[Amps] RF Chokes

[Tom W8JI]

> Peter, I recall an article I saw 10 or more years ago 
> where a guy was trying
> to move the resonance of the RFC in an Alpha amp .He did 
> it by using a
> single turn loop of HV insulated wire around the RFC. He 
> was able to move
> the resonance into the CB band where it could do him no 
> harm.

If we remember what a "series resonance" really is, where 
the choke looks like a back-to-back L network group or 
groups with extremely high L and very low C, it's easy to 
understand what will or will not "fix" the choke by moving 
resonant frequency.

With a large number of turns on an air (or non-magnetic) 
form, moving a shorted loop doesn't change much. Changing 
the capacitance at the voltage peak moves things quite 
substantially.

A test with the choke out of the PA is virtually useless 
unless a correction factor and a known test fixture is used. 
It is NOT the stray output C of the tubes that has 
substantial effect, it is all the stuff mounted around the 
choke that primarily controls unwanted resonances. There are 
very distinct areas where the electric field peaks, and 
those are the critical areas.

73 Tom