"Ian Iveson" wrote in message
...
Jim Lesurf 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.

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.

David.