Let’s start by looking at the open-loop frequency response of an op amp: it’s high (more than 100 dB) at low frequencies, all the way down to DC. At some frequency, the gain begins to drop ...

Looking at the circuit diagram it follows the classic op-amp with a long-tailed pair of NPN transistors driving a PNP gain stage and finally a complimentary emitter follower as an output buffer.

The K2-W vacuum-tube op amp has an LTspice-simulated open loop gain of 27,000. The modeled K2-W input impedance is >100 MΩ. The LTspice modeled result shows 1.00 nA of input current. Early SPICE ...