Circuit help

jetbat · 8th May 2009, 10:21 PM

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

ray_moth · 9th May 2009, 01:32 AM

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.

Johan Potgieter · 9th May 2009, 01:39 AM

Yes, I would agree with Ray.

The second triode is properly biased; I have never experienced 'contol noise' under such circumstances.

billr · 9th May 2009, 06:38 AM

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.

azazello · 9th May 2009, 07:31 AM

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 !!

Johan Potgieter · 9th May 2009, 11:19 AM

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.

jerluwoo · 9th May 2009, 08:01 PM

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.

Poindexter · 9th May 2009, 08:19 PM

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

billr · 9th May 2009, 09:13 PM

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

Allen Wright · 9th May 2009, 09:36 PM

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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9th May 2009, 01:32 AM	#2
ray_moth diyAudio Moderator Emeritus Join Date: Jan 2004 Location: Jakarta	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.

9th May 2009, 01:39 AM	#3
Johan Potgieter diyAudio Member Join Date: May 2005 Location: Pretoria, South Africa	Yes, I would agree with Ray. The second triode is properly biased; I have never experienced 'contol noise' under such circumstances.

9th May 2009, 06:38 AM	#4
billr diyAudio Member Join Date: Dec 2003 Location: new zealand	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.

9th May 2009, 08:01 PM	#7
jerluwoo diyAudio Member Join Date: Feb 2006 Location: Ashland,Ky U.S	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.

9th May 2009, 08:19 PM	#8
Poindexter diyAudio Member Join Date: Oct 2004 Location: Maui, Hawai'i, USA	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

9th May 2009, 09:13 PM	#9
billr diyAudio Member Join Date: Dec 2003 Location: new zealand	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

9th May 2009, 09:36 PM	#10
Allen Wright R.I.P. Join Date: Dec 2001 Location: Schaffhausen Switzerland	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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