The AM Forum

I'm in the process of modifying and getting an Elmac A-54 on the air.  In a previous thread there was a discussion about some simple audio mods for the rig, and I'm going to be running line level audio into it.  However, looking at the modulation transformer in the schematic, it looks like Elmac had a simple negative peak limiter designed when zeners weren't made yet.  This could be improved too using the zener negative peak limiter, right? 

I'm thinking this would be needed since I'll probably go over 100% modulation with line level input.  I don't want to destroy the mod transformer, and I want to keep this rig mostly stock except for the few minor audio mods. 

Jon
KA1TDQ

I like the old A54 neg peak limiter ....it is a sort of keep-alive-ckt

what you are proposing is a very 'hard' limiter .... I would consider softening the action with series resistance between the diodes ... you could also add an led / parallel resistor combo that will give visual indication of limiting action ... watch the insulation of that hV ckt

The M-E keep alive does not load the transformer during negative over-modulation.

The zener idea needs some series resistance as stated.

Something simple to meet both needs might be a rectifier in series with a resistor 2-3x the modulator's load impedance, with the diode cathode connected to modulated B+ and the cold end of the resistor to some voltage of 10% the plate supply value.

Taking the original circuit, this could probably be done re-using the same parts.

This would load the modulation transformer a bit on the negative half cycle and also keep negative modulation peaks to 90% or so. There are a few discussions on the board about this, even simulations.

I use this same scheme on a large TX, 3KV supply, 300V keep alive (200mA supply), an 8020 rectifier or SS stick substitute, and a 10K resistor to the 300V keep alive. At first I just hooked the  diode to the 300V supply, but when modulating too hard by accident, the modulator plate current would hit 800mA!! the resistor is needed.

Hi - use this one.  The circuit using the zeners is hazardous to your modulator's health (particularly if it's a good modulator).

The circuit shown here - I have used in 5kw broadcast transmitters and many other tube transformer coupled modulators over the years.  It works very well, and is modulation transformer friendly.  I use this circuit in every tube transmitter I modify.

Regards,  Steve

(http://www.classeradio.com/3-diode.jpg)

As seen on CBJUNKIES! :)

http://www.cbjunkies.com/threads/115-Three-diode-negative-peak-limiter (http://www.cbjunkies.com/threads/115-Three-diode-negative-peak-limiter)

Back in Mass, I built a tube rig that used just the simple Zener negative peak limiter. Everyone said that I sounded great (a genuine compliment in that area of the world). The setup used a modified Heising setup and I used it on the choke portion.

This will just be on a 60 year old mod transformer, so I'll go with the keep alive circuit. I'll resist the urge though to put the rig on 11 meters. :-)

I've started cleaning the chassis and getting the grime off the tubes. They look good and gleam now. It's going to make a nice classic rig and sound freakin' awesome!

Jon
...from phone

I have used Steve's high level negative peak limiter and it works extremely well.  With the keep alive supply being adjustable, you can set the negative peaks anywhere you want.  Also, the secondary of modulation transformer is never unloaded which protects the mod iron very well. 

Joe-GMS

deleated-duplicated post

Joe

man, you gotta just love that attack chicken ..... 10-4 10-4

Yeah, that chicken is really something !  It really went after the guy in the little video.  I used to have some geese that would do that - but only women.  If you were a guy, the geese would leave you alone.

something about hungry eyes .....

https://www.youtube.com/watch?v=2ssCL292DQA&spfreload=10

S,

I sped through the link, but I couldn't find a chicken. There could have been a goose or two, but I didn't see it.


klc

That circuit has been around since the 50's at least, its in a qst using tube rectifiers and a bunch of filament transformers.

When I ran oit with the 14kv brick diodes I got reports I was very wide (pre sdr days).
I was told it had something to do with the attack time of the diodes.
I also worked someone who told me how to improve it, but do not remember who or what.

I think doing it in the low level stages is better then doing it in the high voltage side of things.

