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Old 8th May 2009, 10:21 PM   #1
jetbat is offline jetbat  United States
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Default Circuit help

Hi, I was wondering if the components marked with red on the schematic are necessary or am I being redundant? I do not know if the tone stack would provide the necessary DC coupling and ground reference to make the red components unneeded. Any help would be appreciated.

Thanks
Scott
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Old 9th May 2009, 01:32 AM   #2
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Your observation seems valid to me, I think you could remove the red-marked parts, at least in theory. In practice, though, if the treble pot has a poor track it will cause unpleasant noise.
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Old 9th May 2009, 01:39 AM   #3
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Yes, I would agree with Ray.

The second triode is properly biased; I have never experienced 'contol noise' under such circumstances.
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Old 9th May 2009, 06:38 AM   #4
billr is offline billr  New Zealand
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the 12ax7 needs a grid resistance of about 1 meg, have a look at the specs and you will see this. I would leave it in.
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Old 9th May 2009, 07:31 AM   #5
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Quote:
Originally posted by billr
the 12ax7 needs a grid resistance of about 1 meg, have a look at the specs and you will see this. I would leave it in.
Of course !!
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Old 9th May 2009, 11:19 AM   #6
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Quote:
Originally posted by billr
the 12ax7 needs a grid resistance of about 1 meg, have a look at the specs and you will see this. I would leave it in.
???

Kindly, which specs are you referring to? Unless 'grid leak' bias is used, where minute grid current gives the necessary grid bias with a grid resistance of some 10meg - 20meg (which can be advantageous when fed from a low impedance source), the G1 resistor to common may be anything. It does not play a role with sufficient cathode bias as no grid current would exist.
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Old 9th May 2009, 08:01 PM   #7
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He is talking about the maximum grid resistance when the tube is self biased (has a cathode resistor) or when it is fixed bias (negative supply on the grid). 1meg for self bias and 500k for fixed bias, based on most datasheets.
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Old 9th May 2009, 08:19 PM   #8
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The grid of the second stage already has a Rgk of 250K + 250K + 2.7K at maximum, through the treble stack, if the 0.44µF capacitor is removed.

P
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Old 9th May 2009, 09:13 PM   #9
billr is offline billr  New Zealand
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No it doesn't, if the knobs are turned all the way round the pot is shorted out, turning the grid resistor into a 2.7k on its own.

It's a dc thing for the grid, you need the 1M resistor in there, the cap blocks the parallel action of the tone controls maintaining the grid resistor to it's original value.

hope this helps.

kind regards


bill
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Old 9th May 2009, 09:36 PM   #10
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Bill,
Sorry but you are wrong. With a 2k2 R in the cathode circuit, the grid resistance to ground can be pretty much anything, as long as it has some resistance. One meg would be an optimum MAXIMUM value, but it can be taken dowm to zero ohms with NO problems at all.

Regards, Allen (Vacuum State)
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