[Amps] diode junction temps and thermal resistance

[Kevin Normoyle]

  Good info Ian.

I was looking around trying to learn more about the various bias circuits in use.
maybe people can provide historical views on some things I was curious about:

-This 1971 Eimac paper on a 8877 triode for 144Mhz, had a 1N3311 50 W 12V zener for the bias.
Makes me wonder why R. Sutherland decided on 50W for that diode.
http://www.landfall.net/Radio/8877-1.htm


-The junction to case C/W is much better for the zeners in the  larger DO-5 stud  package..just 2 C/W max (1.5 C/W typical)
  compared to 10 C/W  (typical) for the DO-4 stud package. I wonder if that's what's interesting...not that it can 
tolerate 50W but that it has lower junction temps to start with, for a given DC dissipation..because of the 
better/larger package. Note that it doesn't matter how good your heatsink is, if the junction to case thermal resistance 
is the problem.

-The ability to tolerate power spikes depends on junction temperature rise in the zener, right?. So starting from a low 
junction temp adds margin?


-rfparts.com has the reverse polarity 1N3306B (50W 7.5V) for a reasonable price, and claims to have the mounting hw. So 
I ordered a pair. It seems easier to drill larger 1/4" holes and use the stud diodes, then use say the flatter TO-3 
packages and say TO-3 heatsinks or something like that.  So I ordered a pair of those to try.
It appears that some amps do use the TO-3 packages though?



-The 1971 Eimac paper mentioned using a smaller zener with a 2N3055 transistor on a heatsink, instead of the 50W zener.
I thought that was funny, because some web sites I've read mention something similar, like it's a new idea.
Also saw people mentioning using zener plus mosfet, at least for audio amps...

  I guess you could size a pretty large power transistor? How come we don't see more of these simple lower power 
zener+transistor circuits? or do people do this?

Ah, I'm reading Ian's Triode Board manual. I guess that's an example of taking that to more extremes.

-kevin
AD6Z