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Old 1st May 2012, 03:47 AM   #1
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Default ul - triode coupled screen grid resistor

Hi all,

it just maybe newbie question.

May gurus here explain what did r62 or r63 in this schematic
did both on triode or ul mode?

Thanks for your help.

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Old 1st May 2012, 10:00 AM   #2
DF96 is offline DF96  England
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Possibly damping a high Q HF resonance in the transformer? Normally a snubber network would be used for this, as a plain resistor will absorb power throughout the audio spectrum.
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Old 1st May 2012, 10:12 AM   #3
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Possible the author used a break-before-make type op switch, and added the resistors so he can safely switch while the amp is powered on
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Old 1st May 2012, 01:49 PM   #4
TheGimp is offline TheGimp  United States
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Since the screen switching is controlled by a relay it is unlikely that it is expected for one to turn off power to switch modes, else it won't work.

Therefore I concur with hidnplayr. The resistors insure a bias to the screens during the time the contact is switching.
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Old 1st May 2012, 01:56 PM   #5
DF96 is offline DF96  England
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Yes, I agree. I hadn't spotted the switching. It would never occur to me to design an amp like that so I suppose I don't notice such things.
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Old 2nd May 2012, 12:12 PM   #6
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Hi ,
thanks all for your opinion

btw is there a way we can determine the correct values for this

resistor?

Thanks
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Old 2nd May 2012, 04:20 PM   #7
DF96 is offline DF96  England
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It needs to be low enough to provide some screen current and add some load to prevent switching spikes from getting too big, and high enough not to consume too much audio power under normal conditions. In a sense there is no correct value because this is not really the way to do it. Changing from triode-UL-pentode really needs a change to feedback etc. too otherwise for at least one of the modes you are running with the wrong setup.
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