The AM Forum

I wonder if anyone has a simple circuit to build a one diode negative peak limiter? It should be self contained, function with about 1 volt of audio and have a threshold adjustment.

I'd like to raise my audio level since the 4X1 modulators have the headroom. However, standard audio processing will not limit the negative peaks, thus the need for a neg peak limiter.  

In the past I used one in a class E rig and thought it performed well.

Also, is there a "soft" clipping vs: a "hard" clipping circuit?  


Thanks.

T

YES!  You are on to something here.. I have dreamed of this for the last year or so.  I would like to control my Neg peaks at the Audio rack and not in the transmitter.

Clark

That would interest me as well. Peavey has an interesting circuit they call "DDT compression" which prevents clipping in their audio power amplifiers. As I recall, it utilizes an op-amp that samples the output waveform and at a point slightly before the amp begins clipping, it imposes just enough limiting to keep the amp from bottoming out. I have no clue about the details, but then again, anything with electrons running through it confuses me.

Might make us sound less tubby Tom.

Tom,

 All this can be done, but first consider that asymmetrical modulation done at a low level and then amplified to a high level will be subject to level shift in the r-c coupled stages. This will tend to undo the asymmetry created earlier. Also, if successful with passing the asymmetry through the active stages, then the modulation transformer will look at the asymmetry as AC voice with a DC component on it, so transformer saturation is a concern.

  So yes I think you can achieve what your looking for, but you might have to look at the design of your modulator first. Maybe try it out with a pulse generator that has a variable DC offset, and then see how that pulse shape and asymmetry is preserved on the final RF modulated envelope.

   I tend to believe that with plate modulation, doing the asymmetry thing is done best at a high level, and AFTER the modulation transformer. Just my opinion.


I did what I describe with a super modulated Viking I (200%) with "soft clipped" negative cycle where the progressive (1-2-4-8) attenuation kicked in at about 80-85% negative, and prior to that there was no asymmetry induced.
see reply 8 here, but in my case I had a solid state modulator with Rs=0:
http://amfone.net/Amforum/index.php?topic=22341.0

Jim
WD5JKO

Try a nice slow diode like a LED or two in series. Want it softer add a series resistor

Thanks for the info, Jim.

So you're saying that when using a low-level neg peak limiter, I may have problems cuz the asymmetry is not preserved thru the later chain and I'm using mod iron in the chain vs: using say a class E rig that will not be affected by the DC component, etc?


I've tried the classic high level 3-diode circuit in the past and it worked, but found it created splatter that was not there before. I've tried it on three different rigs with the same poor side-crud results. I think Don/KYV came to the same conclusion.  Though, others seem to have had better results than me.   I noticed your experiments with the 3-diode circuit from back in 1987. What are your conclusions today?  Have you run some modern spec analyzer tests that shows the more complex circuit you described is a better bet?  I notice you had a splatter filter drawn in.


Here's the schematic of the latest  QIX class H modulator he posted recently. The neg peak limiter is in the middle of the page. It uses a few transistors and is more complex than just a single diode and resistor. I wonder if the added complexity is needed, Frank, and what is the advantage?  I'd imagine it would be to soften up the attack time.

http://amfone.net/Amforum/index.php?topic=23014.0




Maybe I'll try the simple LED/ variable pot technique you suggested, Frank, as a first trial to see what happens.

It seems that if boxes like the Volumax, which use a low level diode neg peak limiter in commercial service work OK, then maybe low level limiting might work in my 4X1 mod-transformer-laden rig.

More ideas about this?

T

   Yes!   A speech clipper by itself due to phase shift after the clipper will diminish the clipping action. Add to that asymmetry, and the R-C coupled stages and transformer non-linearities will lessen the offset of the +/- peaks, if not eliminate them. You either need to DC couple, or have some sort of DC restoration. Taking the R-C time constants stage by stage and decreasing them (lower the -3db cutoff frequency down to 10hz or lower) will help for non periodic speech (pauses between syllables). If you do any of this, remember that transformers don't like DC.


