that modulator and it still works great, so thinking I could just go a bit bigger to handle more voltage and current I would sub a 12au7 for the first part of the 6de7 as its very close to the same specs, and a 6B4 as the output tube.
It works fine but will not go past 60% negative with three 4-400 tubes.

As a test, I replaced the 6au6 plate resistor with a 220K to get more voltage swing with no real change.
Changing the voltage input to the modulator does not change anything but the resting carrier power (static screen voltage).  Its on a variac and can go 0 to 900 volts input.
I measured the bias supply at -130 volts and the preamp tubes run off 400 volts.

[N2DTS]

So what would be a good tube to use as a screen modulator in a cathode follower configuration?

[w4bfs]

811

you might also consider screen driving a tetrode sweep tube as a cathode follower ... tie the control grid to the cathode .. this forms a low u triode  that mayl require some drive current but not much with your high supply voltage... also consider driving both grids tied together as a hi u triode...

 great job for a cheap fet too

love it ... think outside of the box

[KD6VXI]

I was going to mention a sweep tube as well.   Gobs of emission,  and lots of volts!

--Shane
KD6VXI

[N2DTS]

What about distortion though?
I have some 6dq5's and 6dq6's that I used as spares for the G76 when I had it, plus a boat load of 6080's, and all the usual hifi tubes, 6l6, 6550, kt88, etc.
Got some 811's as well.
What would be BEST though, and why?

Voltage and current ratings are light, I might have 700 or 800 volts on the plate, but I have 150 or 200 on the cathode, and while peak currents might equal what you would have in class C plate modulated values, average currents are very low.

[DMOD]

BRETT,

Experimentation is at the heart of ham radio and I applaud your sense of wanting to experiment.

Sometimes throwing pieces of different circuits together may not always work.

It would be interesting to know what your voltages and waveforms were with the 4X150 circuit that allowed them to work.

Another approach may be to use an 813 as the CF in a Triode configuration.

Suggest you use the "characterization" circuit to determine what negative voltage on the grid that gives you about +200V or so on the cathode.

In addition, some modification of the biasing and clipping circuit is in order to attempt to make it stiffer and to work for the higher voltages.

Others may have better ideas for tubes and circuits.

[N2DTS]

Yes, this is the fun part for me.
There are some nice simple circuits in the original 'broadcast audio out of your dx60' thread.
No peak limiter, some do not even have adjustable power output (screen voltage).
I want to adjustable screen voltage setup, should be easy to do, plus the bias supply to get 100% negative modulation.
I need to make a breadboard setup to try different values and voltages, and maybe make the bias supply use a variac.

http://amfone.net/Amforum/index.php?action=dlattach;topic=30451.0;attach=30870;image

That N1UVI circuit looks very simple, just needs the adjustable screen voltage setup.
Then I need to pick a clean strapping tube to use as a cathode follower.
Fun with radio.

[DMOD]

This one shouldn't be too difficult but the choice is yours.

Good luck.

[N2DTS]

As a test, I put a 6080 in place of the 6B4, both triodes.
About the same results as the 6B4.
More fun to follow.

[N2DTS]

Changed it to this circuit, but with a 6B4 as the output tube:

file:///home/chronos/u-ef22acca4e21d388f7e443bc0e09ba923be098dd/Downloads/DX-60%20Modulator%20Schematic%20NG%20For%20AMFONE.pdf

Works on the 4x150 rf deck, but the carrier control does not give enough positive voltage on the screens to get full power and its got some distortion.
Seems to modulate well though.

[w8khk]

Brett, you posted a link that points to your local hard drive file structure.  Perhaps you could post the link from which you downloaded the file so we could review it.  Thanks!   73, Rick

Your link:
file:///home/chronos/u-ef22acca4e21d388f7e443bc0e09ba923be098dd/Downloads/DX-60%20Modulator%20Schematic%20NG%20For%20AMFONE.pdf

Or perhaps you could attach a copy of the actual file.

[w1vtp]

Brett, you posted a link that points to your local hard drive file structure.  Perhaps you could post the link from which you downloaded the file so we could review it.  Thanks!   73, Rick

Your link:
file:///home/chronos/u-ef22acca4e21d388f7e443bc0e09ba923be098dd/Downloads/DX-60%20Modulator%20Schematic%20NG%20For%20AMFONE.pdf

Or perhaps you could attach a copy of the actual file.

Yeah, Brett.  That link will only work on your machine.  How 'bout uploading it here?

Al

[DMOD]

As a test, I put a 6080 in place of the 6B4, both triodes.
About the same results as the 6B4.
More fun to follow.

You mean something like this?

You keep changing things around and never taking measurements of any voltages or waveforms?

Have you thought about just trying to modulate QTY 1 4-400 and see what happens?  

Phil - AC0OB