MODULATION DIODE

[W2PFY]

What are the pro's and con's of a diode in series with the modulation transformer to the PA final? In mobile rigs one simple solid state diode was used. In larger home stations, I have seen an 866 or 872 in series with the mod transformer to the PA.

I know about the three diodes arrangements so I'm not looking for something that's better. I just want to know how well this worked in it's simple form?

Did anyone try it. I did in an old tube CB years ago and it seemed to work fine. I guess the idea was to keep out the negative cycle? Someone said it would more effectively modulate the carrier.

[K1JJ]

Hi Terry,

Recently I've added a high level 3-diode ultra modulation (negative cycle loading) circuit to my 4-1000A  modulated by a pair of 4-1000A's.  I ran some tests and found it did indeed help contain splatter from overmodulation.  In the past I thought it caused splatter on a few of my rigs, but have since decided it is a good safety net when used sparingly.

I know some guys use it to hard limit negative peaks to increase positive peaks as a result - however, I use it simply as protection against modulation transformer damage from 100% negative peaks and to protect against occasional splatter as a result.  In this job it works FB.

I have mine set to work at about 98% negative, so it produces a very thin line at the center line (on the scope) when working. My audio is set for it to work maybe 5% of the time or less, during normal speech.

As for a single diode - I don't like that idea since it creates no matched load for the mod transfomer. The modulation transformer is at risk, especially if the diode is leaned on for bigger audio. Maybe a small rig will survive, but I wouldn't be too agressive using it in a big KW mawl. In contrast, the 3-diode  circuit has a rather nice loading effect as produced by the loading resistor.  The resistor is selected by calculating the normal modulator load into the RF final.

Because of phase shift from the average modulation-transformer AM rig, negative peak limiting should be done in the high level area, thus the 3-diode circuit. Low level limiting is not as effective - unless the rig has exceptional linearity like a low level balanced modulator, SDR or class E rig - all transformerless.  My 4X1 limiter decision was based on this concept.


T

[KC4VWU]

Wasn't that what Lafayette called "Range Boost" on their 11M tube rigs back in the day?

Phil

[k4kyv]

As for a single diode - I don't like that idea since it creates no matched load for the mod transfomer. The modulation transformer is at risk, especially if the diode is leaned on for bigger audio. Maybe a small rig will survive, but I wouldn't be too agressive using it in a big KW mawl.

The single diode in series with the modulation transformer may actually produce more splatter than would simple overmodulation. The problem occurs during the part of the audio cycle when the instantaneous modulated DC voltage is rapidly decreasing towards zero. The series diode effectively disconnects the modulator from the final as the rf bypass caps in the final discharge at a slower rate than the waveform out of the modulator descends. Then, as the instantaneous audio voltage from the modulator begins its upward swing as the audio cycle continues to progress, the diode conducts once again, and the descending voltage from the discharging capacitors clashes head-on with the rising voltage from the modulator. The result is a sharp spike at the negative peak, rather than the normal flattened negative peak due to the PA being cut off for a split second while the plate is driven negative during overmodulation.

This produces splatter, even though the plate of the final is never actually driven negative at the bottom of the modulation peak.

[WD5JKO]

I have sometimes used a single diode from the modulated B+ to ground (reverse biased) with a resistor in series equal to the modulator load impedance at about ~5% of the modulator rms power.

Why would I do that? Well, for one this keeps the modulator loaded at all times, even during periods of overmodulating when the modulated B+ swings negative with respect to ground. This helps keep modulating transformers alive, and reduces any increase in distortion from the modulator due to the small portion of the  modulation cycle that would otherwise go unloaded. Sure the scope baseline will still go to zero, but my experience with this single diode-resistor combo is that splatter is much reduced.

What I present does not prevent over modulation, but it does protect the modulation transformer, and it does reduce splatter.

Jim
WD5JKO

[W2PFY]

f it were me I’d use the 3-diode scheme with the external supply just to be safe.

Your correct, I want to thank everyone who replied. I would use the three diode scheme but it helps to be on the air when doing it.

have sometimes used a single diode from the modulated B+ to ground (reverse biased) with a resistor in series equal to the modulator load impedance at about ~5% of the modulator rms power.

I thought this was interesting but it would seem to me that you would really need to hammer the audio for the diode & resistor to work. Didn't someone use a similar scheme with a mill-amp meter in series with the diode. when the meter started to waggle, your are hitting the base line??

[Opcom]

f it were me I’d use the 3-diode scheme with the external supply just to be safe.

Your correct, I want to thank everyone who replied. I would use the three diode scheme but it helps to be on the air when doing it.

