wrote:
Paul Stamler wrote:

Not really -- you want it to be a good deal higher than that.
Typically a
cathode follower circuit has an output impedance of some 300-600
ohms, but
hang 3k on its output and suddenly it sucks, because you've eaten up
most of
its open-loop gain.

Hi Paul! how much higher? and what 'loop' are you refering to?
there's no global feedback happening in this pre.

There is... that's what a cathode follower is.... it's a stage with
100% feedback. The feedback through the grid circuit is what causes
the cathode to precisely follow the grid. And when you load it too
much, the cathode suddenly isn't following any more.

I think Mr. Stamler is exaggerating a little bit about a load that
is a factor of ten higher than the effective output Z being a problem.
Note that when the effective output Z is calculated, the gain of the
tube is taken into account... the more gain you have, the more the
feedback is helping you.

and what
performance change are you refering to when you say 'it sucks'? a load
that's 10x greater Z than the follower will have a small effect on its
output level. but of course, it may turn into a fuzz box. soon i
will investigate the effects of loading on distortion behavior with
the power of FFTs, i just haven't gotten the unit to my practice space
yet where my computer is.

You don't need an FFT, although it can be nice. Just stick a 1kc square wave
through the thing and watch what happens to the edges of the waveform.

also, can you recommend a transparent sounding zener diode series, for
the purpose of output clamping? with the swing available from tubes,
they really could wipe out any subsequent solid state stage that wasn't
input-protected. go over to a friends house, "dude check out
my...fftfftttttttssszzzz". oops, toasted your AD. sorry!

I don't like zener clamps if they can be avoided at all. And quite frankly,
you don't get very much output swing out of a cathode follower. At least
not if you want low output Z.

You really need to read the Radiotron Handbook section on followers.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."