[Amps] qrp-excited amps

[John Finner]

John

The only amplifier from my several Orr handbooks that you might be thinking of is from the 21st edition.  It is a class AB1 passive grid driven configuration 4CX1500.  What lends it to your qrp level is the grid is loaded with 6 1200 ohm paralleled 2 watt carbon composition resistors for 200 ohms at 12 watts load fed from a 4 to 1 broad band step-up transformer.  The 4CX1500 only requires 35 volts peak drive.  Across 200 ohms this a little over 3 watts PEP from your K2.  The technique is well explained by Rich Measures on his www.vcnet.com/measures figure 5 page.  Instead of a tetrode with handles requiring 300 volts peak drive from a 150 - 200 watt exciter, here it is scaled down to 5 watts PEP drive to a 35 volt grid tetrode.  You don't need a 2.4 inch toroid for the transformer, a 1.14 or 0.82 inch will do nicely at this power level.  You could use any tetrode with a 50 volt peak or less grid drive requirement i.e. 2CX250's 4CX1000.  One or two Russian 4CX800's at about $100 each also look interesting here.  A 450 ohm grid load and a 9 to 1 broad band transformer would lower a 50 volt peak grid drive requirement to about 3 watts.

You could cheat and buy the 100 watt brick but that would move you into the 21st century.

John Finner WA4EPI
***********************************************
Date: Fri, 14 Jan 2005 18:59:59 -0500
From: John D'Ausilio <jdausilio at gmail.com>
Subject: [Amps] qrp-excited amps
To: amps at contesting.com
Message-ID: <aa9c82bf05011415594c586c75 at mail.gmail.com>
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I'm interested in using my K2 as an exciter for an amp for occassional
use. I remember seeing a design for a tetrode in grounded-cathode that
only needed 5 watts excitation .. maybe in an old Bill Orr handbook.
Anyone have any info/comments on qrp excitation? Should I just plan on
getting Elecraft's brick so I can drive it with 100W?

de John/W1RT