Empirically it seems that bootstrapped the output impedance (at the anode of the bootstrapped tube) is lower than you'd get from the same load not bootstrapped. At least, I can see it doesn't get loaded down as much.
I'm assuming that bootstrapping is cutting the impedance by something around a factor of 2 looking at it naively. I'm searching for an analysis that works it out more formally. Maybe it's time to crack the books and try to work it out properly, instead of just kind of testing it into existence.
Frankly I'm just surprised how little fuss there is in adjusting this circuit, given it's kind of exotic appearance. Mainly just making sure you don't end up at some point where one of the tubes is biased too low and overheats. Other than that it pretty much just does it's thing with very little distortion over a wide range of reasonable resistor values.
Are you saying that the output impedance of the valve seems to have reduced? That is what it might look like, but what has actually happened is that the resistance of the anode load has increased. To a first approximation, the anode load resistor (i.e. the lower one) is boosted by a ratio of 1/(1-'Cf gain'). This means that the voltage gain of the valve approaches mu, as it sees less loading from the anode resistor.geezertron said:
As I said earlier, the downside is that the output impedance of the CF is also boosted, although not by the same ratio.
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