811 P-P Z

[K9ACT]

I am building up an 811A modulator for my 811 project but have trouble determining the P-P Impedence so I can set up the mod iron.

All the numbers I see are for much higher voltage or audio power.

The RF 811 will run at about 100W out on the same 1Kv power supply as the mods.

The HV trans is rated at at 250 ma so I am limited to the 100 or so left from the RF deck.

So in general terms I have 1000v at 100 ma peak on the mods.  How do I determine the Z?

js

[WD8BIL]

Push Pull AF Power Amplifier & Modulator - Class B (ICAS)
Plate Voltage .......................... 1000 V
Grid No. 1 Voltage ........................ 0V
Peak Grid No. 1 Voltage............... 185V
Plate Current (Zero Signal) ............ 44mA
Plate Current (Maximum Signal) ...... 350 mA
Driving Power .............................. 7.5 W
Load Resistance .......................... 7.4K ohms
Power Output (approx) .................. 248 W

811A data

[K9ACT]

Plate Current (Maximum Signal) ...... 350 mA
Driving Power .............................. 7.5 W
Load Resistance .......................... 7.4K ohms
Power Output (approx) .................. 248 W


Roger but isn't the load resistance a function of plate current and voltage and power out.  I can't make any of these number come out to 7.4k.

As mentioned, I only need about 50W and so the plate current will only be on the order of 125 ma.

js

[WBear2GCR]

The trick is that you have to look at the peak plate current and plate voltage combo. Ideally, you run the loadline and that tells you the impedance.

But in genearal,  little higher is easier to get "right".

I would say 8-9k range for 811a running ~1kv. as a "rule of thumb".

Assume you have headroom available from the modulator, AND that you will use it for a rig that runs more than 50w at some time?? 

                               _-_-bear

[WD8BIL]

John...... every data sheet I've been able find today on the web puts the 1000VDC load Z at 7 - 7.5K ohms.

Perhaps someone has the time to put the proceedure here.

[w5omr]

As mentioned, I only need about 50W and so the plate current will only be on the order of 125 ma.

No, that's incorrect.  You need as much audio power as necessary to properly modulate the final.  If you're running 100w of carrier output, you'll need a modulator capable of -at least- 100w to 'properly' modulate the carrier, probably more.

It would be correct for a sine-wave, sure.  But, we don't speak in sine-waves.

You want audio?  read this page.  I simply can not convey the importance of, nor the amount of information that is there!
www.qsl.net/wa5bxo/asyam/aam3.html

[The Slab Bacon]

Jack,
         If this discussion is leading up to the selection of a mod tranny, you will also have to consider the turns ratio as this is just as (if not more) important than the static impedance of the transformer. The P-P impedance that the modders see is directly related to the the turns ratio of the transformer and how much current the finals are loaded for.

General rule of thumb for a pair of something modded by another pair and operaring from a common power source is 2:1 step down (full pri to full sec). the dynamic impedance that the modder plates will see is proportional to the frequency of the audio, the loading of the finals, and the turns ratio of the transformer.

                                                                                         the Slab Bacon

[The Slab Bacon]

As mentioned, I only need about 50W and so the plate current will only be on the order of 125 ma.

No, that's incorrect.  You need as much audio power as necessary to properly modulate the final.  If you're running 100w of carrier output, you'll need a modulator capable of -at least- 100w to 'properly' modulate the carrier, probably more.

It would be correct for a sine-wave, sure.  But, we don't speak in sine-waves.

You want audio?  read this page.  I simply can not convey the importance of, nor the amount of information that is there!
www.qsl.net/wa5bxo/asyam/aam3.html

Just FWIW the general rule of thumb for making 100% modulation is simple.  whatever your DC INPUT to the final is you need half that much audio power to make 100% modulation. Not half your carrier output. One more good rule of thumb to keep in mind is when designing a good home brew transmitter, make your modders capable of making audio power equal to the carrier output. this gives you a little headroom and "wiggle room" on heavy audio peaks.

