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Old 16th April 2012, 06:22 PM   #11
Yvesm is offline Yvesm  France
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Because of the DC coupling with the previus stage, this resistor just reduces the idle current in the PI by some 3 mA and, yes, at the expense of an extra load.
Gain is more probably halved than doubled.

Comprend pas / Can't understand . . .

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Old 16th April 2012, 06:23 PM   #12
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It is an extra load independent on grid currents of output tubes. It decreases linearity of the stage that is somehow compensated by global negative feedback, no big deal, but when the amp is overdriven it sounds les nasty as the result.
I.e. it is the way to make the amp little worse in normal conditions, and less worse when overdriven.

All author's explanations sound like marketing speech to me, total nonsense mixed together from technical words.
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Old 16th April 2012, 06:23 PM   #13
SY is offline SY  United States
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Quote:
Originally Posted by Tubologic View Post
Nevertheless, JH wrote (p.14):

... (la résistance) introduisant de la sorte une réaction positive de tension,...

which means:

... (the resistor) introducing in this way a positive voltage reaction...
He was wrong.
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Old 16th April 2012, 06:28 PM   #14
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Quote:
Originally Posted by SY View Post
He was wrong.
Not completely. Feedback becomes positive when upper load hit grid current of output tube, i.e. resistance of anode load becomes less than resistance of cathode load. In all other conditions it is just a shunt.
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Old 16th April 2012, 06:30 PM   #15
SY is offline SY  United States
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The phase splitter couples to a driver tube, not the output stage.
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Old 16th April 2012, 06:30 PM   #16
DF96 is offline DF96  England
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Yes, when all is balanced (the ideal case) it will increase the load while simultaneously reducing the quiescent current in the valve. Guaranteed recipe for increased distortion?

Quote:
Originally Posted by SY
Maybe he means that the gain from cathode to plate is 2?
Surely he can't mean that, because he compares it with the normal case of 0.7-0.9 which is clearly from grid to cathode. I suspect he has misapplied the feedback equation, or something similar. It is easy to get a daft answer by blindly plugging things into a formula.

To get useful positive feedback from anode to cathode you need a transformer. I won't say any more because I don't want to resurrect an old debate!
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Old 16th April 2012, 07:41 PM   #17
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Quote:
Originally Posted by SY View Post
The phase splitter couples to a driver tube, not the output stage.
So, in this particular case it was a cliche. I.e. result of generalizing of something that was useful in certain context. It happens very often, in electronics design it is called Rule of Dumbs. Like, shunting of electrolytic cap by a film one with 10% of capacitance, like Zobel networks in output of transistor amps, and so on, including whole topologies that are suboptimal in many cases except certain ones.
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