NFB normally converts any current source into a voltage source.

This depends upon the open loop impedance, amount of feedback, etc.

Even if the Ro of the open loop amp is infinite,
series voltage NFB converts it into a voltage source.
It doesn't take much NFB to get a DF of 10.



The snag is that my impression is that some valve designs with output
transformers only use a modest amount of feedback. As a result they can
have output impedance of the order of 0.5 Ohms. (e.g. the amp Iain
mentioned in another thread recently). This level of output impedance
allows for quite audible changes in the response with loudspeakers whose
input impedance varies with frequency.

Rubbish, imho.

99% of ppl would remain totally unaware if a 1 ohm resistor was placed betwen
their speakers
and a low Ro amp.

Regarding SS reliablity, all I know is that there are many failures,
and mainly due to shorted speaker leads.

Patrick Turner.