possible modulation

[K5IIA]

I have a couple of questions about modulation.

i have heard people say that the ratio of the modulation transformer is what decides maximum possible modulation.

i understand there are more factors to modulation but i think i have a decent understnading of them. as far as headroom and stuff like that but here is basicaly waht i am asking if someone can explain in somewhat simple terms.

1.how does the turns ratio of a mod transformer affect max modulation percentage. that is if that is even true.

2. lets say you have a pair of 813's modulated by a pair. would putting the modulator tubes on a seperate voltage source and putting addiction voltage on them increase the maximum modulation. i understand that additional voltage would give more audio power but i am more wondering if it does more then just add audio power if it somehow gives addition modulation possible.

i'm not really working on anything that i need this info, it is more just out of curriosity. and i learn stuff more talking about it in a less formal matter and just wondered if anyone could shed some light on this or even if they have any other info that kind of fits in with this that i may not even know of to ask about.

 

[W7SOE]

I copied this from another thread, words of wisdom from Don.  I am not sure it answers your questions but it is good reading anyway.

73

Rich

Snip----->

The optimum design of a modulator and final is more involved than simply calculating the modulation transformer impedances and turns ratio based on the published impedance properties of the tubes. 

If a common power supply is used for modulator and final, regardless of the impedance level, the modulation transformer needs to have a turns ratio of about 1.4:1, or a 2:1 impedance ratio.  The only case in which a "multi-match" transformer, with a choice of several turns ratios, is useful is when separate supplies are used for modulator and final, and different plate voltages can be applied to the modulator and final tubes.  Otherwise, an appropriate fixed-impedance transformer is satisfactory, and may actually give better performance.

The  reason is simple.  In a class-B or AB circuit, the peak audio voltage capability of each tube is roughly 80% of the DC power supply voltage.  It is very difficult to drive the grid of a tube hard enough to bring the instantaneous plate voltage below about 20% of the DC plate voltage.  One of the reasons is that, to get the tube to conduct to saturation, substantial positive voltage must be applied to the grid, and this peak grid voltage is likely to be close to 20% of the plate voltage.  With a triode tube, it is impossible to bring the plate voltage below the grid voltage, no matter how hard the tube is driven.

With two tubes in push-pull, the total peak a.c. (audio) voltage across the primary of the modulation transformer is the sum of the peak voltages generated by each of the two tubes.  If we assume the figure of 80% of the DC plate voltage, that means that the the total peak audio voltage developed by the modulator tubes cannot exceed 1.6 times the DC plate voltage.  Therefore, for exactly 100% modulation capability, the turns ratio, total primary to secondary, would be approximately 1.6:1, or 2.56:1 impedance ratio.  But to avoid distortion near the 100% modulation point, we need some head-room, so that the tube is not being driven to saturation right at the instant that 100% modulation occurs; therefore it is preferable to have a little less step down, maybe 1.4:1 or 1.5:1.  If you are looking for extended positive peaks,  a ratio of 1.3:1, 1.2:1 or even 1:1 would be necessary.

Where the plate-to-plate impedance comes in, involves how much current is run on the final.  By Ohm's law, modulation impedance = plate voltage/plate current.  This impedance is reflected back to the modulator tubes via the transformer.  So you need to choose a plate current that will give the proper modulating impedance that when reflected back to the modulator tubes through the transformer, will allow the modulator tubes to work into a satisfactory plate-to-plate load.

There is nothing sacred about the p-to-p load recommendations given in the tube charts; they are just that, recommendations. With most good tubes, the p-p load they work into can be varied considerably, maybe as much as 2 to 1 and still get good results.  When working the tubes into a lower p-p load impedance, the peak plate current will be higher, the plate dissipation will increase and the stage may become less efficient.  Taken to extreme, the linearity of the tube may suffer.  But within reason, the main thing to watch for is plate dissipation and maximum peak plate current.  The maximum peak output from the tubes may be reduced when one veers too far from the recommended p-p impedance, but if peak plate current and plate dissipation are kept within the manufacturers ratings, the tube will perform just fine.  The same goes for running at a higher-than-recommended p-p load.  In this case, for 100% modulation capability at the full power rating, the DC plate voltage will be increased.  Again, this causes no problem as long as the maximum plate voltage rating of the tubes is not exceeded.  The tube may run more efficiently at a higher p-p load, but if too much step down is used in the transformer, the peak output capability and therefore modulation capability will be reduced.