                                                                                              the Slab Bacon

[w5omr]

Just FWIW the general rule of thumb for making 100% modulation is simple.  whatever your DC INPUT to the final is
you need half that much audio power to make 100% modulation. Not half your carrier output. One more good rule of
thumb to keep in mind is when designing a good home brew transmitter, make your modders capable of making audio
power equal to the carrier output. this gives you a little headroom and "wiggle room" on heavy audio peaks.

Nope.  That's just not true, for spoken-word audio.

Check out the asymmetrical audio page at the link I posted before.

[Bacon, WA3WDR]

Impedance is proportional to the square of the turns ratio.  So with B+ to the center tap and one plate to each end, the impedance seen by each tube will be 1/4 of the 7400 ohm plate-to-plate impedance, or about 1,850 ohms.

The 248W output spec at 1000 plate volts is a bit optimistic, I think. 248W RMS is 496W peak, and 496w with 1850 ohms is 957 volts, so the 811 would have to pull 518 mA with its plate down to 43 volts, and that's assuming zero ohms of winding resistance.  Your maximum undistorted modulator power will probably be more like 200 to 220 watts RMS.

If you want less output, you can just not drive as hard.  You can also go to a higher plate-to-plate impedance, which will reduce modulator power, and also reduce maximum modulation percentage.  A lot of AM gear was designed to flatten out at just under 100% mod with this kind of voltage-limited high-level clipping.  This also reduces tube emission requirements.  If you have adjustable taps, you can try connecting the tubes at different taps and see how they work.  Within limits, pretty much the only effect is more or less maximum modulator output power.

[K9ACT]

Lots of good stuff here.... good discussion.

Bottom line is, I have an SS22 mod trans and am trying to determine how to hook it up. 
From this discussion, I see 7000 primary and 6000 as the secondary as the nearest available options.

Make sense?

Oh yes.... it is not grounded grid.  I posted a link to the schematic and assumed interested persons looked at it.

For the current version, http://schmidling.com/radio.htm scroll down to the bottom and click on the hires button if you need it.

js

[w5omr]

Lots of good stuff here.... good discussion.

What I don't see, is the modulator, Jack.

What you will probably find you need, is a separate power supply for the modulator.

If you don't have enough current handling capability from the B+ supply for the final, AND modulator, then you simply need to build a separate supply for the modulator.

[K9ACT]

>What I don't see, is the modulator, Jack.

Oh yeh... forgot that part but I thought the question was about the final.

>If you don't have enough current handling capability from the B+ supply for the final, AND modulator, then you simply need to build a separate supply for the modulator.

I am working on that but to keep it simple and  get it on the air, I thought that 250 ma would be enough for both if I keep the power to a minimum

js

[Bacon, WA3WDR]

This time I paid more attention to detail...

For 100W output, you'll need about 140W input.  With 1KV on the plate that means 140 mA plate, and about 7K of load Z for the mod transformer.

With 140W input you'll need about 70W RMS undistorted audio for 100% mod, and 70W RMS is 140W peak.  If each modulator tube can pull its side of the mod transformer primary down 900V when it conducts, then you have 900 V producing lets say 150W to allow for some losses.  That translates to 150/900 = 167 mA peak, and 167 mA and 900V means Z load = 5390 ohms.  Doing it directly, 150W and 900V run through  R = (E^2)/P says R = 5400 ohms.  OK, so there was a little rounding error between 166.6666666666...mA and 167 mA.

So one side of the mod transformer primary would be 5400 ohms, and plate to plate impedance would be 4 times that, or 21,600 ohms.  So for 100% maximum modulation in these conditions, you would want about a 21K to 7K ratio in the mod transformer, about a 3:1 ratio on the mod transformer.

If you polarize your transmit audio to favor the upward direction, you can probably get about 100% positive and maybe 70% negative modulation this way.  If you had two times the audio power, you could then modulate 140% positive and 100% negative.

2X 70 = 140W RMS, 280W peak.  280W peak and 900V negative swing means a load impedance of 2893 ohms, or 10.3K plate to plate on the primary of the mod transformer, and and 7K on the secondary, about 1.4 to 1 mod transformer ratio.

