screen modulation again...

[N2DTS]

I can not copy it here, its in the 23 post by DMOD at the bottom.


http://amfone.net/Amforum/index.php?topic=26603.25

Its a simple cathode follower, I did not include the negative cycle clipper/indicator.

Just fooling around the easy way, I need to make a test harness so I can have the modulators out of the rack for measurements.
Here is the transmitter (lower half) with an 811 modulator in it.
 

[N2DTS]

I did a lot of experimenting and could get it to sound very good at about 150 watts out, but not at 200 watts out.
It also had a lot more positive modulation then negative, 90% negative and 150 or 200% positive.

[N2DTS]

I did some tests and clean up of the QIX/dx60 modulator, tests show some problems and very strange results, with a sine wave tone into the modulator, it looks good till about 2000 Hz and above that the carrier level goes up, plate current goes up a lot, very odd.
The solid state modulator does NOT do that.

I like the modulator though, it sounds good, gives a good peak power output, and is a bit more stable in carrier power then the solid state modulator, and it handles reverse screen current better.
That is an issue with 4x150 tubes at high power.
I improved the negative feedback some, and added a bit of a load on the output and it sounds/works very well with voice.

I put aside the higher power 6B4 version and may fool around with series modulation using a mess of 6080's or some such.
Make up some sort of class A series test modulator.
Looking at it, the downside is the need for 2X the plate voltage on the RF tube, not a big deal at lower voltages.
Maybe the 3x 4D32 rf deck would work well at 1700 volts.
300 watts carrier at 850 volts and about 400ma should not be a problem.
How many 6080's would it take?
How much drive power?
Let the fun begin!

[DMOD]

I did some tests and clean up of the QIX/dx60 modulator, tests show some problems and very strange results, with a sine wave tone into the modulator, it looks good till about 2000 Hz and above that the carrier level goes up, plate current goes up a lot, very odd.
The solid state modulator does NOT do that.

Show us your tube circuit and where the voltages are at each point when it took off? We could care less about the SS circuit since you're using tubes.

I like the modulator though, it sounds good, gives a good peak power output, and is a bit more stable in carrier power then the solid state modulator, and it handles reverse screen current better.

How can it sound good if it takes off and the audio is limited?

That is an issue with 4x150 tubes at high power.
I improved the negative feedback some, and added a bit of a load on the output and it sounds/works very well with voice.

I thought you were using the 4-400 in some such configuration?

I put aside the higher power 6B4 version and may fool around with series modulation using a mess of 6080's or some such.
Make up some sort of class A series test modulator.
Looking at it, the downside is the need for 2X the plate voltage on the RF tube, not a big deal at lower voltages.

What about the 813 circuit? You said you had some 813s.

Maybe the 3x 4D32 rf deck would work well at 1700 volts.
300 watts carrier at 850 volts and about 400ma should not be a problem.
How many 6080's would it take?
How much drive power?
Let the fun begin!

Well, we have attempted to help you, but all we have seen is 10 pages of vacillation, and no circuit diagrams, descriptions, or voltage measurements.

Phil - AC0OB

[N2DTS]

I am not really asking for help, just talking radio.
All the stuff is in use except the 6B4 modulator which does not work well enough yet.

The tube modulator I was talking about is the qix/dx60 modulator design.
It works fine and sounds good, does 100% modulation both ways.
I did not say anything 'takes off', if you put a sine wave into it, it looks good on the scope and the transmitter acts normal, but put a 3000 Hz sine wave (or higher) into it and it does odd things, it increases the resting screen voltage, the rf tube plate current goes up from 300 ma to 400 and 500 ma.

Design here:
http://amfone.net/Amforum/index.php?topic=26603.0

I am just posting results of various ways of screen modulating various RF decks.
I find it interesting I can modulate a pair of 4X150 tubes with a slightly modified DX60 modulator.
Two 9 pin tubes modulate a 200 watt carrier just fine.

[N2DTS]

Converted the experimental big tube modulator to the standard dx60/wa1qix design and ran it all day Sunday, worked fine at 200 watts output with the pair of 4X150's.

Thinking of building an RF deck with parts I have around that runs three 4x150's to be screen modulated.
Should run 300 watts carrier and modulate well.
I have all the parts except the vernier drive knobs I use.
They have gotten very expensive.