The third factor to be considered is the nominal impedance rating of the modulation transformer itself.  This is determined by the amount of iron in the core, the type of  iron used, and the number of turns in each winding. Most good transformers can be run at least +/- 100% of the nominal value, again as long as maximum current and voltage ratings are not exceeded.  If you get too far away from the nominal impedances, frequency response may be affected.  Running a transformer at a  much higher impedance than normal will limit low-frequency response due to the lower inductance of a low impedance winding.  Conversely, running a transformer at a much lower impedance than recommended may limit the high frequency response due to the combination of stray inductances and capacitances in the windings.  Also, the core is more likely to saturate on peaks due to higher currents through the windings. But within reason, the transformer should work OK. 

In fact, this is the principle of operation of the multi-match "universal" modulation transformers.  The same turns  ratios are used for many different sets of impedance walues.  The CVM-5 for example is rated for something like a range of 2000 to 20,000 ohms on both the primary and secondary.  Looking at the charts, you will see the same turns ratio connections repeated over and over to transform widely different impedance levels.

So, getting back to the topic, if a common power supply is to be used, a mod transformer turns ratio of somewhere between 1.2:1 and 1.4:1 should be used, and tube types, DC plate voltage, final amplifier plate current and nominal modulation transformer ratio should be juggled for the best fit. Some compromise may be necessary, regarding both performance and power output, when one is limited to using components on hand.

Calculating the mod xfmr turns ratio based on published modulator impedances and final amplifier plate voltages/currents listed in the tube charts may or may not meet the above criteria.  If not, it is better to push the load and modulating impedances a little beyond the tube chart recommendations, or live with slightly less power output capability, than to veer too far from the optimum modulation transformer turns ratio as described above when using a common power supply for the modulator and final.

[K5IIA]

i looked and was like wow that guy can type. hahaah

thanks man i will read it when i get back. my daughter brianna is in the second grade presidents day play tonight and we are leaving now.  i figured i was would ask before i left to have some good reading tonight.

thanks

[k4kyv]

i looked and was like wow that guy can type. hahaah

Learn the Dvorak keyboard layout.  It takes about 3 weeks to master it, with about 20 min. of practice per day, and then a couple of months to bring your speed up to par. After getting over that hurdle, typing on a keyboard becomes as effortless as talking into a microphone.

http://gigliwood.com/abcd/

[W7SOE]

Don,
   Is that the voice of experience or a suggestion?  

My typing is terrible, a hyrid of hunt and peck, watching my fingers and watching the screen.  I really should do something about it.

Rich  

[w5omr]

i'm not really working on anything that i need this info, it is more just out of curriosity. and i learn stuff more talking about it in a less formal matter and just wondered if anyone could shed some light on this or even if they have any other info that kind of fits in with this that i may not even know of to ask about.

I can't add anything to what Don has already said, in his usual elegant manner.

As he said, it's a lot to do with the impedance of the final, and what the plate-to-plate load impedance of the modulating tubes are.

Let's assume we're working with a 1kW DC input rig. (because 1kW is easier to represent with numbers)

2000v @ 500mA = 1kW

As Don said, the impedance formula is simple ohms law, where
Z = Ep/Ip
Z = 2000v / 500mA
Z = 4k ohms.

But, suppose you wanted to run lower voltage and higher current... something along the lines of a 304TH/TL...

Z = 1000v / 1A
Z = 1000ohms.

You've got the same power output at 1kW, but your plate impedance has been reduced by a factor of 4.

So, lets plug in some real-world numbers...




The Single 813 in Class C has a typical operating voltage of 1600v @ 150mA.
That's 11k Ohms of Impedance.

