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Re: [Amps] EB104 600W HF amplifier questions

To: amps@contesting.com
Subject: Re: [Amps] EB104 600W HF amplifier questions
From: Steve Thompson <g8gsq@eltac.co.uk>
Date: Tue, 05 Feb 2008 08:44:38 +0000
List-post: <mailto:amps@contesting.com>
 >Swamping Resistor:

The Ameritron circuit has some differences - there are low value 
gate loading resistors not in EB104, and the feedback resistance 
is lower. This is going to lower the impedance on the transistor 
side of the input transformer. I don't know the turns ratio in the 
ALS600, but I'd guess that the amp circuit might have an input 
impedance lower than 50 ohms so that the 35 ohm series resistor 
give 50ish overall as well as attenuating the input.

 >Output filter:
 >I like the idea of using a diplexer instead of a basic low pass 
 >filter since you can burn up a lot of the harmonics by sending 
it >through a high pass filter and into a 50ohm power resistor.

There's a more subtle reason for doing it this way - at low 
frequencies harmonics lie within the passband of the amplifier, 
and you can sometimes get some very stange effects if they are 
simply reflected back from a filter.

 >Power:
For best performance you want a regulated supply. You can play 
games with transformer ratings, ripple voltages etc. ahead of the 
regulator. Bear in mind that the peak current in the transformer 
could be 7-8x dc, and the rms (heating effect) current is 
typically 1.7-2.5x dc. You might consider hunting for surplus 
switcher supplies - 48V is common in telecomms installations, or a 
couple of 24/28V supplies can be stacked.

Steve
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