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When you put that jumper from pin 1 (cathode) to pins 3 & 9, it shunts the contrast control and R109 completely out-of-circuit, giving the tube all the gain it's gonna deliver. The tube's at max gain and doesn't "see" the contrast control and R109 at all.
That may be all it's capable of with the low B+. What is the actual plate voltage BTW?
You might try raising the value of R87 a little, to allow the plate a little more 'swing'. For the heck of it, also try lowering the value a bit.
Last edited by old_coot88; 01-27-2017 at 01:27 PM.
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01-27-2017, 11:40 AM
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