Tube CAD covers 13 tube circuit types, each of which subdivides into four variations. Some of the variation differ from the textbook archetype in  only secondary support devices, while others are topologically so different that they only share a common function with the textbook circuit.

Cascode Amplifier
The Cascode amplifier works by realizing all of the bottom triode's transconductance by isolating it from the plate resistor and load resistance. This allows the grid to fully control the current through the triode and thus define a gain across the plate resistor. This increased gain, which can easily exceed the amplification factor of the triode, comes at a price; increased distortion and virtually no power supply rejection (PSRR). This circuit enjoyed great popularity during the 1980's. Today, the circuit is not as popular, as it often makes for a more transistor-like sound. Still, it remains a good choice when low input capacitance or high gain is required.

1. Textbook Cascode
   
2. Self Biased Cascode
   
3. Very High Gain Cascode
   
4. Ultra-Linear Cascode
   

1. Textbook Cathode Follower
   
2. Cathode Follower with Negative PS
   
3. Capacitor Coupled Cathode Follower
   
4. Current Sourced Cathode Follower
   

Common Cathode Amplifier
This amplifier boasts some impressive attributes: fairly high gain, no phase inversion, low input capacitance, and a high impedance feedback port. Imagine a differential amp with the first tube's plate resistor bypassed by a capacitor and you will better understand how this circuit works. In fact, build the aforementioned circuit and connect the bypass cap to the plate of the first tube and then to ground.

1. Textbook Common Cathode
   
2. Improved Common Cathode
   
3. Common Cathode with Feedback
   
4. Common Cathode with Hum-Bucking
   

1. Textbook Differential Amplifier
   
2. Current Sourced Differential Amplifier
   
3. Cascoded Differential Amplifier