And maximum modulator output, at least more or less according to spec, is (I think) about 200 to 220W with 7400 plate to plate and 7K secondary, about 1:1 mod transformer ratio.  Assuming 210 watts of audio on a 140W (input) carrier, you would get about 173% peak modulation.

Now, your 1KV 250 mA supply will be providing 140mA to the RF final, leaving 110 mA for the modulator.  Assuming for a moment that you have sine wave audio, that would be about 110W input and (at about 60% efficiency) 66W RMS out.  But your voice is not a sine wave... depending on the amount of speech processing you use, I'd suspect that for the same peak power, your voice might have pull about 1/2 as much modulator power as a sine wave would.  So you would have peaks equivalent to about 132W RMS audio.  This is in the ballpark of the 1.4:1 140W RMS setting from earlier, and you might want to be on the higher primary impedance side of that number.  So maybe about 1.5:1 impedance ratio with 7K output, that would be 10.5K to 7K.

Try that and see how it works.  You should have plenty of modulation, and you can back down on the audio level to draw less modulator current if necessary.  But if you have to back down very far, then try a higher plate to plate impedance.

You need to think about modulator efficiency because of the power supply limitations.  You can run 7.4K plate to plate impedance, but then you need to turn down the level to hold down the modulator current, and the efficiency of the modulator is proportional to output level compared to maximum undistorted output, so you'll get less audio that way.  I think you'll do better, and actually fine, with 10 to 11K plate to plate impedance and 7K load impedance.  There are a lot of unknowns, but I think that something right around there will give you maximum modulation at the 110 mA modulator plate current level.

I didn't see the RF amp schematic earlier.  Looking at it now, I am not sure about the cap to ground on the neutralization (bottom) side of the grid circuit.  It appears in the low-res schematic, I can't read the note on it, and it does not appear in the hi-res schematic.  If it is in there, I think it should be small, equal to grid-filament capacitance in the 811 final.  Otherwise you'll have to retweak the neutralizing cap at different frequencies on a band because the capacitive unbalance will change depending on the grid tuning cap setting.

I don't think you should have appreciable feedthrough output with the 811 plate voltage turned off.  If there is no output load, any coupling will look worse, though.  There may be something goofy with the neutralization, especially if that cap to ground is much larger than the grid to filament capacitance of the 811.  But that one has me scratching my head...

  Bacon, WA3WDR

[KA8WTK]

Bacon,
  There seems to be a difference between the low res and high res schematics. I think the cap to ground you mention off the grid tune cap is not in the high res version.

[Bacon, WA3WDR]

That's what I see too, Bill.  I'm hoping that it is the problem.

[The Slab Bacon]

Bacon,
            dont forget that actual turns ratio and impedance ratio are 2 different animals! He will need to be able to have the AC audio P-P voltage equal to something close to the DC voltage fed to the final. For 100% mod you need to be able to swing that voltage from 0v on negative peaks to 2Xv on positive peaks. (at least for 100% symetrical modulation).
The impedance seen by modder plates will be a function of the turns ratio and voltage and current drawn by the final. (I hate complicated math) If using a 3:1 stepdown mod transformer (turns ratio) (full primary to full secondary) he may need to have a little more voltage on the modders than the finals to get enough munky swing.

I have an S-22 mod transformer on my old buzzard P-P triodes rig. P-P 8005s modded by
P-P 811s (not 811-As) It runs right around 1000v on the plates. The taps are set for both segments of the primary in series with the junction point as the CT and both segments of the secondary in parallel. This gives a 2:1 voltage stepdown. I put 120vac on the primary to test it and read 60vac on the secondary.  Looks like 2:1 to me.   

If anyone wants, I'll see if I can take a pix of it and post it here if Jack wants to see it.

                                                                                          The Slab Bacon

[K9ACT]

I would love to see a pic of anything like what I am doing.

The cap in the grid is  gone.. Youse guys have good eyes.  I also change all the tank values when I got a proper plate cap.