  I agree, I don't like the three diode circuit either. The circuit I linked to is different. Sorry for the crude schematic, but hey that was drawn in 1987 to Hoisy W4CJL on his provided letterhead.


Ok, just remember that you said line level audio at 1 volt. I presume RMS? The threshold of an LED varies on color, but is around 1.2 volts. The cool thing about LED's is the threshold is abrupt with a near perfect kneee. I'm not talking about the bright ones, but just the red/green/yellows that we've known for years and years. You might need to boost the signal to 10v peak, do your diode/resistor stuff, and then go back down to 1v. Could be done with a dual op-amp.


Give consideration to using a function generator or pulse generator with DC offset capability. See what your modulator can do. if the asymmetry goes away in the modulator, there is little point playing with low level clipping/asymmetry circuits.

One more idea to accomplish what you want. Take the 4-1000 RF tube screen voltage, and since it is modulated, add a clipper diode there with variable threshold, and series resistance. This will make a "soft-clipper". I tried this with a 6146 RF tube, and limiting G2 downward swing to +30v with some series resistance did the trick for me. The action was smooth without an abrupt change in the RF output envelope pattern as 100% negative modulation was approached. The idea could be scaled up to a big RF PA tetrode.

Jim
WD5JKO

Jim,

Good info, tnx.

I will try your 4X1 screen diode idea after the low level LED idea.  Using the 4X1, (screen voltage = 450v at 140ma with a series 15H self-modulating choke) could you draw out the circuit you described with suggested values and perhaps scan and post it?  Or describe it in detail like a schematic? I'll bet others here might like to try something similar too.


T

Hi Tom,

Actually the CBS Audimax and Volumax (referred to within the industry as the "Max Brothers") were designed in the days of plate modulated transmitters, and their effectiveness with these rigs was supposedly quite significant. They were considered a major step forward in terms of AM radio audio processing, compared with the wideband AGC limiters available a the time, such as the RCA BA-6A, the Collins 26U-1, and others of that ilk. The Audimax and Volumax really were the first processors designed to the unique requirements imposed by AM broadcasting, and CBS sold a boatload of them.

Their hold on the industry was largely complete, until the advent of the Dorrough DAP-310 in the early '70s, and later, the Orban Optimod.

I had the pair, but I did not like the audio quality thru them. As always, your mileage may vary. Sold them off a few years ago.

73,

Bruce

Hi Bruce,
Now that I think of it, it was the Dorrough DAP 310 I had that used the diode neg peak limiter. I don't remember if the Volumax had one or not.  But the same question is posed: Did the BC rigs maintain the asymmetrical waveform all the way to the high level modulators?

Right now I'm trying out the LED idea Frank suggested. I got it to limit the negative audio peaks at the 4V level really nicely. I'm warming up the 4X1 rig now to see if it works in the high level modulators as well. Will let ya know what happens..

T

Hi Tom,

Yes, they did.

When I was a young BC engineer during my college days in the early '70s, we had the CBS Volumax and Audimax pair in the station audio chain. We could routinely modulate in excess of 100% in the positive direction after the FCC amended the rules limiting positive-going modulation to 100%.

I hope that this answers your question. If not, perhaps I don't understand it.

73,

Bruce

Update:

I just tried using a low level LED as a neg peak limiter for the 4X1 rig.  It required a good 5+ volts of audio to operate as Jim suggested.

I was able to effectively limit the negative peaks of the class C final amplifier as shown by adjusting the pot on the LED and watching the 100% line of the neg peaks get fatter on the scope as I watched the RF envelope.  So it appears the low end coupling is good, though not DC coupled. However, it added a tiny amount of audio "fuzz" as expected.

Anyway, I want to try your screen diode idea too, Jim, and see if that is an improvement.  Though, I wonder if the phase shift caused by the self-modulation screen choke will create a problem...

T

Ifn ya want a project to mess around with, Eric Babour (used to work for Svetlana tubes) ginned up a soft limiter using a 6BN6 gated discriminator tube.  It was part of a tube audio synthesizer.  It shows up on google.