I like to work 'em hot too. Keeps me alert and creates a unique experience.

have sometimes used a single diode from the modulated B+ to ground (reverse biased) with a resistor in series equal to the modulator load impedance at about ~5% of the modulator rms power.

I thought this was interesting but it would seem to me that you would really need to hammer the audio for the diode & resistor to work. Didn't someone use a similar scheme with a mill-amp meter in series with the diode. when the meter started to waggle, your are hitting the base line??

[WD5JKO]

have sometimes used a single diode from the modulated B+ to ground (reverse biased) with a resistor in series equal to the modulator load impedance at about ~5% of the modulator rms power.

I thought this was interesting but it would seem to me that you would really need to hammer the audio for the diode & resistor to work. Didn't someone use a similar scheme with a mill-amp meter in series with the diode. when the meter started to waggle, your are hitting the base line??

Patrick,  Yes the single diode-resistor idea is only there to transfer the modulator load from the RF tube to the resistor whenever the Mod B+ goes below zero; a mere fraction of an AC speech cycle.

So what causes splatter when we overmodulate? In my opinion splatter comes from two main issues:

1.) Modulator, if using high plate resistance tubes, no NFB, and being partially unloaded or an instant during the modulation cycle. The peak modulator AC out put might try to go 2-3x or more higher than when loaded properly. This will generate the 'click-click' we hear 5-10 Khz away from the carrier when the modulation exceeds these levels.
2.) The sudden loss of conduction of the RF tube when the peak mod B+ drops to/below zero volts. The RF tube acts much like the single series diode concept that started this thread.

So guys, does #1 dominate over #2 above, or is there another explanation? My series diode-resistor concept tries to address #1 above. I've never been told I'm wide when doing this even though the scope is 'white-lining' at the baseline frequently.

The series diode-resistor combo seeded expansion of the concept as explained at the following thread:

see reply 8:
http://amfone.net/Amforum/index.php?topic=22341.0

"super modulation with progressive negative cycle unloading with carrier boost". 

Here we started a progressive negative cycle attenuation that can be set to whatever ratio and thresholds you want. This could be transparent to -90% mod, then kick in a divide by x ratio for the negative peak for -90%, then 2X at -95%, 4X at -97%, etc. Then low pass filter to minimize diode switching transients. The circuit shown will show a 200% + sine wave at -100% sine wave without any hard clipping. Back off the audio to 90% +, and the negative modulation is also -90%. The asymmetry produced when driving hard is primarily 2nd harmonic distortion which many folks like to hear..

None of the other circuits I've seen do this. Sure the 3 diode circuit most prefer keeps the mod iron loaded, and addresses both #1 and #2 reasons for splatter above. That said, the audio waveform is hard clipped, and the distortion will therefore have a lot of odd order distortion products.

Jim
WD5JKO 



 

[K1JJ]

Jim,

Your progressive circuit attached below looks very interesting. I like the use of the progressive negative peak limiting as the positive modulation is increased.  I was thinking that the three voltages could be created using three simple zener diode circuits off the same supply.  

One must be sure they have the modulator headroom to make use of the 150% - 200% modulation. That's not an easy task for the average plate modulated rig.

I'd be curious how the splatter filter looks on a simulator. Hopefully it's not too big of a load on the mod xfmr.


I may try a modified version of this circuit later with my 4X1 rig - once the new E rig is working.  


T

[WD8BIL]

Looks like this cutoff is around 12Khz.
(I assumed termination Rs at 8K)

[WD5JKO]

Looks like this cutoff is around 12Khz.
(I assumed termination Rs at 8K)

I bet the splatter is really bad without it.

Hoisy had some good ideas, but this may have not been one of them. The thing I don't like (besides the filter) is the big resistor in series (the 25K). I understand the principle and how it works, but it’s probably better suited for a low power transmitter than it would be for a high powered one. Try finding high power low mu tubes for your modulator. I don’t think 810’s, 833’s, or 4-1000’s are going to work very well and if the feedback alone would be enough.

Brian,

   The splatter really wasn't too bad with HV fast recovery diodes in there. As for the filter, all I did was play with the C-L-C values while listening to my SP600 which was tuned away from center using narrow IF bandwidth looking for splatter. I did not model it. That choke used has the iron core removed to lower the inductance. I do not recall now how I measured that inductance since I built this rig in the 1980's. So maybe the actual inductance was larger.

    If I were to try this circuit with a tube modulator at high power, I would add the series diode-resistor combo discussed earlier in this thread at the left side of the circuit, and return the network to the highest bias supply voltage. This will insure the mod transformer always has a good load upon it. Maybe we could call this the '5 diode ultra modulation circuit'.  

