A class B amplifier produces many harmonics as it switches from
conducting to non-conducting, and the input tuned circuit must provide
an excellent input termination impendance not only at the fundamental
frequency, but also at the 3rd harmonic and ideally at the 5th harmonic
as well. This would not be a problem if it were a Class A amplifier,
but that's certainly not practical for a high power RF amplifier!
Even a short piece of RG-58 between the input tuned circuit and the
tube socket significantly increases the input termination impedance at
the harmonic frequencies. I discovered this 25 years ago when I
originally built these amps, I had only about 12 inches of RG-58 between
the tuned circuit and the socket. The 10M amp produced less than 1000
watts output with 100 watts drive and the tube ran brilliant red! The
15M amp produced only 1200 watts and the 20M amp had only1300-1400 watts
output with 100 watts drive. Moving the tuned circuits to the tube
sockets made all of the amps perform essentially identical with 1500
watts output with 80 watts drive.
By the way, my mother's maiden name is Ogden!
From: Jon Ogden <email@example.com>
To: Frank Donovan W3LPL <firstname.lastname@example.org>; Amps Reflector
Date: Monday, February 08, 1999 10:42 PM
Subject: Re: [AMPS] Input Tuned Circuits with Powdered Iron Toroids
>>As many builders of G-G amps have discovered, the efficiency of a G-G
>>amp is significantly better if the input tuned circuit is located
>>immediately adjacent to the tube socket. Even six inches of RG-58
>>between the tuned circuit and the tube socket significantly reduces
>>efficiency, especially on 10 meters.
>Interesting. Why is this? It seems like it could make for a rather
>crowded tube compartment. The loss of any coax at HF seems minimal,
>all that having a transmission line in there would do would be to
>you on the Smith Chart back toward the source a slight amount before
>apply your matching networks. So why does it make a difference in
>efficiency? What am I missing?
>http://www.qsl.net/ke9na <--- CHECK IT OUT! It's been updated!!!!!
>"A life lived in fear is a life half lived."
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