LF: Electronics amps]

[Andre' Kesteloot]

drassew2@interalpha.co.uk wrote:

> >I have a B K Electronics 300W amplifier which I have modified to
> >give some 250W at 136kHz. This is the second one I have used,
> >and have also run the 200W version.
> >
> >Recently I have noticed that the power drops off by about 25 per
> >cent when it gets hot. Has anyone else come across this and/or
> >knows the cure? I do have a dead amplifier of the same type I can
> >canibalise. I tried freezer spray but this does not reverse the effect.
>
> The channel resistance of power Mosfets increases with temperature, so as
> the feedback within the amplifier is less effective at these frequencies it
> cannot compensate and so the amplifier output impedance does actually
> increase.  The Maplin modules are very bad for this giving a Zout of < 2
> ohms at turn on and rising to 4 ohms when hot.  I recently popped one of the
> amplifier modules when transmitting for the transatlantic tests and the
> other went a few days later on brief overload so won't bother rebuilding
> using the Maplin modules - just use a G0MRF type design instead.
>
> Andy  G4JNT / G8IMR