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

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