Measurements of Amplifier Peak Output
Voltages Under Dynamic Conditions and
Into a Real Loudspeaker Load
by Peter van Willenswaard
http://www.audionote.co.uk/technical/amppeak/
The conundrum within the enigma is so paradoxical about tube circuits that it is important to wrestle with what we know but can't measure, which is why this article is such great reading. Enjoy