by dcgillespie » Tue Mar 20, 2012 3:08 pm
Jules -- When output tubes are operated under certain conditions, the use of screen stability resistors are mandatory. This primarily includes the use of fixed bias operation, and very low impedance screen grid power sources. Under those conditions, EL84 class tubes require screen stability resistors of at least 47 ohms, with 68-100 ohms being optimum. However, for the Heath amplifier, those conditions do not exist, so no screen stability resistors are really required.
As for my background, I fell intensely in love with vacuum tube amplifiers when I was about 6 years old back in the 50s. It is a hobby I have never deviated from, even when virtually everybody had abandoned them. My training has been from the giants of golden era: I've read and reread just about everything every published by the likes of Crowhurst, Hafler, Langford Smith, Tremaine, Keros, McIntosh, Williamson, and so many others I could name, until I thoroughly understood the material, and basically the covers fell off. Along the way, I have built and designed more amplifiers than I really can remember, but still find this hobby as enjoyable today as I first did so many years ago.
I certainly remember what it was like when I had more questions than answers, but unlike today's new crop of diyers, I had no Internet to help get the answers. As a result, part of my library now includes shelves of journals where I've done countless experiments to find my own answers. Now I simply enjoy the fruits of my own labor, and try to help others as I can to help them get the most from their equipment, and keep the hobby alive.
Thanks for the interest!
Dave