So I have 4 screen modulators ,all work well, two solid state modulators that are very clean and take no bias voltage, line level input, and two tube modulators of the dx60/qix design that take D104 or line input and have a negative peak limiter built in, all have adjustable power output and meters.
The dx60 modulator does odd things if you put a sine wave into it.
Looks fine at low and mid frequencies, but when anything over about 2000 hz starts shifting the carrier up (increases the screen voltage) but seems to be fine with voice. Odd.

The solid state modulators (design by John, WA2IMX ) is slightly thermally unstable, the power output goes up a bit as it warms up. Not much, about 20 watts maybe.

[w4bfs]

hi Brett ... I may have some vernier drives .... let me know

I looked at his design .... he is using a bootstrap biasing arrangement which is thermally unstable .... addition of two more resistors ( one from the base to output and one in the emitter leg to output ) would accomplish this...

[N2DTS]

That would be interesting if it does not impact the circuit otherwise.
Its only on the 4x150 decks that its an issue because I have to swamp the output to prevent negative screen current runaway. On 4-125/400's its not an issue as they need no swamping.

Got an idea about the values of resistance to try?

[N2DTS]

I did some playing around with the 3x 4-400 rf deck and the solid state modulator at 400 watts carrier out, the 4-400's modulate different from the 4X150 (4cx250b) tubes.

At 3000 volts on the plates of the 4-400's and 700 to 800 volts into the modulator, its hard to get the peak power above about 1300 watts no matter how I adjust things.
I even tried different coils in the pie net, different drive levels, different voltage levels on the 4-400 plates and into the modulator.

The 4x150's like to modulate a lot of peak power, a pair at 1700 to 1800 volts likes to do 200 watts carrier and at least 800 or 900 watts pep.
The screen in those tubes seems to have a LOT of control over the plate current/power output.

Time to play with the audio generator into the 4-400 rig and measure things.
At 400 watts carrier the tubes are showing slight color on the plates.

[DMOD]

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?

Got an idea about the values of resistance to try?

I am not really asking for help, just talking radio.

ok! 

[N2DTS]

Bet you are a lot of fun at parties!

I may experiment at a later date with a big tube screen modulator, I tried the 6B4 and the 6080 and both worked, but I need to eliminate the negative cycle limiter and maybe increase the output voltage swing.

As a test, I put the one I just finished in the three 4-400 rig.
It modulates the rig fine but is not clean, sounds distorted.
It likely can not handle the current that three 4-400's need on peaks.

An 813 will not fit in that modulator, but I could test KT88's, 6DQ5's, 6146's, and other hifi tubes.
A redesign and I could try 811a's or a 4x150 and other larger tubes.
A 4x150 seems very interesting, very low grid drive needed, lots of voltage and current abilities, small package.
Wonder how triode connection would work....

I doubt anyone has ever tried to screen modulate a bunch of 4x150's with one as a cathode follower...
.

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?

Got an idea about the values of resistance to try?

I am not really asking for help, just talking radio.

ok! 

[DMOD]

Bet you are a lot of fun at parties!

Actually, I am pretty boring since I only talk tubes!  

Here is another schematic using a 6EW7 (Triode Unit No. 1) fed by a conventional 12AX7A speech amplifier stage, with a 6B4 audio driver, and a 6DQ5 triode cathode follower.



See the notes on the schematic.


NOTE: Updated with a 6EW7 intermediate amplification stage.


Phil - AC0OB

[N2DTS]

That looks VERY interesting, I will have to print that out.

[N2DTS]

Ran the 4-400 deck with the solid state screen modulator today at 400 watts carrier out, that gives around 1400 watts pep which is just about perfect if you want to be legal.
Tubes showing only slight color and it sounds very clean.
3000 volts, 320/340 ma roughly.

[N2DTS]

I tried this today on one of the screen modulators and it worked fine.
I do have a bunch of capacitance on the input anyway, but it did not bother the way the modulator worked, and is likely a good idea to do anyway.

The entire transmitter has hum on the carrier, 120 cycles I think and I am trying to figure out where its coming from.
Lots of filtering on the high voltage supply, the screen supply and the bias supply.

Brett

Good!

A suggestion:

If there is hum on the B+ (with respect to the screen modulator's ground), then, by far, the largest effect of that hum on the output of the modulator will be from the base of the first transistor. All of the voltage gain is in the 1st stage, and the audio voltage on the base of the first stage's transistor is directly proportional to hum on the B+

To save space, and the avoid the use of the oil filled capacitor...

