power tube mismatch reassessed
- Subject: power tube mismatch reassessed
- From: "James D Rossen and Dina Janzen" <dlj@xxxxxx>
- Date: Sun, 23 Feb 2003 09:16:34 -0600
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
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.
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