Andre Jute explains why Bode plots are intolerably old-fashioned, imprecise and time-consuming, then shows the single formula you can program in a spreadsheet to tell you the amplitude and phase response of your tube ampllifier over the entire frequency range.
KISS 141
Just the Facts, Mam: The Complete Tube Transfer Function
by Andre Jute
In the dark ages before computers the response of an amplifier design was painfully approximated by days of work on a slide rule to allow a rough vizualition on a Bode plot. Today you don’t need Bode Plots and other approximations. You can calculate the complete transfer function of a tube stage, or many linked tube stages, perfectly precisely simply by programming your computer. BASIC will do the job but most of us will find it faster to program the computer in some pre-existing programme like Microsoft Excel, and that is the one I chose because it will not only do the math but draw a graph as well. The two results of most interest is amplification and phase, so I therefore developed my formulation to calculate both at any frequency of interest. Here is the formula for a RC triode stage which, when fed into a calculator or correctly programmed, will for any arbitrary frequency or range of frequencies calculate amplification and phase:
And that is everything you need to calculate and graph the response of your amplifier at any frequency of interest, or that matter for every single frequency. Good luck.
THE VOLTAGES IN THIS AMP WILL KILL YOU.
GET EXPERIENCED SUPERVISION IF IT IS YOUR FIRST TUBE AMP
All text and illustration is Copyright © Andre Jute 1992-2007
and may not be reproduced except in the thread KISS xxx on rec.audio.tubes