G3LDO wrote:
It wasn't always so. When I used the BK amp originally I used a tapped
loading coil arangement and got some strange effects. Things got better
when
I started using a multi-tapped torroid transformer, which isolated the BK
amp from the antenna system. This transformer was located at the base of
the
coil and fed at 4ohms from the amp using heavy duty electrical cable. The
BK
amplifier is prone to instability when experimenting with different loads.
This is caused by the high gain of the amplifier (the negative feedback
might also become positive feedback with some loads) and could be fixed by
putting some heavy resistive loading at the input of the amplifier.
That's exactly my experience. Since using toroidal transformers and a
resistor across the input I have a very stable BK amp. I use the amp on
73kHz (though it will, of course, also run on 136 if I want) and I have a
toroid to take the impedance from 4ohms to about 50 ohms, then this feeds a
multitapped toroidal transformer that I use on 136kHz to take the Decca from
50ohms to the 150-200ohms of the antenna system.
Since the final output is completely isolated, any current is due to the
earths that I know about, not any stray earthing through the rig.
In the history of my station, one of the great breakthroughs (several years
ago) was to separate the matching and resonating. They can be done together
but it leads to much fiddling and some confusion as to what is happening.
Mike, G3XDV
http://www.lf.thersgb.net
====================
| 