"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.