Grid Resistors

**Ian Thompson-Bell[_2_]** · November 14th 07, 10:05 AM posted to rec.audio.tubes,uk.rec.audio

I often see volume control pots with their wiper connected directly to
the grid of the following tube. Trouble is as they wear they get a bit
scratchy and often lead to the grid becoming disconnected. Would you
agree it is good practice to add a resistor (say 1Meg) directly across
the grid?

Cheers

Ian

**Don Pearce** · November 14th 07, 10:15 AM posted to rec.audio.tubes,uk.rec.audio

On Wed, 14 Nov 2007 11:05:10 +0000, Ian Thompson-Bell
wrote:

I often see volume control pots with their wiper connected directly to
the grid of the following tube. Trouble is as they wear they get a bit
scratchy and often lead to the grid becoming disconnected. Would you
agree it is good practice to add a resistor (say 1Meg) directly across
the grid?

Cheers

Ian

Yes, it is a good idea. Make the resistor about ten times the value of
the pot, or you will be changing the way the volume control reacts.

Disconnected grids are bad news.

d

--
Pearce Consulting
http://www.pearce.uk.com

**Iain Churches[_2_]** · November 14th 07, 04:13 PM posted to rec.audio.tubes,uk.rec.audio

"Don Pearce" wrote in message
...
On Wed, 14 Nov 2007 11:05:10 +0000, Ian Thompson-Bell
wrote:

I often see volume control pots with their wiper connected directly to
the grid of the following tube. Trouble is as they wear they get a bit
scratchy and often lead to the grid becoming disconnected. Would you
agree it is good practice to add a resistor (say 1Meg) directly across
the grid?

Cheers

Ian

Yes, it is a good idea. Make the resistor about ten times the value of
the pot, or you will be changing the way the volume control reacts.

Yes. This was discussed some time ago on RAT. I think it was Sander
who raised the subject. He too recommended R to be 10 x the pot
value.

Iain

**Ian Iveson** · November 14th 07, 05:55 PM posted to rec.audio.tubes,uk.rec.audio

Don Pearce said

Yes, it is a good idea. Make the resistor about ten times
the value of
the pot, or you will be changing the way the volume
control reacts.

10-1 rule again :-)

I often see volume control pots with their wiper connected
directly to
the grid of the following tube. Trouble is as they wear
they get a bit
scratchy and often lead to the grid becoming disconnected.
Would you
agree it is good practice to add a resistor (say 1Meg)
directly across
the grid?

Disconnected grids are bad news.

Why, in this case?

cheers, Ian

**Don Pearce** · November 14th 07, 09:41 PM posted to rec.audio.tubes,uk.rec.audio

On Wed, 14 Nov 2007 18:55:28 GMT, "Ian Iveson"
wrote:

Don Pearce said

Yes, it is a good idea. Make the resistor about ten times
the value of
the pot, or you will be changing the way the volume
control reacts.

10-1 rule again :-)

I was considering 9.72, but on balance...

I often see volume control pots with their wiper connected
directly to
the grid of the following tube. Trouble is as they wear
they get a bit
scratchy and often lead to the grid becoming disconnected.
Would you
agree it is good practice to add a resistor (say 1Meg)
directly across
the grid?

Disconnected grids are bad news.

Why, in this case?

cheers, Ian

Because the anode current will rise until the valve hits saturation.

d

--
Pearce Consulting
http://www.pearce.uk.com

**Iain Churches[_2_]** · November 15th 07, 05:02 AM posted to rec.audio.tubes,uk.rec.audio

"Don Pearce" wrote in message
...
On Wed, 14 Nov 2007 18:55:28 GMT, "Ian Iveson"
wrote:

Don Pearce said

Yes, it is a good idea. Make the resistor about ten times
the value of
the pot, or you will be changing the way the volume
control reacts.

10-1 rule again :-)

I was considering 9.72, but on balance...

I often see volume control pots with their wiper connected
directly to
the grid of the following tube. Trouble is as they wear
they get a bit
scratchy and often lead to the grid becoming disconnected.
Would you
agree it is good practice to add a resistor (say 1Meg)
directly across
the grid?

