Topband: w7iuv preamp last comment

[Lee K7TJR]

Wow. Larry thanks for all you have done with these amps. No one ever meant any of this to be personal, or to imply your amp does not work for amateurs as designed. As for the bottom line if your heat sink is not big enough because of the power dissipated make it bigger, reduce the current or reduce the voltage. No one ever meant to pick apart your design but to explain why it might be too hot. I have used nearly the same configuration in aerospace and commercial interests as well. It is a good design. The current in this device can vary as much as 20% due to Beta changes in the 2n5109 from various manufacturers and my friend Rons amp had way too much power for his heat sink. Easy way was to vary the 10 ohm resistor just as you did to increase the current for your own use. My original errored calculations and assumption that the current would be higher for the average device was mostly based on a common practice of ignoring the base current drawn from the bias string. On the average in this design it lowers the bias current significantly except for the occasional case of a high Beta or substitute transistor. If one were forced to do something stupid as I suggested like changing the bias resistor to get the amp to a recomended current or maybe they had to substitute a device I am confident it would help lower the power dissipation and prevent a premature device failure, of course at the expense of lower IMD specs. All my post was ever meant to be about. We all know there are many of these around and ON4UN thought enough of it to include it in his book, a testimony in itself. I have 4 similar design amps in my 8 element receiving array operating 24/7 as we speak as well.  A lot of good things and some bad have resulted from discussions such as these, I would bet some new faces got exposed to transistor junction temperature problems and maybe somebody found out how to vary the operating current in this amp as a result would you not agree? 

Lee K7TJR Oregon  http://home.comcast.net/~k7tjr  www.k7tjr.com