You don't need three diodes for the keep alive circuit.  I use one diode (made up with a string of TV damper diodes, very high speed).  The power supply voltage is made to sag when a little current is drawn from it.  I use about 10% of plate voltage.  The supply is made with a small xfmr and a filter circuit.  The filter is a RCRCRC, the resistors are about 3K each and the last cap is only about 1mfd.  I also have a LED in line which will flicker when current is pulled from the supply.  The supply is left on at all times.  On standby, when the plate voltage is off, the LED will light showing the final is drawing some small current from the supply.  As soon as the plate voltage returns the LED goes out until over modulating.  The circuit works perfect and there is no splatter due to the sagging keep alive voltage.  You also don't need any load resistor as the internal resistance of the keep alive supply presents that load to the mod iron.

Fred

Mine is also a single diode and runs like that save the supply is a regular LCLC filter type, the rectifier is an 8020 so that's about 1K resistance.
If I wanted to seriously use >100% modulation I would add 7-9K in series with the rectifier.
Mine's just set up for 'normal' negative peak limiting so the rectifier's 'resistance' is enough. If I do hit it hard, the modulator plate current meter will hit 800+mA due the negative peaks.

The advantage of the one diode setup is a smooth limit up to a point, whereas the 3 diode gives a hard limit all the time. I don't like it because the corners where the waveform goes to keep-alive may contain pretty high frequencies. Granted the power's very low there. It's just a preference.

I used the Zener method on my AF67.  A large voltage one and a series of lower voltages.  Used a scope to find a tap on the diode string just above the clipping point.  Later I added a resistor to smooth the clipping.  I use the limiter only to protect the modulator, not to increase positive peaks.   The complete circuit was in ER a few years ago.

Patrick.

Exactly,  you want to avoid that hard clip, those corners is where the splatter comes from.  My supply allows for a soft clip, the corners look rounded.  I designed the set-up and it seems to work FB.

Fred

I use the 3-diode keep alive circuit on my single FET rig on 40 meters and I haven't heard any complaints about splatter.  Granted, I'm only running 30 volts on the drain and 25 watts carrier, but my linear puts me at legal limit with 150% positive modulation. (I use an RF attenuator between the transmitter and amplifier input)

Tooting my own horn, someone told me in Tucson that I sounded absolutely wonderful on the air last night.  Broadcast quality audio, yadda yadda.  Gosh, I'm such a great guy... :-)

Anyway, seems to work.

Jon

Jon,

   The modulated B+ decoupling capacitor on your rig will attenuate some of the higher harmonics caused by the abrupt limiter you use. The bypassing here can be used as a tool to hide the transient effect of the occasional voice peak that gets clipped. The mere mention of adding a C-L-C Pi low pass filter to the modulated B+ is guaranteed to generate a fire storm of responses. So hide that 'L' and let the 'C' work with the leakage inductance of the modulation transformer. That way nobody knows there is an 'L' in the circuit.

   One thing for sure, we will not get a consensus on which way is best. Many folks have had bad experiences with these circuits, and therefore "they do not work". Then someone else comes along to refute that claim with something that not only works, is powerful, and sounds great. Jon, today that be you. :-) 

   Since we are comparing whose circuit is best, let me toss my circuit into the mix!  ;D

   Remember the Japanese transistor radio's? They were rated by the number of transistors. Same might be true of high level negative peak limiters. Mine has 4 diodes, how many does your have!!

http://amfone.net/Amforum/index.php?topic=30945.0

   I make no claims which way is best, only that for a given circuit, take the time to make it work as intended with your particular transmitter.

Jim
Wd5JKO

Why not just do the negative peak limiting in the low level audio stages? I don't know if you're using the Elmac to drive an amp etc. You could use 1N34 diodes set up in the speech amp, and have them clip the negative peaks and let the positive go as high as your voice characteristics allow etc?

There are lots of of outboard devices out there too as you know... CRL etc etc., and freeware multiband processor programs and analogue hardware to do what you want, especially at line level? Its personal choice...  In that case you don't have to deal with any potential crap out issues with components connected across the mod transformer with zener diodes etc?


Al VE3AJM

In theory this approach works well IF the transmitter has a flat frequency response over a very wide range, and there is no overshoot.

Negative peak limiting is best done with a DC coupled system (or at least something that comes close).  Otherwise you get waveform "tilt" and waveform integration - integration because asymmetrical limiting causes a DC shift.