Disconnected grids are bad news.

Why, in this case?

cheers, Ian

Because the anode current will rise until the valve hits saturation.

d

Most input stages are cathode biased.

Iain

**Don Pearce** · November 15th 07, 05:12 AM posted to rec.audio.tubes,uk.rec.audio

On Thu, 15 Nov 2007 08:02:14 +0200, "Iain Churches"
wrote:

"Don Pearce" wrote in message
...
On Wed, 14 Nov 2007 18:55:28 GMT, "Ian Iveson"
wrote:

Don Pearce said

Yes, it is a good idea. Make the resistor about ten times
the value of
the pot, or you will be changing the way the volume
control reacts.

10-1 rule again :-)

I was considering 9.72, but on balance...

I often see volume control pots with their wiper connected
directly to
the grid of the following tube. Trouble is as they wear
they get a bit
scratchy and often lead to the grid becoming disconnected.
Would you
agree it is good practice to add a resistor (say 1Meg)
directly across
the grid?

Disconnected grids are bad news.

Why, in this case?

cheers, Ian

Because the anode current will rise until the valve hits saturation.

d

Most input stages are cathode biased.

Iain

Cathode biasing needs the grid to be held down. What happens if the
grid becomes disconnected is that the valve (I am assuming a triode)
becomes effectively a diode, wired across HT to ground, with current
limiting provided only by the anode and cathode resistors in series.
Generally the anode resistor is much bigger than the cathode, so it
does most of the work in this regard.

d

--
Pearce Consulting
http://www.pearce.uk.com

**Patrick Turner** · November 14th 07, 12:44 PM posted to rec.audio.tubes,uk.rec.audio

Ian Thompson-Bell wrote:

I often see volume control pots with their wiper connected directly to
the grid of the following tube. Trouble is as they wear they get a bit
scratchy and often lead to the grid becoming disconnected. Would you
agree it is good practice to add a resistor (say 1Meg) directly across
the grid?

Cheers

Ian

Yes. From 0V or bias voltage to the grid.

AND better you replace the pot after cleaning it if that doesn't stop
the sratchiness.

Patrick Turner.

**David Looser** · November 14th 07, 01:06 PM posted to rec.audio.tubes,uk.rec.audio

"Patrick Turner" wrote in message
...

Ian Thompson-Bell wrote:

I often see volume control pots with their wiper connected directly to
the grid of the following tube. Trouble is as they wear they get a bit
scratchy and often lead to the grid becoming disconnected. Would you
agree it is good practice to add a resistor (say 1Meg) directly across
the grid?

Cheers

Ian

Yes. From 0V or bias voltage to the grid.

AND better you replace the pot after cleaning it if that doesn't stop
the sratchiness.

And I would also suggest ensuring that there is no DC across the pot (such
as that due to a leaky coupling cap from a previous amplifier stage). Even a
pot in pristine condition will create noise when adjusted if there is DC
across it.

David.

**Ian Thompson-Bell[_2_]** · November 14th 07, 07:08 PM posted to rec.audio.tubes,uk.rec.audio

David Looser wrote:
"Patrick Turner" wrote in message
...

Ian Thompson-Bell wrote:
I often see volume control pots with their wiper connected directly to
the grid of the following tube. Trouble is as they wear they get a bit
scratchy and often lead to the grid becoming disconnected. Would you
agree it is good practice to add a resistor (say 1Meg) directly across
the grid?

Cheers

Ian
Yes. From 0V or bias voltage to the grid.

AND better you replace the pot after cleaning it if that doesn't stop
the sratchiness.

And I would also suggest ensuring that there is no DC across the pot (such
as that due to a leaky coupling cap from a previous amplifier stage). Even a
pot in pristine condition will create noise when adjusted if there is DC
across it.

David.

Yes I have come across that on old broadcast receivers. Leaky coupling
cap buggers up grid bias on output tube - result distortion and hot
tube. often need to replace cap and tube to repair.

Ian