Pre-Amp tube specs

[dearborndave]

I have a chart of 9 pin "12" type (12ax7,12au7,etc) tubes that list their Mu numbers and also a couple other items. One is TC which I figure is transconductance, and the other is HC which I have no idea what that stands for, but a 12ax7 is listed with a MU of 100, and HC of 300 and a TC of 1250. Does anyone know what these numbers mean?

[PRR]

Heater Current of 12AX7 is 300mA.

[dearborndave]

Okay, that makes sense. I was thinking it was heater current, but I thought it couldn't be that easy!