[Amps] stability of amps

[Colin Lamb]

>From time to time, we get back to basic arguments that involve philosophy of design.  Many years ago, I remember that Tektronix often designed their scopes to use tubes from certain manufacturers, and found that other brands were not suitable.  In order to meet their specifications, they insisted that certain brands be used in certain positions.  And, if the tube was replaced, the tube needed to meet design specifications.  Was that good engineering practice to build a device knowing that plugging in a replacement tube not approved by Tektronix might cause the instrument to fail to meet specifications?  Tektronix thrived during that time period, so it appears the consumers answered the question.  

And, then there is the Gee Bee airplane.  Jimmy Doolittle, flying a Gee Bee, won a major air race.  Problem was that almost every pilot who flew it died.  The design finally disappeared, and all of the history books recorded it as a flawed design.  About 10 years ago, someone from Oregon built a replica of the Gee Bee.  It has a large engine and looks something like a potato with something like a 10 foot wingspan.  It has been pleasing air show fans ever since, even though it would probably instantly kill most competent pilots.  So, is that well engineered or poorly engineered? The roll rate is fantastic. 

Looking at the issue pragmatically, if Carl and Lou are converting amplifiers that satisfy their customers, isn't that good enough?  Perhaps the debate about whether or not it is good engineering is no longer relevant. 

Many years ago, I converted an SB-200 to a 6 meter only amp.  I used a separate input and output and tried to reduce lead length to minimum.  Now, that was an example of poor engineering, because when I got done, it was unstable.  That was before I had much test equipment and I never solved the unique problems of that amp - created mainly by the 572B tube.  Things would have been much easier had I been using a 3-400 or 3-500 tube.  I had just built one of those amps that was very stable.  

Colin  K7FM