"John Stumbles" schreef in bericht
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On Tue, 18 Oct 2011 19:52:01 +0200, petrus bitbyter wrote:

---snip--- (hint! :-) )

Makes me think of the old audio tube output amplifiers. The Miller
capacitor could make them yell like an HF-transmiter. Usually a stop
resistor in the grid circuit prevented this oscillation. So adding (or
increasing) the resistance in the gate circuit may solve the problem.

Some ancient neurons in my brain stirred. I think you mean Miller
capacitance? Not an external component but the effective capacitance at
the grid of a valve (or gate of a FET) due to the actual capacitance
between grid (gate) and anode within the device amplified by the voltage
gain of the circuit it forms.
http://en.wikipedia.org/wiki/Miller_effect

--
John Stumbles

I don't want to be part of a club that would have someone like me as a
member.\

Sure. I should have written capacitance to make things more clear. This
capacitance was - and still is - made up by the grid and the anode of the
tube, especially in triodes. This *is* a capacitor as it consists of to
conductors separated by an insulator. Even though the remedy - a stop
resistor in the grid circuit - was known, it was sometimes left out for cost
reduction. It is known that many FETs act in the same way triodes do.

There are other things that can make an audio amplifier oscillate. Last year
I got a modern tube amplifier. The ultra linear amplifier oscillated due to
too low a resistance in the screen grid circuit.

Modern semiconducters suffer from even more capacitances then the old tubes.
I once was told that SMPS-designers consumate their components not by the
number but by the bucket as heavy duty switchers oscillate very easily and
than blow themselfs.

petrus bitbyter