Hi Tom,

The reason the single diode works in the class H modulator for the class E rigs is that the system is DC coupled, and the diode (in the new class H modulator) is working with very low power levels (it's ahead of the modulator MOSFETs).

I use a low level negative peak limiter in the PWM modulators as well, but again - DC coupled.

I have used the 3 diode high level neg peak limiter for YEARS in broadcast and other transformer-coupled transmitters with no addition of garbarge products.  Once the system starts clipping, you will obviously generate some audio harmonics which can make the signal wider, but this will occur with ALL waveform modification systems  ;)

Regards,

Steve

Tom,
Most LEDs have a breakdown voltage of about 1.7V so I am surprised it took 5 volts unless there is an internal resistor or multiple leds in series. You might try a cap across the led to soften the effect of the clipping. Also a series resistor ahead of the clipper may help. I use a similar NPL as Steve uses in my PDM modulator.
There are a number of ways to build a low level clipper. Biased diode is pretty good but might be too fast causing distortion.
As Steve said clipping action causes wavefrom modification which is distortion.

OH, BTW, your rig sounds great
 

Yep, nothing comes free... ;D We'll have to see how little crud we can get away with with the limiting.

I think the 4X1's audio is close to what I've worked for.  Check out this recording made by Al / W1VTP today using his Flex and sync detector. I was limiting the highs to about 6kc with the new digital low pass filter.  The reverb is configured to work on the extreme highs only - here's the results below.

Al/VTP said: "Bandpass was set for 7 KC.  Your sibilants are real punchy.  A lot like WBZ (1030 AM). Detection was SAM"



T

Sounds very FB - crisp, well balanced and loud, but not obnoxious or in-your-face loud.

I agree. The highs were nice and sharp and lows sounded like you were sitting in the same room.

A vacuum diode has a nice soft curve. Perhaps an instrument diode? or any diode with a very close spacing.
A small signal low V_Gth FET might work.

Your signal sounded extremely clear BTW, very admirable!

Thanks very much for the comments guys...

After a week of bothering everyone for audio reports, I'm finally happy. I just knew there were a set of optimum settings that would make my average voice sound decent.  With the 6-band processor and 31 band EQ, it gets overwhelming and frustrating at times... ;D

To continue the thread...others are wanting to do a low level limiter for their plate modulated rigs too.  I did more reading about neg peak limiting and saw an older thread that discussed this.  As already discussed above, to neg peak limit a transformer based rig (including mod transformer) it must be done with a HIGH LEVEL 3-diode circuit (or equivalent) due to phase shift and DC offset problems. I may install the 3-diode high level circuit and use it as a safety net for occassional overmodulation and to protect the mod xfmr.  To use it for heavier modulation may work OK, but at a slight cost of some minor crud. I'm also thinking about doing it in the screen circuit as a simpler, lower voltage alternative.

I'll post the final results, whatever they are.

T

Tom,   I am attaching what I did with my Gonset G50. A Heising modulated 6146 is a far cry from a 4-1000 modulated by a pair, but the screen diode clamp may work in a similar way..

    On a big rig, I might try the diode (or two in series), a series resistor like 5K 10w, and a bench variable DC supply that goes zero to +150v. The idea is to see the effect of clamping the downward G2 swing to some minimum voltage. I'm not sure how that 15H choke will react to the clamp....

Jim
WD5JKO

OK Jim,

Let's see if I can adapt this to the 4X1 rig....

So, the cathode of the diode(s) goes to the screen pin (after or before the screen choke?)  and the anode of the diode connects to a 5K 10W resistor - the other end of the 5K resistor goes to the plus of the variable 0-150V supply. The supply negative goes to ground.    Is this correct?

Do I need any bypass caps or resistors anywhere else?  Should it be a flat function when I freq sweep it looking for neg peak operation - and if not, do I put a compensation cap across the diode, etc?

T

   Tom, yea I think you got the idea. The diode cathode directly to the G2 pin. The 5K resistor is just a precaution against too much G2 current if you hook it up with the plate voltage off. That said, some series resistance with the diode might be desirable.

