[AMPS] Re: 6m amp blocking caps

[Ian White, G3SEK]

2 wrote:
>
>When DC blocking caps are paralleled, a parallel-resonance is created 
>between the capacitors in the RF output path.  The additional resonance 
>may be problematic.
>
This is only relevant when the two capacitors are very *different*. You 
then get a parallel resonance, between the lower capacitance and the 
self-inductance of the higher-value one.

For example, if you connect a typical 0.1uF polyester cap in parallel 
with the 1000pF screen bypass cap built into a tube socket, you're 
likely to get a parallel resonance at HF. This is from 1000pF with 
several hundred nH self-inductance of the polyester cap. The resonance 
is very sharp, but it means that the screen grid is completely 
un-bypassed at that frequency!

Fortunately these sharp resonances are easily damped out. Often the 
losses in the components (generally the higher-valued capacitor) or 
elsewhere in the circuit will do it automatically, so you never even 
notice. A more positive cure is a low-inductance resistor of 10-100 ohms 
inserted between the two capacitors, or in series with the higher-valued 
one. (The resistor can be shunted with an RF choke if DC continuity is 
required.)

But none of this applies when *identical* capacitors are paralleled.

-- 
73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                           'In Practice' columnist for RadCom (RSGB)
                            http://www.ifwtech.com/g3sek

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