Valiant and other clipper circuits: The magic, the mystery, and the scorn

"The first thing you need to do it tear out that circuit!"

I've heard this stated over and over to those looking to improve the audio of their Johnson Valiants, Eico 730 Modulators and others that employ such a circuit. Dutifully, these ops drill and blast the circuit out without an understanding of why they were included in the design to begin with.

As is well known, an increase in average modulation will result in an increase in loudness which is vital for making the most of a transmitted signal. This can be done to an extent with audio compression, but it involves a bit of circuitry and audio peaks still restrict the benefit of compression. The problem is that peak energy in the waveform is sampled to determine attenuation even though it doesn't contribute to loudness. Think of audio peaks like static discharge in a dry room when you touch a grounded piece of metal. That spark can be in the tens of thousands of volts but it's of such short duration that you remain alive due to the lack of energy.

In a transmitted waveform, the amplitude of peaks can extend far beyond the average waveform energy, but the modulation has to be restricted to prevent them from cutting off the carrier or damaging an amplifier. Average modulation would rise significantly without these peaks, so they need to go. The engineers at E.F. Johnson and EICO knew this and included a clipper circuit that achieves loudness by clipping off the undesirable peaks thus raising the average energy of the waveform.

So why the universal hatred?

The downside of clipper circuits is that they generate harmonic energy proportional to the amount of waveform clipping, so a filter must follow to pass desired audio but eliminate everything above in frequency. Yep, there's the issue, and "desired audio" is a subjective term.

Hamateur transmitter manufacturers through the years focused on tightly restricted audio for two reasons. First, to do it right, broadband audio requires more expensive components, such as with the modulation transformer. Second, signal to noise ratios are higher if receiver filters are narrow, and there's no sense in wasting modulation on frequencies that won't be received. While they were at it, they rolled off the lows as well since receivers tend to do that also. Yeah! Smaller mod iron! This is how the campaign for "hyellowy audio" became so popular with designers.

Interestingly, clipper circuits have been universally used in AM and FM broadcast processors for decades and their ability to achieve competitive loudness is crucial, so how are they great for Part 73 stations and hated for Part 97 stations? The love/hate relationship with clipping is in the choice of filtering component values.

Most clipper circuits are comprised of two sections: a dual diode that clips both polarities of audio peaks and a low pass filter that is needed to remove harmonic energy generated by the clipped waveform. It's the aggressiveness of the low pass filter that needs attention.

The L/C clipper circuits in the Valiant (L45, C94) and EICO (L1, C6) are nearly identical, and while I could not determine the value of the Valiant's L45, EICO used a 5 Henry reactor for L1.

Valiant clipping circuit:


EICO 730 clipping circuit:


Both employ a 6AL5 dual diode where each section clips a polarity of the waveform. Some have recommended that one diode be bypassed so that only negative peaks are clipped. While this results in unleashed positive peaks, I doubt the consequences to the mod iron and RF components justify the effort, and, you're only letting peaks with no audio energy through. Leave the peak power meter for SSB use.

By the way, a quick tip for Valiant jockeys: If you find that your negative peaks exceed your positive peaks, reversing the plate caps on the 6146 modulator tubes will reverse the modulation polarity. No fuss, no muss, and no soldering!

Using the online calculator for passive low pass 2nd order filters, an L value of 5H paired with a C value of 0.001 uF gives a cornering frequency (Fc) of 2.25 kHz. Y-I-K-E-S! At least EICO gave low frequencies a better deal by using 10 times the capacitance (C4) for coupling from the clipper compared to Johnson's 0.01uf (C84).

So I'll bet you're ready to grab those cutters and dispense with these circuits, but before you do, remember that very similar circuits are used in FM broadcasting. The difference is a significantly higher Fc and very tight control over how much clipping is used. As mentioned, the amount of filtering needed is proportional to the amount of clipping. Broadcast processors use compression *and* limiting prior to the clipping stage, so the amount of clipping and resulting harmonic energy generated is significantly reduced.

