Phil Allison wrote:

"Patrick Turner" wrote in message
...

Occasionally, one does see RF oscillations at higher F than 20 MHz,
and a sixth sense is needed to detect them, or a meter.
A cascode preamp I recently built did oscillate at some high RF,
and just touching parts where the voltage was supposed to be fully
bypassed with a screw driver caused a loud click at the output speaker.

** A simple voltage regulator using a BD139 and 18 volt zener built for a
disco mixer proved effective at wiping channel 9 completely off the screen
of a TV set in the same room. No sign of trouble on a 10 MHz scope.

Power mosfets make nice HF and VHF oscillators as well - the Hitachi
2SJ /K ones - it pays to keep leads to them short with minimal inductance.

I once built a 2 x 300 watt mosfet class AB amp,
http://www.turneraudio.com.au/webpic...et400w302h.jpg
It has 6 hitachi mosfets per channel, 2SK1058, 2SJ162,
and yes, I did have some trouble with oscillations, and with making sure the
rails were fully bypassed right up to RF.
Eventually, I got a very nice looking error signal,
even with a 20 kHz square wave, from a fast sig gene,
and this amp sounds indistinguishable from many decent tube amps.

Sloppily built mosfet prototypes usually always oscillate,
until you learn how to set things up as you say.
But there is fun getting it right, and going better than many commercial
amps.....
Sometimes one has to introduce inductance, just at the right place,
to obstruct HF signals.

Patrick Turner.



.............. Phil