OK, here's the 'final' circuit. This is for the 'standard' model, no variable to mess with.
The unlabeled diode is the existing 'anti spike' diode all radios have coming from the modulation transformer. You wrap the two other diodes around it.
This is the way the circuit works.
The diodes and resistors form a divider that allows the D2 to ONLY conduct during NEGATIVE mod peaks.... The HARDER the negative mod peak, the HARDER the diode conducts. During this conduction, it allows B+ (tube speak... Same as "the hot lead" in 12 volt stuff) from the resistor and capacitor to conduct through D2. By varying the resistance of the approximately 50 ohm resistor, you can vary the amount of negative peak limiting that the circuit provides.
I did NOT invent this... This is an adaptation of different peak limiters.
This circuit does NOT work 'just like npc' on the 148 or other class A series limiter radios.... IE, it DOES work at an audio rate, but it will NOT prevent negative 'peaks'... It prevents the carrier from being COMPLETELY pinched off. THIS is what causes the MAJORITY of splatter and garbage. This circuit will NOT clean up the waveform... I can add a splatter filter (to clean up the pos and neg peaks), but those add distortion of their own.
Also, the approximate 50 ohm resistor. It will run COLD if you don't 'bang on the limiter', so to speak. The MORE negative peaks you add (ie, the HOTTER you run the audio gain / power mic / etc), the HARDER this resistor has to conduct..... With a stock mic, a pair of 1/2 watt resistors in parallel (100 ohms) can be held in the hand. With the 'typical' levels a power mic produces, they will get WARM....
FOR THOSE THAT WISH TO MODIFY THE CIRCUIT!!!
If you want variable negative peak limiting, then take the "approximate" 50 ohm resistor and replace it with a simple LM317 or other 'variable' regulator. Copy any typical schematic for the voltage regulator.....
The 5.6 ohm resistor I made with a pair of 10 ohm half watts. The total package needs to be able to pass 1/20th of the total AUDIO power fed to the final... In a typical CB
, this means it needs to pass ABOUT a half watt, average. The diodes need to be able to stand 4 times the unmodulated input voltage (13.8 on a transistor rig).
IF YOU WANT TO DO THIS TO A DIFFERENT 'STYLE' CHASSIS, here is the math:
The diodes are 1/2 the final plate impedance.... IE, (Voltage x voltage) / (2 X Power out) and take that answer and divide it by two. For a 50 watt MOSFET, it would work like this:
13X13 = 169. 169 / 100 = 1.69 ohms. Half that. .8 ohms. This is the MATHEMATICAL way to find the resistor value. once it is found, you can place a RHEOSTAT or pot there to vary it to get the level you want. Be careful, in tube stuff, full B+ can be across this resistor, depending on mod index. You can use the same LM317 to feed the circuit..... Tubed or transistor, you only need a few volts to ensure the carrier is NEVER pinched off.
Want to put this in a series modulated radio? Like the 148 / etc? Do the standard NPC volt the final mod, and the wire that goes to the driver, you cut. Insert a "anti spike diode" in series, and otherwise wire up the circuit above. Is it easier than a cap? No. BUT, if you have a buttload of these made up, it makes it easier, and if you put a variable power supply on it, you have a LOT better method of keepalive than NPC...
Incidentally, the power supply that feeds the keepalive circuit MUST be low impedance. Therefore, even though most other schematics that are similiar to this show it with the cap being unnecessary, I keep mine in circuit ALL the time.... Low ESR cap means you get a low R power supply.
Enjoy. Any questions, please feel free to ask. Simplest circuit I've come up with , and this will do all that is needed.