Pluses and Minuses
   Last month we took a look at the Common Cathode amplifier. Of course, a quick reread of last month's article is a good starting point. Additionally, if you are running a PC, it would be a good idea to download and install this month's Live Curves program, as the tube profiled, the 5687, is the same tube used in all the following examples. This month, we will examine some alternative solutions to the biasing problem.

   A quick recap: last month we saw how the Common Cathode amplifier boasted several advantages and one key disadvantage. On the plus side, it did not invert the phase of the input signal, an d fairly high gain, and a convenient high impedance feedback input port, and very low input capacitance. On the down side, the dissimilar plate voltages demand dissimilar grid bias voltages, which is not always easy to implement. Last, month, we offered one possible answer to the problem of biasing the two triodes correctly: keep the plate voltages the same between triodes so that a equal DC grid voltage can be used. The trick was to give the first triode's plate a fixed voltage from a Cathode Follower.

Choke Loads
   Alternatively, if we still wish to avoid the use of dissimilar plate voltages, then the most elegant solution might be to forgo the use of a plate resistor and use a choke, aka inductor, in its stead. A perfect choke holds no DC resistance between its leads. Thus, the inclusion of a choke in the plate's current path to the power supply, will not subtract from the plate voltage. (Of course, all wire has some DC resistance, but so slight are the trivial currents used in a line stage amplifier, 5-10 mA, that the drop across the choke's low DCR, 5-500 ohms, is meager.) The choke makes an excellent plate load in that it essentially halves the power supply voltage requirement, while dissipating no heat. It also serves to buffer the plate from power supply perturbations, as the choke resembles a nearly infinite impedance (i.e. a constant current source) at high frequencies. 

   The needed value of the choke is set by the lowest frequency needed to be reproduced and the rp (plate resistance) of the triodes used. The lower the frequency, the bigger the choke; the lower the rp, the smaller the choke. The formula is roughly:

      (inductance in Henries) H = 2rp / 6.28F

where F is the lowest frequency required. Here is where a tube with a low rp, such as the 6BX7, 7119,