I was pretty astonished (huge understatement) to read about setting the clipping control in the Valiant manual where it is recommended that 12dB of clipping "is not at all objectionable, on the contrary, speech sounds as though the speaker is enunciating with special care".

Okay, my fellow broadcast engineers and audiophiles, you're probably making faces right now, I was too when I first read that, but we think of this as clipping a processed waveform, not a raw waveform as would be the case in these devices. That said, I feel that the real focus should be on the low pass filter and responsible clipping levels.

The value of L is WAY too high, I believe, but audio reactors are hard to come by. However, simply changing the C in the filter to 250pF would increase Fc to 4.5 kHz. Adding a compressor such as the popular Symetrix 528 would allow much lower levels of clipping to be used, and together, would add significant punch to the Valiant or EICO.

None of this addresses the many other components that deserve scrutiny when improving audio, but I felt that someone should stick up for a circuit and function that has drawn much criticism, and mostly undeserved as I hope you will agree.

 

Most of the hammy AM rigs of the era of the Valiant used too much step-down at the modulation transformer, so the modulator couldn't deliver enough voltage swing to modulate beyond 100% (positive or negative) anyway. A good compressor and peak limiter are a far better way to control modulation peaks, but far more expensive. Speech clipping and restricted audio were a cheap way to make an AM rig competitive with SSB.

Remember, back in those days, amateur radio audio was supposed to sound tinny, scratchy and distorted. Those who did try to achieve pleasant audio quality (mostly AMers), were frequently derided by the mainstream amateur community.

 

A friend may have a Valiant that he is picking up on a road trip next summer. He doesn't want it, and has tentatively offered it to me. Do I want it? I have a restored B&W 5100, but the Johnson has 160 meters. Is the Valiant worth the effort? Too many large radios here already, so I have to be selective. I used to think the BW was the ugliest SOB I have ever seen, but it has grown on me after seeing its battleship construction.

Pete

 

I understand this point being made and don't disagree with it, since the transmitters in question are under 200 watts and need help, especially when the antennas are often compromises. Ham rigs in the '50s were designed with the following constraints: No one has a way to monitor modulation peaks. Everyone is using a D104 straight into the rig. U.S. regulations limit up and down modulation to 100%. ARRL wants bandwidth limited. This is why a GK500 for example can barely hit 95% on a good day.

Broadcast clippers were big in the days of the Max twins. I have no idea about FM processing and don't follow its relevance here. For AM what followed was multiband processing and look ahead VCA limiting to a soft knee clip but now it's all completely digital (which I don't know anything about) for AM broadcast and clipping, as in diodes chopping off peaks is probably an utterly antiquated concept.

 

Hi Don and all, I've heard similar comments even today from older hams. The ones that compare good audio to shipboard like audio using a carbon mic! Another reason Johnson and Eico use these clipper circuits back in the day was for chasing DX so the audio response wasn't going to be wide. They wanted talk power, not rag-chew audio talking your bubs that are 20 over S9 on 75m. I've seen some operators abuse just about all tools or common sense if chasing a new one or a weak station. I DX as well, but I always want low distortion audio. Clipping or compression set to 10 dB is a lot! I've seen some ops on YouTube operate their rigs full tilt to the right, hi. 73 de jay/w5jay..

 

The Optimod 8600 (FM) and 9200 (AM) have clipper settings for each of the five bands plus separate HF and Final clipper settings. It’s done digitally, but these functions mimic what is considered an indispensable feature to this day.

 

Back in the 60s I experimented with a type of high-level hard clipping that was very effective. The plate supply to the modulator tubes was run off a variac, and the modulator tube grids were driven hard enough to push the tubes into saturation, causing peak clipping, much more cleanly than doing it in a low level stage using a diode clipper. The +HV was adjusted with the variac, until the clipped waveforms were just barely shy of breaking base line on negative peaks. The modulation transformer was followed by a high level splatter filter, consisting of a low inductance choke in series with the modulated high voltage line, and a capacitor at each end of the choke to ground. The values of inductance and capacitance were calculated for a 3000~ cut-off.

The filter served to attenuate the harmonics generated by the hard clipping of the saturated modulator tubes. It inevitably had enough phase shift to tilt the clipped waveforms slightly, but the clipped and filtered audio still carried a lot of punch. Much more effective than the typical low-level hard clipping like what's in the Eico 730 and Valiant. The audio still sounded clipped, but the distortion was less conspicuous.

It worked well, but there was one problem. It blew modulation transformers. In less than a year of fooling with that thing, I blew a half dozen kilowatt class transformers. At first, I thought it must be because I was using old transformers, many of them WWII vintage or earlier, and maybe the insulation had deteriorated. But when I blew a brand new Thordarson, unused right out of the box, rated for 1000 watts of audio, I knew something was wrong.

I had been testing the clipper/filter with a sine wave tone generator. The rig was a single 304-TL modulated by a pair of 833As. I ran the tone modulation up to 100%, with visible flat-topping at low audio frequencies. Then the tone frequency was increased, starting at 30~ and rising in frequency up past the cut-off frequency of the filter. It was right at 3000~ that suddenly the modulation disappeared and the plates of the 833As turned bright orange. I glanced at the big open-frame modulation transformer just in time to see smoke rising from the winding.

Then it occurred to me what was happening. Up to the cut-off frequency, the splatter filter had little effect on the resistive load the modulation transformer was looking into. But right at the knee of the cut-off curve, the splatter filter added a lot of reactance to the load. Modulation transformers are designed to work into a purely resistive load, not a highly reactive one. That's why operating a modulator with no load causes peak voltages to develop across the winding and may destroy the transformer. Running the modulator at 100% sine wave modulation right at cut-off frequency developed enough peak voltage to arc through the insulation in transformer winding.

The previous modulation transformer failures likely occurred when the audio happened to randomly contain high amplitude peaks near cut-off frequency. Right away, I discontinued the high level clipping, removed the splatter filter, and never blew another modulation transformer. I was able to repair the big Thardarson, since the arc had occurred between the top layer of the winding and one of the terminal leads that were laid over it, and the damaged turns were accessible. I was able to find some of the same size wire, carefully count turns (a few hundred) and re-wind the damaged two layers, and the transformer worked fine.

The Collins KW-1 uses a similar splatter filter, and the stock modulation transformer taps are set for an impedance ratio of about 3:1, just enough step-down to limit the modulation capability to right at 100% - probably intentional design to reduce the likelihood of accidentally over-modulating. I suspect a lot of KW-1 owners, inadvertently or not, have driven their modulator stage into saturation, and a rogue peak near filter cut-off caused the transformer to go, just as it did in my case. The KW-1 has a reputation for blowing transformers, while the Johnson Kilowatt uses the exact same transformer, plate voltage and PA/modulator tube line-up, and modulation transformer failures in the Johnson Kilowatt have rarely been reported.

Another lesson from this story is to never try to run a plate-modulated transmitter, for a sustained period, at 100% modulation, with a sine-wave tone. And it would be a good idea for KW-1 owners to permanently remove the splatter filter from the circuit, and re-adjust the modulation transformer taps to the turns ratio with less step-down, allowing some positive peak headroom.

 

Too many unknowns. Do you want to operate 160? What are your goals on 160? Just work hams on phone within 200 miles? I don't particularly care for any rig with a built-in VFO. Ignoring that, when 160 conditions are good as they have been lately, a Valiant with a good antenna can work out okay. But 160 condx aren't always good, and most hams have no interest in putting forth the effort to achieve a good antenna. It's a bit like when 10 is hot, it doesn't take much, but 8 or 9 out of 11 years, a beam and more power is really nice. Right now, a D104, Valiant and low half wave dipole can work. Most of the time though, more power, processing and a vertical with a lot of radials and a separate rx antenna are again, really nice (especially for the ham at the other end). Ignoring 160, the 5100 may very well be a better rig (I have never owned or operated one). The RF power difference isn't enough to matter much.

