[Moon-net] Amplifier technical questions

[Chris Bartram]

> IF you have used cobber as a line, change it to Brass.

Alhough I'd agree wholeheartedly with Kim about needing LOTs of air to cool a 
K2RIW amplifier: if the tubes aren't trying to jump out of their sockets, you 
don't have enough! But, I'm afraid he's simply wrong about the use of brass 
for the anode line. The effective resistivity of brass at UHF is very much 
higher than copper. Not only will you loose output power in dissipation 
losses in the line, but you'll also increase the temperature in the anode 
cavity.

The use of about ten skin-depths of low-resistivity (NOT standard commercial) 
silver plating on brass would restore the performance to about that of 
copper, but it's actually quite difficult to beat ordinary copper sheet which 
has been polished and lacquered.

Many years ago I had a commercial contract where I had to supply a triode 
amplifier in the 2GHz range. From memory, the loaded Q of the output cavity 
was about 20. When I initially assembled the amplifier, as it arrived from 
the machine shop as a set of plain brass machined parts, the efficiency was 
about 20%. Getting the amplifier parts heavily plated with low-resistivity 
silver increased the efficiency to around 37%.

Vy 73

Chris
GW4DGU