Common Cathode Amplifier
This amplifier boast some impressive attributes: fairly high gain, no phase inversion, low input capacitance, and a high impedance feedback port. Imagine a Diff-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.

Grounded Cathode Amplifier
The Grounded Cathode amplifier is the most used of tube circuit; with good cause. It is both simple and effective. With just one triode, four resistors and a capacitor, a perfectly usable line stage amplifier can be built. Whereas, using any other single device would require many more parts to achieve similar performance.

Grounded Grid Amplifier
Although this is one the three basic tube amplifier circuits, it is seldom used in audio equipment, because of its very low input resistance. Still it offer slightly greater gain than the Gnd cathode amp and much lower input capacitance and it does not invert the phase of the signal.

Totem Pole Amplifier
Very hip, very popular. This compound amplifier works by using the top triode as both a cathode follower and a current source for the triode below it. The positives: high gain, good PSRR, low Zo, one current path. The negatives: high cathode to heater voltages, phase inversion (could be a plus).

White Cathode Follower
Earlier, we said the that the Cathode Follower is the work horse of tube circuits, well then this must be the Clydesdale. This buffer circuit offers a very low output impedance, almost unity gain, and a symmetrical push-pull current swing. How? Feedback from the the plate of the top tube is the answer.

Parallel Triodes
Most small envelope tubes contain two triodes. Giving both the same signal to amplify, prevents any mixed signal over-spill into the other, which should improve the imaging. The worst-case example is to have one envelope used by both channels. If the increased heater and plate current demands are too great to use parallel triodes, then use two envelopes and leave one triode in each disconnected.

Rk Bypassing
The cathode resistor is often bypassed with a large value electrolytic capacitor, which will serve to increase the gain and the distortion. This capacitor should be further bypassed by a small value, high quality film capacitor. The increased distortion is due not to the cap (well, maybe a little) but to the elimination of the linearizing effect of the cathode resistor on the triode. The triode behaves as if it were hooked up in a fixed bias setup, except for better clipping characteristics as the positive grid bias, which causes current to be drawn through the grid when it becomes positive relative to the cathode, is mitigated by the slowly increased cathode voltage.

Tube Selection
Audio Gadgets lists five common audio tube types: 6DJ8, 6SN7, 12AT7, 12AU7, and 12AX7. The Hint Line message at the top of the program provides the important specifications for each tube type. Further information can be found in a good tube manual. This list is not exhaustive in the least. Other tubes worth looking into are the 6BL7, 6BX7, 6SL7, 6SU7, 12B4, 12BH7, 5751, and 5965.

User Defined Tube Data
So you have a dozen 6BQ7's waiting to be used in a project--no problem. Check this box to bring up the user defined data entry boxes. These input boxes can also be used to define tubes that don't exist, but maybe should. How about a tube with a mu of 200 and a rp of 100 ohms.