Input Stage

One of things I didn't like about my orignal SOHA design was the trimpot. This problem is solved nicely in the SOHA II with an input stage that permits a small amount of feedback which, in the SOHA II, really helps with tube rolling and gives excellent performance.

However, in this amp we don't have room for the nice front end in the SOHA II that needs two tubes. We can only have one tube.

So how do we set the plate voltage for the tube without loading it down too much (destroying the high frequency response)? First, let's take another look at the SOHA front end:

SOHA Input Stage

The triode is in a grounded cathode configuration. In order to set V_p we simply adjust the cathode resistor using a trimpot to set V_k. This changes the bias conditions on the grid until we get the desired plate voltage. If we can find a way to auto-adjust V_k by sensing the plate voltage we can eliminate the trimpot.

Since the plate is loaded with a CCS the effect of the trimpot is to change the cathode voltage for a constant I_p. Another way to adjust V_k, however, would be to leave the resistance a constant value and either sink or source current from it. Changing the current in the resistor will effectively adjust V_k. If we can create a current source that senses V_p and knows what value to set V_p to this scheme will work. In other words, we need a current source that has a reference voltage, V_r, that it can use to compare V_p to and then adjust the current in the cathode resistor. This idea is illustrated here:

Input Stage with Sensing Current Source

Notice that because the cathode resistor is bypassed the current source will be shielded from AC variations in V_k during normal operation.

This particular circuit feature begs to use an opamp for the current source. The opamp can source or sink current from the cathode resistor as needed. We just need to set V_r and to find a way to sense Vp. Sensing V_p is tricky because in order to sense V_p we have to attach something to the plate. When we attach anyting to the plate we immediately add a load to the tube. If we attach a load that is too small the tube's performance will suffer at high frequencies. Thus, if we use the most straightforward sensing circuit, a voltage divider, the resistors must be large - in the MΩ range.

With a voltage divider to sense V_p and an opamp current source the circuit looks like this:

Input Stage with Opamp Current Source

The value of V_r is somewhat arbitrary needing only to fall within the opamp rails. But we can add another condition which is that most of the cathode current should flow through the resistor so that the opamp is not supplying the major portion of the current. Even this condition is arbitrary. We must make some additional choices. Anticipating the design of the PS we can expect to have V_p=80V and CRD=2mA (1N5305). Looking at plate curves for the 12AU7 running at 80V and 2mA we guess V_k about 2.5V. And knowing that the buffers will be powered by V+=24V we can make things simple and set V_r to one tenth of V+ or 2.4V so that the opamp's input and output voltages are nearly the same. Normally, we would use a zener or other reference device for setting V_r, but since the V+ will already be regulated we can use a simple resistive voltage divider.

Now, to get 2.4V from 24V we need a one tenth voltage divider. I like to set dividers to run at 1mA when possible (arbitrary). The closest 1/10 divider will be to use 18kΩ/2kΩ for the reference divider from 24V.

For the V_p sensing divider we can also arbitrarily set R_p1 at 1MΩ. It remains to calculate R_p2. The opamp will try to equalize its input voltages and, because it's a jfet opamp, will draw essentially no current. This means that V_pd will also be 2.4V. With this final value we can compute R_p2:

V_p = V_pd * (R_p1 + R_p2) / R_p2

Putting in the values we get:

80V = 2.4V * (1MΩ + R_p2) / R_p2

Solving we get R_p2=30.9kΩ

Which value just happens to be a standard value for Vishay RN55D resistors. And the final servo circuit now looks like this:

Input Stage with Resistor Values

Additionally, the 1MΩ resistor is large enough not to steal any plate current from the CRD. Thus, nearly all of the CRD current becomes I_p. The opamp has a rail cap to prevent oscillation and an integrating cap to make it operate like a servo.