 

I think a lot of the audio controversy stems from hams with experience limited to one kind of AM rig making general statements about audio gear and circuits that encompass all AM rigs. I have been on the receiving end of "all you need is a D104 right into the rig" a few times, and I note this always comes from hams who's experience is limited to restored vintage manufactured-for-ham-radio rigs that are either stock or very near it. They are not thinking of solid state rigs, or broadcast rigs, or homebrew built with broadcast designs and parts. In my opinion, they are correct, if they confine their comment to vintage tube ham rigs with little or no modifications. If I were to put a Viking 500 on the air, I'd probably run a D104 right into it (with a 5 to 10 meg grid resistor) and call it done. Unless you go all out to make a crystal element that's in good shape sound great with external circuitry, the ham gear sounds listenable, because the limitations of the stock design (single ended stages, cheap audio iron) mask the limitations of an unbalanced xtal element at a single ended mic preamp input. On the other hand, if you run a rig that can support some processing, your microphone etc. options expand and you can wind up with some gear and mics that work well. It's just a matter of taking into consideration what you have for a transmitter and setting up the audio source accordingly. I have a homebrew rig built for broadcast, and I initially tried running a couple of xtal mics right into it. The result wasn't real good. It was nearly impossible to not clip the carrier, and the D104 to my ears sounded kind of harsh. Almost of necessity, I put a couple of processors in between for audio intervention. Some rigs almost have to be run with processing; others, you don't want it.

 

Interesting discussion.

I used the clipper in my Viking 500 for well over a year back in the 80s and am familiar with it. I had friends with Griefkit A Patch Me transmitters who did the same. Yes, they knew which knobs to turn where so we can leave that variable out.

Those who subscribe to "engineers knew what they were doing back in the day" mantra may wish to consider factory built AM transmitter sales was a business. Economy and features maximize profit.

The EFJ clipper circuit was the least expensive way to increase communications effectiveness back when these transmitters were sold new.

Marketing types probably insisted on features which made the sales more competitive. The Valiant had a number of extras like an extra 6146 in the RF output stage, same final and modulator tube type, phony patch input, capability of ssb "linear"(god awful in practice) operation, 160 meter capability, very wide range output network, really good CW keying and built in VFO.

Use of the clipper necessitates throwing away a good part of the voice spectrum mostly because of the choke following the clipper.

Today there are much better ways of maintaining audio effectiveness without the necessary audio low and upper low end audio rolloff.

Operators who know how to do basic repair and have some patience can make this happen without spending wheelbarrows of cash on brand name professional broadcast processors.

One way to put this discussion to the ultimate test would be for someone who has a transmitter which otherwise proofs excellent frequency response and modulation capability to utilize the EFJ clipper/filter for a period of time. I know exactly what will happen : )

Part two would be to take a fairly unmodified Valiant, bypass the clipper/filter and feed that with multi band compressor/limiter. Now all we need is a volunteer.

 

Funny you should mention this. My Valiant is driven with a Hnat-Hindes CSP-2013 wideband compressor/clipper (with B'cast mods per the manufacturer). The audio is inserted at the grids of the 6146s using a 600:8,000Z CT transformer.

I was actually thinking of running the Class E transmitter with just a clipper + LPF circuit, possibly tonight. Just need to set up a bal/unbal interface and be very mindful of mic gain.

 

Well Clark you are safe for now. I couldn't get on the air as the tower was nested due to high winds.

I was going to listen to a Hendrix CD during my 45 minute cardio workout but chose to listen to you guys instead. I figured you would have better harmonic content.

Alas the Valiant clipper was not in use.

 

Is that the Mic Maze?

That mental state seems to especially affect certain Collins owners. Art was a genius, Collins stock, is perfection, any non-factory mods an abomination.....

 

Yes, with the Steve Hnat mods for AM which drive the compressor and clipper harder in addition to modified pre-emphasis and LPF values.

 

http://www.tubebooks.org/Books/orr_radio.pdf

Chapter 15-7 describes the Valiant type clipper/filter and reminds us that it is not at all similar to clippers used on much newer multi-band broadcrap processors.