The pair of 813's in push-pull at 1500v has a plate-to-plate load impedance of 9300.  Probably a bit closer to 10k when you reach 1600v.  You could get by with using a 1:1 modulation transformer, or as Don says, if you wanted to put a little bit more audio, a turns ratio of 1.4:1 would help.

remember, this is for -1- tube in the final.  Depending on the type of circuit you use, the plate current could either stay the same with two tubes (classic balanced push-pull, cross-neutralized tank) or double with the finals in parallel like in a Pi-L network.  If the voltage stays the same, but the current doubles, then the Impedance of the final will be reduced.  now you're looking at 1600v @ 300mA giving a Z of 5333 ohms, instead of 11k, and your ~10k ohm plate impedance modulator doesn't quite match.

There are some other variable factors that are involved, including how asymmetrical your voice is.  On my push-pull 250TH rig, modulated by 250TH's, I run somewhere down around 1000vDC on the plates of the final.  I run 1500vDC on the modulators, at 0v bias.  Mostly I run low-power (~150w) with my full-wave delta loop to keep the peace in the neighborhood.  Both plate input voltages on the 2 different plate transformers are controlled by a VariAC

I have another power supply that's in series with the modulator supply that if I wanted to crank up the VariAC for "night-time" power (when everyone's gone to bed) then I need -that much more- voltage on the modulators to get the positive peaks up where they belong, without flat-topping.  Night time power is typically ~300w of carrier output, which equates to ~1600v @ 220mA.  At that point, there's close to 2500v on the 250TH's in the modulator, and bias voltage has to be adjusted for ~50 to ~60'ish volts to maintain ~200mA of resting current, but the positive peaks -really- respond to the increase in modulator voltage.

[w5omr]

there were supposed to be a couple if images that I had put into the message...

I'll upload 'em as attachments.

[k4kyv]

Don,
   Is that the voice of experience or a suggestion?  

My typing is terrible, a hyrid of hunt and peck, watching my fingers and watching the screen.  I really should do something about it.

From experience.  For decades I was a lousy typist.  I tried to teach myself touch typing while I was in high school with a cheap portable typewriter and instruction book, but I learnt so many bad habits in the process that the best I could ever do was just a step above hunt-and-peck.  I even completed a two-semester university typing  course, but the best I could do was a "C", and my typing skills still didn't improve very much.

I had read about the Dvorak keyboard back then, but the only way to use it was to purchase a special mechanical typewriter, or have one modified, neither of which was cheap. But years later when I got a decent computer, I immediately noticed the Dvorak keyboard layout was built into the operating system; it was just a matter of activating it.  At first it was only on Windows machines, but later on, Apple started putting it on their Macintosh OS.  About a dozen years ago, over a summer I downloaded the on-line tutorial and attempted to learn the new layout.  By summer's end, I could type with Dvorak with fewer mistakes, but my speed wasn't much better than before, but I still found it easier, and after a few months I was typing faster than I ever had before, and my speed has continued to improve with fewer errors.

With the standard keyboard I always looked at at the keys while typing, but with Dvorak I truly learnt to touch-type.  I was forced to, since the letters printed on the keys no longer correspond to the letter each key actually types.  I  have my computer set so I can toggle between the standard keyboard and Dvorak by simultaneously pressing the "ctrl" and "shift" keys. I usually toggle back to the standard keyboard to enter stuff like passwords and product keys,  hunting-and-pecking one letter at a time, since sometimes just a single wrong character can screw things up and require a long involved procedure to correct the error.

At work a few times I used it to play with students' and co-workers' heads.  Someone would ask to use my computer to type something or send a quick message via the in-house email system.  They would try to type, but all that would come out would be gibberish.  They would call me over to show me that something was wrong with my computer.  I would try to type it myself, and everything would come out right, so I would look puzzled and show them there was nothing wrong with it. They would try again, and still just gibberish.  Sometimes I would cycle this through several times before I let them know what was happening.  

What was really funny was sometimes one of the IT guys would come in after hours to check my computer or install new software, and it might take him an extra 45 minutes to figure out why he couldn't make anything work.