All about Tubes, Tube Circuits, Tube Gear

Thursday, June 14, 2012

Ultralinear Guitar Amp (4): Screen Resistors

In regard to screen resistors,
I've read a lot of internet discussions on it,
and some of them actually had some 'meat', i.e., somebody actually tried different values in a commercial amp or two.

What I read left me with this impression:

(1) Although higher resistors limited screen current, something happened to the guitar 'sound'. SR values higher than 600 ohms made the output stage "too stiff", harsh and non-musical in the judgment of the players.

(2) This was thought at first to be some kind of distortion, but turned out to be the opposite! Specifically, the 'attack' was too sharp, possibly reproducing the dismal performance of the pickups or previous stages. No clear results could be given in regard to the 'decay' of a note, but since that was less important it was less noticable.

(3) The cure was to go back to lower screen resistors, to preserve a 'softer' attack, but then make some other adjustment (presumably fixed bias adjustments or lower B+) to protect or enhance tube life.

I was really interested to find out that this approach (increasing screen resistors) was effective from an engineering standpoint but was disastrous musically.

Another case of solving a problem without regard to the overview and/or preserving some desirable flaw.

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