David Looser wrote:

When valve amps were the norm, true "slew rate limiting"
was unheard of, AFAIK. Not enough gain or feedback.

It may have been "unheard of", but it existed all the
same. Any amplifier, with or without feedback, can exhibit
slew-rate limiting if the bandwidth is insufficient to
cope with the rate of rise or fall of the input signal.

Staggered. I really don't know what to say...where could I
begin?

One effect that I have seen referred to as slew rate
limiting can happen when a cathode follower with
inadequate bias current fails to keep up with the demands
of a capacitive load. Like real slew rate limiting, it is
an effect of feedback, and results when the triode
approaches turn-off only when the signal is both high
amplitude and high frequency...which is when the slew
rate is highest. However, for small amplitudes, the same
CF wouldn't suffer from the same problem even if the
input slew rate were the same.

Nope, CF failure can occur even with small amplitude
signals. The cathode voltage can only fall at the rate
determined by the time constant of the cathode resistor
and load capacitance. If the input falls faster than this
the output will not follow regardless of the signal
amplitude. CF failure can be a problem with video signals;
to drive a high-capacitance load with a CF may require a
load resistor so low in value as to represent a real
problem. In the original TV transmitter at Alexandra
Palace the cathode resistor of the CF at the output of the
modulation amplifier required water-cooling as it
dissipated over a kilowatt. Since the valve was a DH type
the filament supply for it came from a motor-generator set
mounted on tall insulators to minimise it's capacitance to
earth! More recent TV transmitter design has used White
cathode-followers or other forms of push-pull drive for
this function to reduce the power dissipation. The
transmitter I am currently working on uses 4 PL38s in a
White cathode-follower.


Staggered, once again. Next time I see you, you should be in
sackcloth and ashes.

In the meantime I suggest you check out how a cathode
follower works, and in particular how its output impedance
is defined.

Then look into the meaning of the phrase "slew rate
limiting". I guess Wikipedia would be an appropriate place
to start. You may argue that it has acquired a wider, more
sloppily-defined meaning, but I prefer to keep the useful
distinctions that a disciplined use of language is able to
convey. In any case, no matter how sloppily defined it may
have become, your own interpretation is far beyond the pale.

Looking at Wiki myself, note the reference to op amps, high
transconductance, and the implication of the presence of
feedback because of the role of the compensation capacitor.

Engineers weren't daft in the days of valve amps. Had slew
rate limiting been a problem, it would have been recognised.
Yes there are various forms and causes of current limiting,
which may appear similar to slew rate limiting, but that's
not what they are.

Ian