A problem I had with keep-alive experiment was that when the modulation ws forced negative enough to activate it, it created some splatter as the modulated B+ transitioned from the downward-slope of the modulating wave to hit the hard floor of the keep-alive voltage. The only way I can describe it is that when the switching point was crossed, that little 'corner' in the waveform was full of high frequencies. Maybe I did it wrong but I couldn't avoid it.
Patrick,
You might remember when you and I were doing modeling of the three diode circuit, and variations thereof. I was using a LPF for the reasons you and Gito have mentioned. When using SS diodes, the reverse recovery time is a consideration as well. When I actually ran a multi-diode circuit years ago, I did have a LPF, and splatter was not an issue even at 200% positive peaks. I did use HV fast diodes surplus from HV switching power supplies.
More recently, I developed a Ultra-Mod circuit for the SS Retro-75 XCVR. Here I was able to get by with only one diode, and still had a keep alive supply which acted somewhat as a LPF as well. This arrangement with a larger Mod transformer, allowed clean peaks to 150% while limiting the negative peaks to just under 100%. I attach that circuit here. I wonder if this idea would be workable when scaled to high impedance tube circuitry.
Also, the idea to process the audio prior to going in the rig might work when the modulator is transparent input to output. Broadcast transmitters might do this, as well as some of the Class E setups. On the other hand, take the average Ham transmitter like a Viking II, DX-100, etc. where we have multiple transformers in the audio chain, along with frequency limiting (hi and lo) R-C circuits. Here we simply cannot gain much from upstream processing, especially if we wish to enhance the positive peaks by embracing asymmetrical modulation. Therefore we must 'process' the audio at a point where there will be little to no phase shift, or level shift following the processing point in the audio chain. With amateur transmitters like the ones I mentioned, the only place to minimize the phase shift, and level shift problems is AFTER the modulation transformer.
Jim
WD5JKO