    For kicks I went into the shed and took a snapshot of the modified Viking I that I did this to. see attachment. I wish I had room to set up a bigger station.

Jim
WD5JKO

[Opcom]

In the "page 3" circuit, how are the resistor values determined?

[WD5JKO]

In the "page 3" circuit, how are the resistor values determined?

  Good question! I did this in 1986... But it went something like this:

* What is the maximum % upward modulation you can achieve? In my case it was over 200%..
* When do you wish to start attenuating the negative modulation? a 150v threshold on a 750v supply for    the first diode says we kick in at 80% downward modulation. For 100v - 87%, for 50v - 94%.
* Figure the parallel equiv of the three resistors (each in series with a diode) versus the upstream resistor, and make that resistor ratio fill out but not quite clip the downward modulation peak. The more bias supplies, and resistor-diode networks the greater the detail you can preserve in the compressed downward peak.

Here is an example of a low level AM rig using this concept running 150% upward modulation and about 95% downward modulation with sine wave drive:
http://pages.prodigy.net/jcandela/CE20AQRO/j.jpg.JPG

The circuit, and whole story is at:
http://pages.prodigy.net/jcandela/CE20AQRO/
The PNCL circuit is a little different being low level, and the upstream diode/resistor is the 12AT7 plate resistance. This rig sounds quite nice in the station monitor even when driven hard.

Jim
WD5JKO

[WD5JKO]

It’s fun to think there are betters ways, but...

   Yes you are right! I've made a tweak to Heising modulation overcoming a longstanding problem (look for my Gonset G50 posts), and on this post I've introduced an alternate method to ultra modulation citing two real world examples that I've built. Maybe I should have listened to you Brian, it would have been easier to just accept that the state of the art with AM was reached decades ago. Sorry, but I just don't accept that idea.

Jim
WD5JKO

[WD5JKO]

I'm just looking at it from a practical standpoint Jim. The amount of work it would take and the number of bias supplies needed to make it right would be rather large.

  Brian, The reason I corresponded with Hoisy, W4CJL was because he wanted to know 'what the heck I was doing'. In a long 80m roundtable one noisy evening, my lowly Viking I stood out from the rest. If you can boost the audio on AM by 3 db without splatter, and still sound clear, then that is huge. At that time I also had carrier boost in addition to the ultra mod circuit. That kept the diode detectors in receivers happier. I am not aware of this ever being done before, or since.

Jim
WD5JKO

[Opcom]

Jim I noticed a screen resistor for the final. Would a choke in series with the screen supply work as well in cases where the plate volts are much higher than screen, like a 4-1000?

[WD5JKO]

Jim I noticed a screen resistor for the final. Would a choke in series with the screen supply work as well in cases where the plate volts are much higher than screen, like a 4-1000?

Patrick,  You made a good point. I know that the screen grid dissipation was high. With both the carrier boost + the heavy audio, the screen would turn red visible to the eye with a sustained audio note.

 The carrier boost was adding 300-500v to the B+ depending on speech level. I did that because 200% mod + to many receivers came through raspy mostly due to the distortion curve of a rcvr diode detector. So with just carrier control (AC mod transformer disconnected), the carrier would go from 100w to 200w by whistling into the microphone (B+ jumps to about 1200v or so).  

  I doubt it would be easy to do this with a tube modulator. The big Crown M600 I used had zero source impedance, and essentially infinite power (1340 watts RMS at 4 ohm load). So adding on two mod transformers (1 for carrier boost, other for modulation) was easy..just hook it up and go! I had a variac in there too for the carrier boost transformer.

   Anyway back to the screen supply for the 4D32. Aside from a g2 dissipation issue, the upward modulation was perfectly linear up to 200% and beyond (clipping was ~ 250% as I recall). Ironically the modulation transformer came from a Harmon Kardon Citation V tube amp (NO DC through secondary). This amp was rated at only 40 watts RMS, but this rating went down to 7 Hz. The iron was big and heavy, and I didn't modulate much power below 100 hz. I forget the transformer math, but doesn't the iron area designed for 40w at 7Hz allow for 400w at 70 hz? I know it goes something like that. So for 200% mod with 150w DC input, I needed 4X that of 100%, or 4 * 75W = 300w RMS audio. The carrier control sure complicates that.

    I hope to resurrect the old Viking I someday. Storage has resulted in front panel rust, and frozen knob shafts. The Crown M600 is still here. Powering that brute up will either cause smoke, or a successful power up.

Here is a slightly off topic, but interesting take on screen voltage:
http://oestex.com/tubes/screens.htm


Jim
WD5JKO

[K1JJ]

As used in the 3-diode high level Ultra Mod circuit:

Has anyone done actual tests on standard power supply diodes vs: high speed diodes for AUDIO use?

