R-C Coupled Amplifier

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Resistance-Capacitance coupling is the important method of coupling the signal from one stage of amplification to next stage. In this method the signal developed across the collector resistor of each stage is coupled through capacitor in to the base of the next stage. the cascaded stage amplify the signal.the overall gain is equal to product of individual stage gains. the amplifier using R-C coupling is called R-Coupled amplifier.

Having designed and built a couple basic RC coupled vacuum tube amps with inexpensive parts, I wanted to take things to the next level, both in design and parts quality. I wanted to work with some new tubes and employ some of the many advanced circuit design techniques I have been studying. I also came across some big surplus iron, and wanted to put it to use along with good Magnequest outputs and plate chokes.

First I was going to build a Morrison Micro 2a3, having slowly figured out how to design a direct coupled circuit. Direct coupling of course is where the output tube sits at a relatively high voltage, such that the cathode is at a voltage equal to the plate voltage of the driver, minus the bias voltage of the output tube. This eliminates the need for a coupling capacitor. Then I saw the various Monkey designs, where the voltage supply for the driver comes from the cathode circuit of the output tube. This reduces the amount of current the circuit draws because the driver recycles some of the current that would normally be dissipated in the cathode resistor. This recycling effect is what had led to Jeremy Epstein calling his design "Free Lunch". In a circuit like this, the amount of voltage dropped across the plate load of the driver is exactly equal to the bias voltage of the output tube. Usually you would want some margin of voltage to allow the driver to swing more voltage than the output tube would require to drive it to full output, but in this design it is not possible. This led me to believe that a choke was the only appropriate plate load for the driver tube. A resistor would need to be much larger than the small amount of resistance needed to drop the bias voltage, and a current source would need a few volts of headroom, further limiting the drive voltage available. I hoped that a choke would allow the full driver swing, and perhaps a bit more on transients due to the choke's ability to store and release energy. Click here for more details...