Effectively, the current source load means that the triode is being loaded by only its own plate resistance, which when multiplied against its own transconductance results in the final gain of the circuit.
   Of course, this ideal falls apart once a heavy load is attached to the output, but as long as the following stage only represents an impedance of 1 meg, the effective load on the 12AX7 will only drop to 58k (58,000 = 1,000,000 || 62,000). If the following stage is a Cathode Follower, then its input impedance will be much greater than 1 meg and the full gain will be realizable. (The worst case example is not having to drive a low impedance, but a frequency dependent impedance, such as a high capacitance.)   

0            100v         200v         300v         400v

   Let's take a quick look at the advantage of using a current source to load a tube over the voltage regulated approach. We will start with current source approach. The current source loading arrangement consists of a raw B+ power supply voltage of 170 VDC even with a 12AX7 as the tube being used with 150 volts of plate voltage and an idle current of 1 mA. The 12AX7 benefits greatly from the current source loading of its plate, not just from the increased PSRR, but it also yields the maximum possible gain from the tube and it also greatly reduces the distortion. 

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