power tube mismatch reassessed

["James D Rossen and Dina Janzen" <dlj@xxxxxx>]

I acquired an amp that is well suited to investigating the influence of power
tube mismatch on tone- an Ampeg Gemini II with fixed bias 7591 x 2, a bias pot
and a balance control.  The balance control is a 100 ohm pot with the wiper to
ground and outside terminals connected to the cathode of each power tube.  I
added 1 ohm bias test resistors between the balance pot and cathodes.  During
operation the cathode voltage is about 2 V and plate voltage about 470 volts.
The balance pot allows precise balancing or un-balancing of idle current during
listening.

Result was that I found the harmonica tone to be fatter and warmer with
mismatched idle current.  My ears favored mismatch of about 6 ma.   Above this
and the tone seemed to get muddy/harsh.

Seems like this sort of balance adjustment would be easy to implement on fixed
bias amps, and provide another way of tweaking tone to preference.

Jim R