The OP asked in the context of valve amps. I do work on valve amps
exclusively, and they are usually made by
me, and they have grid stoppers where they should have.

If I use overall nfb, then ringing for a power amp is likely to be
around 80kHz and 1MHz. Never thought through why this always seems
the case. The former is dealt with by snubbing the OPT primary
somehow, and the latter by power-valve grid stoppers usually.

If I built pre-amps with feedback then a better scope might attract
me more. As it is I rely on sensible precautions.

cheers, Ian

"Phil Allison" wrote in message
u...

"Ian Iveson" wrote in message
...

"Phil Allison" wrote


** If you can't see them on the scope how do you even know
when
they are there ?

I measure with a voltmeter. Also, the scope's trace goes fuzzy.


** That contradicts my "if" condition .

10 Mhz makes no effect on a 1 Mhz scope.



If you don't know they are there then why would you use
an
(RF) voltmeter ??

To check they are not there :-)


** You are assuming continuous RF oscillations - more usually
ones sees
bursts superimposed on a audio wave.



This is in the context of audio valve amplifiers. When I check
an
amp I put a 25MHz voltmeter across the load. If I get a
significant
reading with no input signal, it is oscillating. If I need to
know
the frequency of oscillation then I use a frequency counter.
The
scope only gets connected when I know the amp is not
oscillating.
Both these instruments are decent, ex-military units and fairly
recently calibrated. I can read them without using a ruler.


To check for ringing above 2MHz, I suppose I could sweep and use
the
voltmeter, but I don't bother. Perhaps for guitar amps, or
other
things with complicated voltage stages and crowded wiring?


Do you often discover anything worth finding above 2MHz? Maybe
I
should save up for a new scope?


** I started out ( as a teenager) with a home brew 3 inch, 2
MHz all
tube scope 35 years ago ( still have it too) then a 10 MHz
single beam and
for the last 16 years a 50 MHz dual beam.

SS amps can have parasitic oscillations up to 40 MHz or more,
especially
if they use MOSFETS.

Probably 2 MHz is enough for someone who works on tube amps
exclusively.




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