   On the 6146 in the G50, I did not have a series R with the diode, and the G2 voltage minimum was around 30v, and the G2 limiting was abrupt. Quite surprising to me though was the RF envelope at the same point just had a slope change suggesting "soft limiting".

75% mod before diode conducts:
http://pages.prodigy.net/jcandela/G-50/75_mod.jpg

85% mod, diode threshold being met:
http://pages.prodigy.net/jcandela/G-50/85_mod.jpg

Normal speech showing diode action. Without the diode, I was brightening at the baseline frequently:
http://pages.prodigy.net/jcandela/G-50/YEA_AM.JPG

The R's and C's on the 6146 G2 where an attempt to compensate for audio phase shift on the modulated G2 voltage. As it is, a triangle wave into the modulator results in an exact shape on the RF envelope anywhere from 50 Hz to 5 Khz. A more typical circuit really messes up the upper frequencies due to phase shift. I believe in your case with a self modulated screen grid that this approach is non applicable (NA).


The old Gonset G50 in stock form had one of the worst high level modulators I've ever encountered. My change using a P-P output transformer the way I did (modified Class A Heising) really cleaned the thing up nicely.

Jim
WD5JKO

Not sure if it will help but one might apply some "soft recovery diodes" to good advantage here...

Thinking out loud, perhaps for the DC "bias" component, if there were two feeds of bias, one from a stiff supply and one from a rather wimpy soft supply (maybe via a high Z resistor) that might cause the initial threshold through the wimpy supply to sag (failing to have enough current) down to the stiff voltage supply causing a kind of "soft knee" clipping? Each voltage would have to be fed through a diode to keep them from interacting...

                      _-_-bear

Tom.  I would suggest you remove that CHoke and modulate the Screen directly with the Dropping resistor.  Your not going to believe the difference in sound quality and punch when you do. I did back to back tests on 4-400s this way. We where shocked to actualy hear the difference on first key up. 

C

Clark,

Yes, the screen dropping resistor works FB for a small rig, but with big HV, the wasted power adds up fast.  At 3KV on the 4X1 final, there wud be 2500V dropped across the resistor.  At 140ma screen current this is 350 watts of heat used up. It would take some big power resistors to handle that continuously. That's why the little 500V screen supply with the choke is used.

In addition, I would lose the ability to fine tune adjust  the screen current on the fly. I use the Variac in the 500V supply now. It helps to adjust screen current as antennas are changed, loading is changed and when HV is stepped up or down.

One last thing to consider:  If by error the HV is turned on before the filament and/or the rig is not keyed to draw the normal 140ma of screen current, there will be 3KV on the screen and associated screen circuitry thru that resistor. Yikes!

Though, weren't screen chokes with LV supplies used all the time in the BC industry? I agree the resistor is better than a choke with high shunt capacitance problems, but think the difference is very small if working correctly.

I'm curious - anyone else using a screen choke for a big rig - opinion?

BTW - I'm gonna try that screen neg peak limiter later today...

T

T,

Agreed.

I too am using a screen reactor in the screen circuit of my 2x 4-400A rig. I think a pair of 4-400As qualifies as a high-power rig. ;) The screen supply in my transmitter is Variac-controlled, so tuning the HPA to the resonant point is a breeze; just reduce the Esg to about 200 volts, and there is no fear of pulling excessive off-resonance plate or screen current. I've been doing it this way for over 30 years. It may not be the most optimum solution in a high-power plate modulated rig, but it is possibly the easiest and it certainly works well. If someone can offer a more elegant solution, aside from using triodes in the HPA stage, I am all ears.

I do not see how there could be a large difference either, in terms of audio quality (read final amplifier stage modulating more linearly) when comparing a screen reactor vs. a screen dropping resistor. Perhaps Clark/KE7TRP, can chime in here and describe the difference or improvements he noted with the screen dropping resistor approach.

As you indicated, the use of the screen dropping resistor would necessitate a resistor of significant dissipation capability, along with an attendent large waste of power. Dropping the Esg for final tuning I think would be more problematic as well; a high-voltage switch and additional series resistors to further drop the Ebb would be required, etc.

