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[AMPS] pi vs. t tuned input

To: <amps@contesting.com>
Subject: [AMPS] pi vs. t tuned input
From: w8ji.tom@MCIONE.com (Tom Rauch)
Date: Wed, 10 Jun 1998 10:10:51 +0000
To: <amps@contesting.com>
> Date:          Wed, 10 Jun 1998 05:45:24 +0000
> From:          Scott Townley <nx7u@primenet.com>
> Subject:       [AMPS] pi vs. t tuned input
> To:            amps@contesting.com

> 
> About tuned input sections for amplifiers:
> Everybody and their dog seems to have an affinity for the pi-net type of
> tuned input.
> Is there some practical reason not to use a T-net?

Absolutely. When conduction angle is less than 360 degrees the tube 
generates harmonics in the cathode. In order to maintain efficiency 
and improve IMD, the cathode MUST see a low impedance at the harmonic 
frequencies.

Worse case performance degradation occurs when the cathode sees a high 
impedance at the third harmonic.

That's why lead length from the cathode to input network is critical, 
and why the TYPE of network is critical. 

> I was thinking of using molded chokes that are easily found from Digikey,
> Mouser, etc., for the inductors...typical values in the 1.0-7.5uH range are
> cheap, are rated for 500-600mAdc, and have RF-tested Q's of 35-40.  Which
> isn't much, but is more than sufficient for a network with a loaded Q of
> between 2 and 5. 

They won't handle the VAR power well.
 
Should I "roll my own" on
> powdered iron toroids?

Yes, it is virtually impossible to saturate powdered iron or 
ferrites because they overheat long before saturation occurs. The 
exception is in pulse duty, where the duty cycle is so low 
saturation can occur without melting down the core.

 
73, Tom W8JI
w8ji.tom@MCIONE.com

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