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

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

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.

"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.

"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

On Nov 14, 11:05 am, 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

Buy good Swiss pots and the problem should not arise. I like the DACT
pots which are Swiss SMD stepped ladders made for medical instruments
and repurposed for audio.

Shunt a high value resistor from the grid to 0V as an additional grid
leak, if it still worries you.

The late Bill May used to say, "Prayer also helps," on occasions when
I ran out of ideas.

Andre Jute
Visit Jute on Amps at http://members.lycos.co.uk/fiultra/
"wonderfully well written and reasoned information
for the tube audio constructor"
John Broskie TubeCAD & GlassWare
"an unbelievably comprehensive web site
containing vital gems of wisdom"
Stuart Perry Hi-Fi News & Record Review

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

I've never had that problem, even using cheaper Alpha pots...

Sealed military grade pots (PEC) or ladder attenuators (as mentioned) should
help.

Andrew


Ian Thompson-Bell wrote in message
...
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

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

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