In the class E rigs, negative peak limiting is indeed accomplished at low level, but the modulator is DC coupled so everything works correctly.

We don't have that luxury here, particularly with amateur grade modulation transformers and probably a less than flat frequency response all the way along.

I am betting one could accomplish negative peak limiting in the low level audio with something like a Flex 5000 or similar transmitter that has a wide, flat frequency response.  This would be a worthy experiment!

The hard limiting of the 3 diode circuit can/may create splatter. Never saw any caveats about that. Very wide response potentially. A low pass  audio filter to limit high freq clipping products would be in order whether high or low level limiting.

Al VE3AJM

Does using the 3-diode negative peak limiter with a modified Heising setup differ from that of just a straight modulation transformer with respect to hard clips?

My single FET rig is modified Heising and I plan to just use the mod transformer with the Elmac.

Jon

What's needed is a negative peak limiter that applies about 30 microamps to the user's carcass when adjusted to produce spurs.

Either that or one of those sissor mechanisms driving a boxing glove like in old cartoons.

Really not sure why people are obsessed with meters swinging when using AM. This is not in reference to the OP.

The ideas of protecting the modulation transformer from failure and minimizing buckshot are fine. Problem is when the protective limits are exceeded.

Most older plate modulated rigs do not have any audio reserve to begin with. adding a waveform bender looking for higher positive peaks is not going to play well with the neighbors.

None of the circuits mentioned here are foolproof. Most would benefit from a plain old hard audio limiter at the end of the audio gizmos.

There's plenty of peak power to be had from a system having dynamic headroom  with enough audio bandwidth to include low notes polarized correctly. If you have a voice like a little girl all bets are off.

If the whole world used advanced forms of AM detection this situation would change.

That's true. But like anything in life, it all boils down to whose meter swings the farthest. Mine swings to 150%.

Jon

Peak meter swings are mostly meaningless. It's the average that will determine loudness.

As Dave noted, most older plate modulated transmitters do not have enough audio reserve to modulate very far over 100% in the positive direction. I recommend using a high-level negative peak limiter only to prevent splatter and to protect the mod tranny. Select the correct audio polarity and the positive modulation will automatically be greater than the negative - no limiters needed. This is all you need with these types of transmitters. Reserve any processing to increase the average, if you want an increase in loudness.

Homebrew tube rigs with big modulators and the proper transformers, appropriately modified amateur grade transmitters, some BC TXs, low level modulated rigs and Class E rigs are different animals.

Here is a schematic of what I did in my AF67. I believe I did a similar thing in my A54 mod. It has been quite some time ago but as I remember in testing it did seem to do the job.

In doing my screen modulation experiments, I found it very interesting that you get a lot more positive modulation because of the lack of phase shift and frequency response limitations.
My plate modulated rigs have plenty of clean audio power, at least as much as carrier output, and they have good mod transformers (best I found outside of broadcast iron), yet 4X carrier is the best I could ever do, and 3X was more likely.

Screen modulation does 5X or even more.

Trying to brute force a marginal rig with a high level NCL circuit seems the wrong way to go.

Yes, no problem with modified heising and the 3 diode circuit.  You insert the 3 diode NPL between the heising reactor and the RF amplifier.  Basically, the NPL goes between the RF amplifier and whatever is supplying it.

That's how I built my current 40 meter rig and it sounds awesome.

Back in Mass, I built a plate modulated single tube rig that was also modified Heising. At first, I just used an LED to indicate negative peaks. I would just speak softer when it lit up. That was really annoying.

I then incorporated just the Zener clipper circuit and that fixed the problem. I'm also sure that splatter wasn't a problem. Living in the northeast, if I were, someone would've told me I'm sure.

I've built two rigs with these circuits with nothing but compliments on my audio. Any future rigs I build will use them too. Modern modulation methods allow for >100% talk power. Why whisper into the microphone?

Jon

You really have to pound on the N.P.L. to generate noticeable splatter.  Splatter from overmodulation in the negative direction is worse.

With special circuits, especially experimenting, Please do not rely on meter swing. Please refer to scope patterns.