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142.206.2.21
In Reply to: RE: Plate dissipation. posted by Nunki on March 17, 2017 at 01:12:47
Hmm, not quite. Bias voltage is one thing, actual current is another. Forget the KT120 or KT150 for the moment, and suppose you're installing a new fresh set of tubes, same model and make. A given bias voltage will not produce (or restrict, rather) the current in an equal fashion between the two sets of tubes. One set might have a higher current output (closer to an open circuit). For the same reasons, this is why one would adjust the bias as tubes age. As the use builds up, they lose life in the form of current output (and I think transconductance as a result), so you have raise your bias voltage (bring up the negative value closer to zero), to restore the output level. Of course, there's a point where this cannot be done anymore (limitation of bias circuit), and a new set of tubes is necessary. And, working from that point where you've adjusted the bias for older tubes, when installing a fresh set, you want to apply more negative bias to restrict the current to prevent an overload condition.
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May your tubes be lively, warm, and long-lasting. Holy be thy heater.
Edits: 03/17/17Follow Ups:
"Of course, there's a point where this cannot be done anymore (limitation of bias circuit), and a new set of tubes is necessary"
By this time the space charge around the cathode will be depleted to the point where the tube is in serious danger of arcing. One of the challenges of trying to suck the last once of life from a set of output tubes. I know from experience. ;~)
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
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