A friend suggested it and it appears in the Currier amp but it did nothing good and seemed to screw up neutrlization.  I tried 5pF and 10pF.

I saved this whole thread for further reference and will start with 10k/7k and review the file if it doesn't work.

The RF deck is actually two 811A's in parallel but that leaves no poop for modulating.  After getting it working, I pulled the second tube and started building the modulator.  When I get this running, I will assemble the other power supply and use it for the mod so I will probably be back for some numbers in that mode.

Great night on 85, BTW.  Actually, it was 80.  Lots of static but good sigs from the East Coast.

js

[Bacon, WA3WDR]

Bacon,
            dont forget that actual turns ratio and impedance ratio are 2 different animals! He will need to be able to have the AC audio P-P voltage equal to something close to the DC voltage fed to the final. For 100% mod you need to be able to swing that voltage from 0v on negative peaks to 2Xv on positive peaks. (at least for 100% symetrical modulation).
The impedance seen by modder plates will be a function of the turns ratio and voltage and current drawn by the final. (I hate complicated math) If using a 3:1 stepdown mod transformer (turns ratio) (full primary to full secondary) he may need to have a little more voltage on the modders than the finals to get enough munky swing.

I have an S-22 mod transformer on my old buzzard P-P triodes rig. P-P 8005s modded by
P-P 811s (not 811-As) It runs right around 1000v on the plates. The taps are set for both segments of the primary in series with the junction point as the CT and both segments of the secondary in parallel. This gives a 2:1 voltage stepdown. I put 120vac on the primary to test it and read 60vac on the secondary.  Looks like 2:1 to me.   

If anyone wants, I'll see if I can take a pix of it and post it here if Jack wants to see it.

                                                                                          The Slab Bacon

Again I was sloppy.  By 3:1, I meant about 3:1 impedance ratio.  That would make the turns and voltage ratio 1.732:1.  A 2:1 voltage ratio is 4:1 impedance ratio.

Figure you have 2:1 voltage ratio, and your modulator tubes can pull all the way down to zero volts on peaks.  Then the peak voltage across 1/2 of the primary would be the modulator B+, and the output would be + and - modulator B+.  So if your RF final B+ was the same, your maximum modulation would be 100%.

The modulator tubes can't make it all the way to zero, because they can't produce plate current at zero plate volts - and that would be where they would need to produce the maximum current.  So I tap down a little on the primary side, so I get a little more output than a 2:1 voltage stepdown.  About 1.75:1 voltage ratio (about 3:1 impedance ratio) will allow the tubes to make 100% mod without flattening out.  Then the tube doesn't have to pull the plate voltage too low, and the output is + and - modulator B+ for 100% modulation.

[k4kyv]

With a common power supply, to maintain some headroom to allow peak modulation percentage somewhat above 100%, you want a mod xfmr with a total primary to secondary turns ratio of somewhere between 1.4:1 and 1.6:1.  If it's greater than 1.6:1, regardless of the load on the modulator, the modulating impedance of the final, or how hard you drive the modulator tubes, the audio waveform will flat-top or clip right at 100% at best, and most likely somewhere between 90 and 95%.  If the ratio is too low, you may exceed the peak plate current rating of the modulator tubes, or introduce excessive distortion, even though you may achieve higher positive peaks.

It might take 200-300 m.a. to fully modulate with a sine wave, but because voice modulation is of such low duty cycle, you will most likely hit well in excess of 100% on voice peaks without the modulator plate current peaks exceeding 75-100 m.a. as read on an analogue meter.  If you get a good clean pattern on the scope and it sounds satisfactory, don't worry about the lack of power suppy current capacity.  Just don't do a sustained whistle into the mic.  To accommodate instantaneous peak loads on the power supply that may be well in excess of the average as displayed on the plate current meter, use as much output capacitance in the power supply filter as you can, short of popping fuses due to the inrush charging current.

This is the principle that SSB linears have used for decades, and the  reason that so many "2 KW pep" linears have such anaemic looking power supplies. It's also the reason that "p.e.p." is virtually irrelevant to the actual loudness of a signal.

