Just be sure that you use "sharp cutoff pentodes". Some TV IF pentodes
were "remote cutoff". Variable gain depending on bias. Great way to do
AVC, but not good for audio work.


Hi Robert,

Why is that anyway?


These tubes have variable gm (or mu if triode), depending
on the DC bias on the control grid. Which means that
the output will not be linear. Not an issue if you are
using such a tube in an IF amplifier (signal level in
the millivolts, and most distortion products fall outside
the IF amp's passband). Bad for audio, which is a wide band
signal and higher levels. Excessive 2nd harmonic for sure.
If you have an audio amp using a 6AU6 try subbing a 6BA6.
It's sure to sound bad. A 6AU6 is a sharp cutoff pentode,
the 6BA6 is a remote cutoff pentode.