The screen reactor in my rig is actually 62 hys; I am using a 30 hy choke in series with a 32 hy modulation reactor out of a Gates BC-1G 1KW AM transmitter. The mod reactor is deliberately positioned closest to the 4-400A screen circuit, as this inductor is probably better in terms of lowest HF loss due to capacitance effects to ground, when compared to your average garden-variety filter choke in this application. For what it is worth, I cannot hear any real improvement with the 32 hy reactor in the circuit, so it may be going on the shelf soon, but I first do want to perform a proof-of-performance on the rig to quantitatively see if indeed there is any real improvement. The original 30 hy Kenyon screen choke I use is mounted on ceramic stand-offs to minimize the HF loss to ground, again due to potential capacitive loss issues, and worked great, with excellent low and high-frequency characteristics. I never did a swept frequency response of the rig with the big 32 hy mod reactor in the screen circuit; too lazy I guess. Without it, and with just the Kenyon 30 hy reactor in the circuit, it was about 35 hz to 12 khz at the -3dB end-points.

With the two reactors in series, the predicted -3dB roll-off in my rig at the normal value of screen voltage and current (i.e. the screen impedance) that the two 4-400As pull, is about 18 hz. This does not include the low-frequency roll-off imposed by the mod xfmr, driver xfmr, or mod choke, etc., but rather is the -3dB low-frequency point for the screen circuit only.

In retrospect, the two series reactors in my rig are gross overkill and most certainly not representative of good or practical design. The 32 hy mod reactor in the HPA screen circuit will probably come out sooner than later.

I agree with you; in a small plate modulated rig (<150 watts DC input), the screen dropping resistor is the way to go.

I do recall seeing, however, more than one AM BC rig that used a variant on the screen dropping resistor approach; perhaps it was RCA or Collins in their 4-400A series of 1000 watt-class transmitters. However, I believe this may have more of a design-to-cost issue, vs any potential improvement in the modulation characteristics of the transmitter.

73,

Bruce

Assuming you have a good choke (low shunt capacitance and sufficient inductance) there will be zero difference when compared to a resistor. Impedance is impedance. The modulating audio doesn't know the difference.  :D

I just tried the screen diode negative peak limiter. It had no effect as I increased the voltage from 0-150V or changed the series diode resistor from 400 ohms to 5k.  It was like it wasn't there.

The circuit:  cathode of diode to screen pin, anode of diode in series with 5K resistor, resistor to 0-150V + supply.  Supply neg to ground.

I felt the resistor getting warm, but no neg peak shunting effect on the scope looking at the modulated RF envelope.


** So... I have enuff HV diodes to build a 3-diode circuit for the plate circuit. Gonna give that a try.  I know it will work cuz I've done it many times b4.  Better to have some protection than none.


T

I put together the 3-diode negative peak limiter in the 4-1000A plate modulator circuit.  I used 6A diodes and made three 15kv legs.  The power resistor is 4K and I found about 50V was needed for the "keepalive" supply.  The negative band will will get as thick or thin as I need depending on the keepalive supply.

The scope pictures below show the intentionally overmodulated signal to show the negative band is slightly thicker, maybe negative -95%.  

It works FB as desired for a safety net.

T

Hmmmmmmm..............Ok, I'm sold. T, you're the man!

I guess I'll do the same thing now, based upon your success. It appears the low-level route does not cut it, much to my dismay. What did you do differently this time around, that you did not do previously with regard to the 3-diode negative peak limiter?

And no audible artifacts in the modulation whatsoever???????

Why the 6A diodes? Availability of the parts on-hand?

I have a bunch of the Amperex 15 KV @ 0.75A stacks I could use, and I'd use a 5K ohm loading resistor to simulate the class C HPA load impedance in my rig.

How about posting a quickie schematic for the great unwashed masses such as myself? I'm looking to implement this as the protection of last resort for my mod xfmr.

Thanks for the good work!

73,

Bruce

Here's the 3-diode circuit as drawn by Steve, QIX.


Bruce:  I didn't spend much time testing it yet, just socked a few Yallos into the dummy load. I'll get some on-air reports for side crud tmw in the quiet day.