I'm using those 6A 1KV standard Silicon Valley diodes for all three legs of the 3-diode circuit in my 4X1 rig. (20 per leg)  I used them cuz I had a bag of them here left over from another project.

I wonder what the difference would be at the higher audio frequencies, like 7kc. I know most feel the slower diodes may produce more splatter, but has anyone run a test or can recommend one to evaluate both type diodes out-of-circuit?

Maybe the difference isn't worth the trouble of my switching over to the high-speed ones.

T

[K1JJ]

Hi Brian,

That's a good point about the diodes blowing out.

I've had a tube PDM rig which used damper diodes to snub the 130kc residual spikes coming out of the filter. I tried using regular diodes and they immediately heated up and shorted. The high-speed ones ran fine.

In addition, I have blown one leg of my 3-diode circuit once already in the 4X1 rig. (Regular diodes) I added more to the leg (now 20) and it's hung in there so far.

So, it sounds logical that if they don't hang in there, the slow response will generate heat and pop them. Or maybe make them more vulnerable to spikes.  Maybe I will think about faster diodes, BUT if they hang in there, I wonder if they are close enuff in performance to the high speed ones, splatter-wise. That's the question.

Maybe someone will suggest an audio sweep test of some kind. I wud think a resistor in series with a diode with a sig gen across the pair wud show the relative drop at various freqs.

T

[W2PFY]

I heard Dirk WA2CYT complain that his diodes were not conducting fast enough and someone was sending him faster diodes. So JJ the next time you hear him on you could ask "Notice I didn't say might ask"  him about his adventurous with the new and old diodes. That is all. Over.

[KE6DF]

I heard Dirk WA2CYT complain that his diodes were not conducting fast enough 

Has anyone tried vacuum tube diodes for this purpose -- like in three diode limiters or just the single diode approach.

Tubes like 866, 3B28, or 836's.

I'm not sure how MV or Xe diodes would work at higher audio frequencies, but 836's (or bigger vacuum recitifiers) would.

[K1JJ]

Has anyone tried vacuum tube diodes for this purpose -- like in three diode limiters or just the single diode approach.

Tubes like 866, 3B28, or 836's.

I'm not sure how MV or Xe diodes would work at higher audio frequencies, but 836's (or bigger vacuum recitifiers) would.

I've used larger vacuum rectifier tubes  as PDM damper diodes. (non-mercury vapor) They showed some color but appeared to work well. In contrast, the standard silicon diodes crapped out immediately.  I don't have an opinion on how fast they are, just that they hung in there.  I've heard that mercury vapor are not as good at high-freqs, but unconfirmed.

T

[w4bfs]

I heard Dirk WA2CYT complain that his diodes were not conducting fast enough 

Has anyone tried vacuum tube diodes for this purpose -- like in three diode limiters or just the single diode approach.

Tubes like 866, 3B28, or 836's.

I'm not sure how MV or Xe diodes would work at higher audio frequencies, but 836's (or bigger vacuum recitifiers) would.

W8VYZ reported using 836 in his ultra-mod circuit ... he said ss rectifiers were not fast enuff ... maybe schottkys or freds

[KE6DF]

One thing I do remember is I would occasionally hear a hum on Bill's signal from time to time. I have not heard him in the past few years, but I do remember that hum and he's had the tube limiter forever.

It’s possible it was caused by something else like his plate supply, but I wonder? Could some 60 cycle interference have been getting induced because its connected on the output side of the mod transformer with both the final AF and RF involved?

It could be, but an 836 has a 2.5 V filament.

So, if it was used in a plate circuit running at 2KV, and the entire 2.5 VAC was added as ripple, it would be only 0.125% ripple.

Most likely the plate power supply had more ripple than that to begin with.

Plus if you ran the 836 filament off a tansformer with a CT and took the plate voltage off the CT, you wouldn't expect even that much ripple to be added to the B+.

That's what you would do in an audio stage using directly heated triodes like 2a3's which operate at lots lower signal levels without a ripple problem.

[KE6DF]

Makes sense, but what happens to that .125% ripple when the tubes are heavily loaded say like when it really hard limits the negative peak?

Is the .125% at idle or at full load?

Well, in this case the 2.5 VAC is the filament voltage for the 836 and that doesn't vary with current through the rectifier. So the ripple introduced by the 836 would be constant under load, it seems to me.

But, on  your side of the argument, that is 2.5 VAC RMS and so it's really P-P of 2.8 times that much. I think ripple precentages are usually calculated as P-P.