[The Slab Bacon]

Bacon & Don,
                     this is true, but since the S-22 has a limited number of taps, if he sets it for 2:1, that will at least give hom a good starting point to work from. He can then experiment with tapping up or down to "fine tune" it afterwards.

                                                                                        the Slab Bacon

[K9ACT]

Getting overload on all the numbers but I appreciate it all never the less.

I took Don's easy method and just concentrated on the voltage ratio.  I started with the 10k/7k suggested by Bacon I believe and came up with about 1.2.  I tried various combinations until I came up with 1.5:1 and wired it up that way.  The numbers on the hookup chart were 9000 and 3200.

I would like a check on my procedure to make sure I did it correctly.

I did it backwards so I could read the ratio directly.

I put 1kc at one volt into the secondary and measured 1.5 on the primary.

Is this right?

I am ready to go as soon as the new tube arrives.

Hopefully be on 7290 Wed morn.

Thanks for all the help.

js

[WBear2GCR]

Ok, think of it this way... albiet simplified somewhat...

To modulate the final 100% you have to swing the B+ voltage how far negative?
All the way to ZERO.

So if you have symmetrical modulation that is limited to 100% positive (the basic and simple case) then the total swing of the secondary of the mod iron has to equal 2x the B+ voltage (you can look at everything as peak or rms, doesn't matter, just stay with one way). But since you have TWO modulator tubes in a P-P modulator, and ONE output tube the ratio becomes self-evident. Again, the simple case is where the modulator B+ = final B+... Think of it this way, in a class B modulator ONE tube will have to swing the full B+ voltage at the secondary side to have 100% modulation in one direction...but there are two tubes and a winding for each one on the primary one swings positive and one negative. Ok?

Better still you can tell if you have it right by seeing if the voltage swing available with a dummy load = plate Z load for the final sitting on the secondary of the modulator (minus some fudge for the DC saturation of the mod iron) is equal or greater (better) than is required to swing the requiste voltage!

This is an empirical way to check it and set it up, at least get into the region that is very close to what is needed.

Just apply the <turns ratio>/<impedance> formula to see what the technical impedance ratio has to be, if ur spec'ing the iron that way.

            _-_-WBear2GCR

[w5omr]

Better still you can tell if you have it right by seeing if the voltage swing available with a dummy load = plate Z load for the final sitting on the secondary of the modulator (minus some fudge for the DC saturation of the mod iron) is equal or greater (better) than is required to swing the requiste voltage!

Yeah... with the hysteresis curve increasing, and developing a 'knee' at a couple of points along the line, with final current load across the secondary of the modulation transformer, it would -really- be best to slap a reactor in line with the DC final supply voltage.  This shunts the DC off of the secondary of the modulation transformer, and allows for a 'more linear' hysteresis line through the transformer.

How much inductance you ask?  According to WB3HUZ and others, the 'rule of thumb' is around 8HY per 1000Ω of Z in the final.  So, if you were running, lets say, 1500v @ 200mA for 300w DC input to the final, you're looking at 7500Ω.

7.5 * 8 = 60Hy @ 200mA. 

Not to worry, you -can- string some chokes in series, but watch the total DC resistance.  3 20Hy chokes at 200mA wouldn't be difficult to come up with.  1 60Hy choke would be a monster, and good luck on finding one.

The configuration that *I* use on a pair of 250TH's modulated by a a pair, is a single 50Hy choke @ 400mA in series with the DC line of the plate B+ power supply.  One side of the modulation transformer secondary is grounded and the other has a 4µF @ 5kVDC (because that's what I had) cap tied to the final side of the connected choke.  This blocks the DC from the final supply, and AC couples the audio to the DC line. 

As far as the coupling capacitor, Don/K4KYV suggested at one time that it shouldn't be more than a couple of µF @ 2x DC voltage + AC/modulated voltage (for safety).  For your 811 situation, a 2µf @ 4kV oil-filled cap would be sufficient. 

So much for the 'compactness' of the rig, huh?

But, it's these 'good engineering practices' that will pay off dividends later down the road.

Best of luck.