It does appear to work and lets the positive peaks continue higher as the neg band stays fat.

I used the 6A diodes cuz I have a bag of them. Your blocks will work FB, of course.

All in all, it's a great way to creep up on the audio while hitting the baseline regularly without generating the predictable trash of -100% negative. Plus the mod xfmr is protected with another layer of safety.

I don't expect to be slamming it regularly like the scope pics show, but will try it anyway and see what the reports show on the air out of curiousity.

T

Tom,

I'm going to guess your success this time around was related to having the correct keep-alive voltage available. It appears that perhaps a greater percentage of voltage is required, at least in your transmitter, compared to what 'QIX had originally recommended.

If 50 VDC did the trick, what is your HPA plate voltage? I'm looking to scale this value for use in my rig.

Thanks,

Bruce

Bruce,

Here's something about the crud level of this circuit as written by Steve/HUZ. His calculations show the crud to be minimal. Tests will tell.

I used 2200V for this particular test. (low tap)  I wud make a supply that will do ~0-150V to have a full range of adjustments. The neg band seems quite firm and you can set the keepalive so that the band is thin and forget about it. I wud set it to a minimum for the job.  The tests will show us.   ie, How little we can use and still not generate band side crud while keeping close to -95% - -98% neg..

T



http://www.amwindow.org/tech/htm/zenlim.htm

"Although the zener regulation action will clip the negative audio peak near 100% negative peak modulation, the resulting distortion is minimal. Remember, at the instant the clipping occurs, the voltage on the final tube(s) is only about 5% the normal B+ voltage. This also means the instantaneous final plate current is also only about 5% of normal (or what you read on the transmitters meter which reads the average value). Knowing power is the product of voltage and current (P = EI), then the input power to the final at the instant the negative peak clipping occurs is only 0.25% normal. Thus, the distortion products created by the clipping will be no worse the 0.25% of the input power or unnoticeable. For example, if you are running 100 watts average input, the power input at the instant the negative peak clipping occurs is just 0.25 watts!
  With the circuit in place, the negative peak modulation is absolutely limited to slightly less than 100%. If your rig has sufficient modulator power, you can crank up the audio gain some, achieve higher levels of positive peak modulation, and never produce splatter. Remember, splatter is produced by modulation in excess of 100% on the negative peaks, not the positive peaks. Modulating over 100% on positive peaks will make you audio sound louder at the receive end or give you more "talk power".
  The circuit also provides protection for the modulation transformer. A big killer of modulation transformers is large transients created by modulation in excess of 100% on negative peaks. Since the zener circuit prevents negative peak modulation from exceeding 100%, no damaging transients are produced. Long live your modulation transformer!"

Tom, Bruce,
You may have had problems in the  past if the keep alive voltage could not supply enough current to the NPL when the mod transformer went beyond the negative peak setting. The negative cycle loading draws current off the keep alive supply. If that voltage pulls too low you will hit baseline. I suggest a big cap on the keep alive input to handle transients. I had the same problem in my PDM rig during over modulation. fc

Referring to Steve's article on NPL above...

He mentioned a 100w transmitter's negative peak crud is 0.25 watts when limiting at about -95%.  This would be down maybe -27db or so? (100w > .25w)   If someone is S9 +40 over, then the NEW side crud generated by the NPL would be S9+ 13 over.  Of course this is better than the crud generated by a -100% negative transmitter, but not as clean as one that uses no NPL at all and naturally stays above -95%.

I ran some tests last night into the dummy load using the local receiver as a spec analyzer and think I could pour on more audio without the bad effects of -100%, but will need to use on-air reports to be sure.

At least the NPL appears transparent when below the threshold use level, acording to crud measurments with it on or totally out of the circuit.  So far I'm pleased with it and will make up a permament ~50V NPL supply.

T

Hi Frank,

Many thanks for the input.

You make a very valid point here. How big of a cap do you suggest?

As I indicated to Tom earlier, the biggest problem I had was finding the real estate to mount this thing. Playing a little hookie from work this morning, I think I found a good location right over the mod reactor. I really just have to get one HV rectifier stack and then start the mechanical layout, etc. I think I have all of the other parts.

