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From: Dimitrios Tsifakis <dtsifakis@gmail.com>
Date: Wed, 13 Feb 2019 16:17:49 +1100
Message-ID: <CAKvcm2eotJOvzSZGmn2Jy_GHM8RbAD=hHH0TWS2OpxqLtA-Akw@mail.gmail.com>
To: rsgb_lf_group@blacksheep.org
Subject: Re: R: Re: VLF: in VK?
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Hi VLF,

> I will do a proper test: an appropriate resistor (50 ohm if I decide
> to do 50 ohm to 4 ohm transformer) at the output winding and the
> amplifier at max power. The only thing that should get hot must be the
> resistor!
>
> I made long time ago a web calculator for this:
>
> http://people.physics.anu.edu.au/~dxt103/calculators/toroid.php
>
> Anyway, I should stop talking about it and just try it out.

Just finished making a pair of impedance transformers. I used some
junk box large (OD=61.5mm, ID=37.5mm, L=12.8mm) ferrite toroids of
unknown material and I stacked two on each transformer. I then made an
autotransformer with a total of 52 turns tapped at 15 turns making
roughly a 4 ohm to 50 ohm transformer (48 ohm to 4 ohm to be pricese).
I wasn't aiming for 50 ohms it just happened to be the number of turns
that neatly fitted on the toroids!! A=3.2cm^2 and the calculated Bmax
for the expected power levels is less than 500 gauss so we are in
business.

I then did some tests with the amplifier, I connected the two 50 ohm
windings back to back and place the amplifier on one end and a 4 ohm
high wattage resistor at the other end. I was pleased to see that the
resistor got super hot and the transformers remained cool as a
cucumber. I was able to get on the resistor more than 100 watts from
f=3 kHz to 20 kHz, which reduced to 80 watts at 30 kHz. I can live
with that. The amplifier is nominally 100 W output per channel.

Fingers crossed now I will get a chance to fire it up on the antenna
this weekend!

73, Dimitris VK2COW/VK1SV