Grid Resistors
 

"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

Grid Resistors
 

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

Grid Resistors
 

"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.

Patrick Turner.

....AND better no pot at all, but if you must, use a step attenuator with
quality resistors.

west

Grid Resistors
 

"West" wrote in message
news:TAS_i.21505$763.20213@trnddc07...

"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.

Patrick Turner.

...AND better no pot at all, but if you must, use a step attenuator with
quality resistors.


Hi West.

Better still, no preamp at all:-) With the addition of a stepped
attenuator, one can feed the CD player straight to the power amp.
Morgan Jones states: "No preamplifier is better than any
preamplifier!" I agree with that sentiment.

Many take this a step further and add an input selector switch
as well as the SA to the power amp. This makes sense too.

Cordially,
Iain

Grid Resistors
 

"Iain Churches" wrote in message
i.fi...

"West" wrote


...AND better no pot at all, but if you must, use a step attenuator
with
quality resistors.


Hi West.

Better still, no preamp at all:-) With the addition of a stepped
attenuator, one can feed the CD player straight to the power amp.
Morgan Jones states: "No preamplifier is better than any
preamplifier!" I agree with that sentiment.


S'what I did with my 2A3 SET which is effectively a power amp with a
volume control - my thinking was that the 'pre' section could be a
nightmare for a beginner to get right (quiet) and that there are/were
any number of 'ready-made' pre's that could be used if necessary, until
such time I built my own! It has worked so well without a pre I never
got round to building one!


Many take this a step further and add an input selector switch
as well as the SA to the power amp. This makes sense too.


Except that it eliminates the option of a 'single source direct' and
condemns the amp to possibly 'inescapable' crosstalk with the extra
internal wiring....

Grid Resistors
 

"Andre Jute" wrote in message
ups.com...
On Nov 14, 11:05 am, Ian Thompson-Bell
wrote:


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

Andre Jute

Is this where the phrase "Plug and pray" originated ?

Grid Resistors
 

In article ,
(Don Pearce) wrote:

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.

But what happens if some of those electrons flying past the grid on
their way from the cathode to the anode get stuck on the grid making it
negative? Couldn't that keep a high mu input stage from saturating?


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/

Grid Resistors
 

On Thu, 15 Nov 2007 09:06:56 -0600, John Byrns
wrote:

In article ,
(Don Pearce) wrote:

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.

But what happens if some of those electrons flying past the grid on
their way from the cathode to the anode get stuck on the grid making it
negative? Couldn't that keep a high mu input stage from saturating?


No. Firstly it would only be electrons that actually chanced to hit
the grid that would stick, and secondly once even quite a small charge
had built up, the rest of the electrons would just get deflected
slightly away from the negative grid wires and no further charge would
build up.

d

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

Grid Resistors
 

"Don Pearce" wrote in message
...

No. Firstly it would only be electrons that actually chanced to hit
the grid that would stick, and secondly once even quite a small charge
had built up, the rest of the electrons would just get deflected
slightly away from the negative grid wires and no further charge would
build up.


If the electrons hit the grid with enough energy to cause secondary emission
then the grid potential would be driven positive until it reached the same
potential as the anode. Whether secondary emission would occur would depend
on the anode potential and the design and material of the grid.

David.

Grid Resistors
 

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?


I was taught that you should *never* allow DC on a volume or tone
control. The input to an amp should follow the sequence of resistor to
ground, isolating cap, pot, isolating cap, grid resistor to ground, grid
(or grid stopper). In light of this I've also wondered the same thing!

--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info http://mixpix.batcave.net