[AMPS] Another Stupid Question

[2]

>
>> >That means you only need  about 300pF blocking C for 160
>> >meters, if the anode system's operating impedance is 3000 ohms.
>> >
>> >I remember laughing at a friend of mine who stuck a 100pF
>> >blocking capacitor (by mistake) in his 3-500Z amplifier for 160.
>> >When he changed the part to 1000pF, nothing happened except
>> >the tuning capacitor slightly changed position. Efficiency and power
>> >out remained the same.
>> >
>> //  Amen, Mr. Rauch,  Another Ham radio old wives' tale bites the dust.
>
>Industrial RF generators (Henry, RF Power Products, et al) operating on
>13.56 mhz at powers from 1 to 5 kw all have something in common. They
>use 200-400 pf blocking caps. I was "brainwashed" at an early age to believe
>that 2000-5000 pf was needed on 160 meters because all of the amateur 
>products
>used that value in the "old days."
>
>To answer the original question, I would say that the values for all tank
>components
>vary with the choice of tubes/transistors, the box chosen to house them, 
>and the
>layout. More experimentation, measurement, and trial-and-error is involved in
>the
>design stage than relying on a formula, or what someone else used in the 
>past.
>
>My guess is that the main cause of failure in a blocking cap is 
>over-heating due to exceeding its RF current rating.
>
Some Ham radio amplifiers use disc-ceramic capacitors for DC blocking.  I 
have never seen a disc-ceramic capcitor that was RF-current rated. 

cheers, Phil

-  R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.  
end


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