The Valve Wizard
The mu-follower is a high gain, low output impedance inverting stage with excellent
PSRR and very low non-linear distortion; making it enormously popular
Operation of the circuit is
simple: The lower triode (V1) is a normal grounded cathode gain stage,
except that it has an active load formed by the upper triode and load
resistors. The upper triode (V2) is an ordinary cathode
There are also two possible outputs from the circuit:
For simplicity, the two triodes are usually the same type, but they do not have to be. High gm valves are well suited to cathode follower part of the circuit (such as an ECC82 / 12AU7). High mu valves tend to be less suited to the lower section since they tend not to operate reliably at low anode voltages.
The lower triode: The quiescent anode voltage of the lower triode (Va1) is not critical, and is usually made to be in the region of 70V to 100V, or one third of the HT voltage. This means that the voltage across the upper triode is equal to HT - Va1. In this case we will set the lower triode's anode voltage at 85V. This leaves 285 - 85 = 200V for the upper triode.
In this case a bias of -1V looks good (green dot). Any less and we would be entering grid current territory, any more and we would be operating in the region of low mu, as well as restricting the anode current to a pathetic trickle.
It is usual to add a cathode bypass capacitor to the cathode of V1. Leaving it unbypassed would hardly affect the gain in this case, but it would increase the anode impedance which reduces the PSRR factor and increases the output impedance of the lower output. Since the frequency response will be hardly affected there is little point calculating the bypass capacitor's value carefully, any value greater than 1uF should do. Alternatively the bias could be obtained using an LED or diodes, negating the need for a bypass capacitor.
The upper triode: We already know that we have 200V of HT available for the upper triode and we can mark this point on the x-axis (blue dot), and we now also know that the anode current is 2.5mA (current through the triodes is the same because they are in series). We can therefore choose any bias point that lies on the 2.5mA line, and it is usual to bias to half HT (200 / 2 = 100V) for maximum headroom in the cathode follower. This ensures the cathode follower acts as a CCS for a long as possible before cutting off or saturating.
In this case at half HT (Va = 100V) and an anode current of 2.5mA, the bias is about -1.3V. We can now draw a load line through the two points and use it to find the total value of load resistance (Rl + Rk2). In this case the load line tells us (Va = 200V / Ia = 5.2mA) that we need a total load of 38k.
The bias resistor is subtracted from the total load we need, making 38000 - 560 = 37440 ohms. The nearest standard is 33k and this is what we will use for Rl.
The quiescent anode voltage shown on the graph is 100V. This is actually the anode to cathode voltage, so the true cathode voltage will be 285 - 100 = 185V.
Bootstrapping, grid-leak and input capacitor: Now we have set the upper triodes conditions we can work out what AC load it present to the lower triode.
Due to bootstrapping, the AC load presented to the lower
valve will be:
As usual the grid-leak resistor on the cathode follower is also bootstrapped and can be made smaller in value than we might normally use, which will reduce resistor noise and blocking distortion. If we use a value of 220k, the effective input impedance of the cathode follower will be:
Since the input impedance of the cathode follower is so high, the input coupling capacitor (Cg2) can be chosen based solely on a desired reactance at a low frequency. For a reactance of 1Meg at 10Hz:
Gain: The gain to the lower output is equal to mu, assuming
the output has a load greater than 5Meg attached. The data sheet quotes
mu for the ECC81 as 70, although looking at our horizontal load line
it looks to be more like 60 because we are operating at such a low anode
Output impedance: The output impedance from the upper output is simply that of the cathode follower, which can be closely approximated as:
Heater elevation: Because the upper cathode is at a high voltage it is usually necessary to elevate the heater supply to avoid exceeding the maximum rated heater-cathode voltage (Vhk). For the ECC81 this is 90V, so we would need to elevate the heater by at least 100V.
The diagram [right] shows how two different triodes might be used, and how the lower output can be used to DC couple to a following stage: