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Old 31st March 2009, 12:35 PM   #1
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Default Bias Supply resistors

Hi - I have attached an image of the bias supply of an amplifier from van der Veens book "Modern High-End Valve Amplifiers". I am using a similar design on my project and was wondering if anybody would be able to shed any light on the function of resistors R35, 37, 39 and 41. Do they merely block AC signals from affecting the bias supply?

Whatever the case may be, how would I go about deciding on a value for them? The values of the ones in the example I have given are 220k.

Thanks
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Old 31st March 2009, 06:07 PM   #2
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Be interestring to see what un1 to un4 actually are....

IF these are the grid-connections, then these resistors would be the grid resistors....

Personally I would add a couple of extra caps to that layout...
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Old 31st March 2009, 07:10 PM   #3
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I think the same....Grid res.....Maybe there is grid selector for using different tubes.....
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Old 31st March 2009, 07:32 PM   #4
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I've attached another image with the rest of that circuit on if thats any help - looks like there is already a grid resistor on there so what else could they be? It says the quiescent current of the valves should be set at 45mA each valve. Where would you add some capacitors Alastair out of interest?
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Old 31st March 2009, 10:11 PM   #5
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R13, 16, 19, 22 - against oscillations....
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Old 31st March 2009, 10:18 PM   #6
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That makes sense but I'm still unsure of the purpose of R35, 37, 39, 41 and what value to use for them
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Old 31st March 2009, 10:40 PM   #7
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Apr. 100 - 200 kohm.
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Old 31st March 2009, 11:29 PM   #8
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Yeah that agrees with the book, but what's the difference between R35-41 and R13-22? They both seem to be some kind of grid stopper to me - how come just one resistor can't be used?

I've just been looking around on the net and it seems that R35-41 are bias feed resistors and R13-22 are grid stoppers. Would people agree with this? I'm still unsure what the purpose of the bias feed resistor is though...
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Old 1st April 2009, 12:16 AM   #9
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The problem as I see it is the ****-eyed way the schematic has been drawn , which makes its function a bit obscure.

R35, 37, 39 and 41 are the grid-leak resistors. Don't think of them as "bias feed resistors", that's a meaningless term and confusing; they are grid leak resistors. Without those resistors in circuit, the incoming signal would be effectively shorted to ground through the bias pots.

R13 - 22 are grid stoppers, as you say.
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Old 1st April 2009, 12:25 AM   #10
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Ah yes - Now that you say that, I think I have encountered the same problem before! All is clear now and knowing that they're grid leak resistors makes good sense. Thanks a lot!
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