A very simple project with hopefully good protective benefits, etc. I'm really looking to protect the mod xfmr in the rig more than anything else, and certainly not to increase the density of the modulation.

73,

Bruce

I=c dv/dt

Since you are charging the cap with 120 Hz pulses it has to carry you through a  charge cycle. This means it needs to hold the voltage stable for 8.3 ms. Say you swing the voltage 500 volts negative across the negative cycle load. Now calculate the current through the resistor. The keep alive needs to hold the voltage so the final stays lit.

In looking at this thread, I seem to remember a 'log speech processor' from back in the 80s marketed by a kit mfg. that did just what people are looking for, create asymmetry in the audio chain and block negative going peaks.

I remember building one and taking it into electronics class on a (also homebrew) AM transmitter.  My electronics teacher, Dan White (KB6TDW, SK), was AMAZED at the pic on the o-scope.  Where was the negative mod?

I remember the name now, it was CBC Intl. that did the kits.  Worked quite well, although I don't think they where anywhere near hi fi.  A friend built one last year, I just got it in the mail to play with.  If anyone's interested, I can do some before and after's on a scope when I get "round toit".

--Shane
 
modified to correct SK callsign

Good job on the limiter.  

I know you will have 350watts of heat.  However,  I ran back to back tests with the choke and the Dropping resistor. The difference in audio quality and sound was huge.  I dont know what else to say. Sometimes people get stuck doing things one way and then defend that solution.  I have an ultra modulation unit that contains three 866s.  That unit is designed for BC use at 1000 watts Am transmitter. In the instructions it tells you to convert the screen to a Dropper and to NOT use the choke or Fixed Screen.  We converted it and where shocked at the results.  I think its because the screen and plate where not in phase. With the choke. There was clear distortion. Not bad but you could hear it. With the Dropper, This cleaned up and flat out talked.

There are obvious dangers here..  You decide if you want those pitfalls.  This might not be the best for Fabio. But now you have a neg peak limiter on the Plate only.  This makes the screen and plate not modulated the same.

I am in the process of changing the T3 over to modulated Screen.  I have studied the schematic and this seems very simple to me since the T3 has a Dopper on the NON modulated B+ now.  I have a feeling when I talk to you next, You are going to notice it :)

Clark

Something else is going on.  Time constant of L & screen C vs. R & screen C?
-shouldn't be that big a difference.  What is the value of your screen bypas caps.?

I don't detect a whole lot of audio loss using 20 H screen reactor and 780 pf/tube screen capacitors on my 813 rig (screen fed w/seperate 350v. supply.)

and,

Right.
After all, it is just audio, not 60 MHz RF.

I just wanted to add the keep alive needs to be stable or it will appear as negative modulation at a phase lag which will be distortion.

Frank,

How would you measure this.... just use a scope and look at the keep-alive DC component for movement under modulation?  Or, are more elaborate tests required?

I just ran a Fabio RF dummy load test with high tap HV on the modulators and final. Gawd, those modulators are capable of some clean and serious audio when the negative peaks are limited. I never ran them that hard cuz of fear of the mod xfmr breaking.  I still plan to keep things naturally near neg -95% despite the NPL, but it's good to know the headroom is there for best linearity.  Still need to do some live on-air side crud tests with the new NPL kicking in.


BTW, just received a pair of new Tron-recommended Sony 7506 headphones today. What a difference over the crap I had before. The listening low end is huge now. No wonder I had trouble EQing my own audio in the mod monitor before.

T

Tom,
Yup just monitor the keep alive voltage to see that it is stable. Any change in voltage will appear as modulation superimposed on the B+. Ripple also needs to be as low as possible. At some point the mod transformer will saturate limiting the peaks. Also keep an eye on the load resistor temperature.
Here is a good test for the transformer. Measure the resistance of each winding cold. Then run the crap out of it for a while. Shut it down and measure the resistance again. Knowing those values you can determine the power in the windings and the real temperature of the wire.