by dcgillespie » Sun Dec 12, 2010 6:57 am
Erick -- It is a somewhat complicated subject, but in general, it is based on the characteristics of each particular tube type, in conjunction with the way it is operated in a circuit. Therefore, there is no one impedance that is optimum for a given tube, but an optimum impedance for a given tube in a given circuit. Change the tube type or change the operating conditions, and it changes the optimum load impedance.
Each vacuum tube has published for it what is known as a "family of plate curves" -- among many curves published for it. It shows the dynamic relationship between the plate current a given tube can pull, and the plate voltage and grid voltage required to produce it. If it is a screen grid tube, then it is additional element on the curves controlling the plate current as well. These curves make up a primary part of a given tube's characteristics.
But circuit conditions also play a part in how a tube functions as well. For example, in triodes, raising or lowering the plate voltage has a significant impact on plate current. But in pentodes, plate voltage has little to do with plate current, since it is primarily a product of screen voltage. Use fixed bias and peak current draw capability is maximized. Use cathode bias, and it is restrained -- and we haven't even applied a signal to the tube yet! Ultra Linear operation changes things even more as now the screen becomes a dynamic element rather than a static element, as AC is now being introduced on it. The product of all these design variables come together in a given design to determine what the optimum load will be.
So the characteristics of a tube, and the conditions it operates under determines its optimum impedance to operate into. As a basic fact, when the external load impedance equals the load of the generator (the tubes in this case), maximum power transfer is achieved. But this usually does not produce the point of lowest distortion. So an engineer draws a set of what is known as "load lines" across the family of plate curves to determine what the power output and the distortion will be for a given tube under a given set of operating conditions. Then, based on his design goals for these qualities, he can chose the load line that best delivers the performance he is trying to achieve.
For the EL84 in a classic UL configuration with cathode bias, when it is operated at a typical B+ of 360-380 vdc, the best compromise between power output and distortion is achieved with a plate to plate load of 10K ohms, and a screen taping point of 50%. If fixed bias operation is used, then even better performance can be had with a plate to plate load of 8K ohms, with a taping point of 23-25%. If you are interested, all of this directly relates to a significant element forming the basis for the improved performance modification I introduced for the SCA35/ST35 amplifiers in the thread in this section titled as such. There is also a followup piece available to further help you, that directly compares the performance of two transformers optimized for these two types of operation.
Keep reading and studying Jone's book -- and there are so many other good books available as well. The more you understand about how a given tube interacts in a given circuit, the more you can improve your sound!
Dave
Last edited by
dcgillespie on Sat Dec 18, 2010 2:43 pm, edited 1 time in total.