Add a 10k ohm resistor between R2 and the base of the first transistor (which, to the extent that it makes a difference, will be compensated by using a value of R2 that is 10k ohms less). Add a 10uF (or larger) capacitor between the junction of these two resistors and ground. The voltage from that junction to ground will be less than 100 volts (measure it before adding the capacitor), so you won't need a very high voltage capacitor.

This will significantly reduce the hum at the output of the modulator, without having to use the oil filled capacitor across the entire B+

Stu

[N2DTS]

Started work on a quad of 4-125 tubes in pie net to screen modulate.

The power supply in the old 3x4D32 transmitter gets up to about 1700 volts so I will see how a bunch of 4-125's work at lower voltages.
Should do close to 200 watts out.
No fans or blowers so it will be quiet.

[N2DTS]

Progress so far:

[N2DTS]

Grid circuit done.

[N2DTS]

Got it done and it works.
But the rig I installed it in is lacking in screen voltage to get much power out.
I will have to swap it with the 3x 4-400 rf deck and see how it sings at full voltage.

[N2DTS]

Put the new quad of 4-125 deck in the other transmitter and it does 150 watts carrier and 800 watts pep with no color on the plates, and it seems to sound good.
2500 volts on the plates, about 200 ma plate current.

I also tested it at 200 watts out carrier with color on the plates, works fine there also, but without any fans/blowers I would keep it at 150 watts.

Tunes nice once there is enough voltage on things.
Really seems to make a lot of positive peaks, a KW on a 200 watt carrier.

I should also try plate modulating it with the 100TH mod deck.
I could run the whole mess off one power supply at 2500 volts.
Would do a KW in that setup.

[N2DTS]

To sum it all up:
The solid state modulators designed by John, W2IMX work very well with a very wide range of tubes and power levels.
813's: not much power output, not real clean sounding in the mod monitor.
4D32: modulate ok, not a lot of power out, under 100 watts for three of them.
4x150a/2cx250b type tubes: work well, lots of power out at lower voltages, reverse screen current can be a problem.
4-125: modulate very well, not a huge amount of power output.
4-400: modulate very well, a good amount of power output, a pair will about do the legal limit.

The WA1QIX DX60 type modulator works quite well, even into a pair of 4x150's which are easy to drive tubes.

Once I got used to how things work, I find it easy to adjust things.
I have a mod monitor (shows carrier shift and the modulation) and a peak reading power meter.
I load the transmitter up past the point of maximum power output until the dip gets broad and I get no carrier shift on the mod monitor, and the peak power is 4X carrier.
You can adjust it on both sides of that point, less carrier and more peak power (carrier shifts up on modulation) or more carrier power and less peak power (carrier shifts down somewhat), say another 100 watts carrier and 3X power on peaks. It does not SOUND much different in the mod monitor.

You can change frequency quite a bit without having to re tune anything, but SWR does change things somewhat.
You may have to change any EQ of the audio as there is nothing that colors it in the screen modulators, 20 Hz passes just as well as 20,000 Hz.

Screen modulation seems to be sensitive to hum, all my rigs have some, I even shielded all the AC wires in the quad of 4-125's and still have some hum, there is nothing to stop it from passing through.
The loudest hum I have is 40 db below carrier and does not show on the mod monitor, can be seen on the scope and the SDR display.

You can ADD a screen modulator to almost any rig, and it makes power output adjustment very easy with just a pot.

[w4bfs]

Brett, these results show how much hard work you have put into this worthy approach ... nice going !

[N2DTS]

Its been a lot of fun.
It started with me working a few guys running good sounding screen modulation, and John, W2IMX with his simple screen modulator design.

[N2DTS]

I like this 4-125 deck, I hooked it up to the PPP 100TH modulator deck for a no fan/blower (silent) transmitter and it works nicely.
The 4-125 will work well from about 1500 volts to over 2500 volts.
So will the 100TH.

[N2DTS]

WA2SQQ sent me a very nice recording of most of my transmitters and all the screen modulated rigs have some hum.
Where does it come from?
Its not the audio, I can disconnect the audio from the modulator with no change.
Its not on the screen supply, I can add a lot of capacitance with no change, same with the grid bias.

Is it coming from filament modulation?