Tube DIY Asylum: Cathode Follower Preamp Outputs by Sasquatch

Cathode followers are commonly used to buffer the output of preamps. It seems that a common practice in the design of these circuits is to load the cathode follower with a resistor that drops a lot of voltage between the cathode bias resistor and the ground. The grid is grounded at the junction of the cathode bias resistor (Rk) and the load resistor. Anyone who has compared the sound of a loaded CF versus one loaded solely by Rk knows how much better they sound. The purpose of adding the extra resistor (or CCS, or choke) is to place a more traditional load on the tube. It would seem that the degenerative feedback of Rk alone cannot compensate for an inadequately loaded tube. However, one issue I have never seen addressed is that the AC load for the tube is not the series resistance of Rk + the load resistor but it is this sum in parallel with the input impedance of the next stage. A CCS load is the best that can be done, and makes the AC load seen by the tube approximately equal to input impedance of the next stage. If so, it would seem an important factor to take into account when picking the operating point for the tube. This becomes quite important if the input impedance of the next stage is low. Of course, exactly the same thing is true of a plate loaded common cathode amplifier driving the next stage of an amp in a chassis, but it is more obvious to the designer that the effective plate load is Rp in parallel with the following stage, and this issue is commonly discussed.

An example of the issue can be seen in the simple preamp linked below. Nominally, the load on this tube is 56.27K. In reality, it is 56.27k in parallel with whatever the preamp is hooked up to (ignoring the 1M resistor). Thatâ€™s 36K for a 100k amp, 26.5K for a 50K amp, and 8.5K for a 10K amp. The impedance ratio is good in every case but the tube is AC loaded very differently. Do I have all this correct? If so, how big does this parallel load need to be relative to ra when we take into account the degenerative feedback of a CF?

The issue has come up recently in a buffered pot preamp that I have been experimenting with. Thanks in advance for any comments on this issue.

http://www.joeltunnah.com/images/12AZ7buffer.JPG (Open in New Window)