Grid Resistors
 

In article ,
(Don Pearce) wrote:

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.

That's fine, but the question is would enough charge buildup to prevent
the tube from saturating? I would think the situation would be similar
to the so called "contact bias" scheme used in the first audio stage of
old US built tube radios?


Regards,

John Byrns

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

Grid Resistors
 

On Thu, 15 Nov 2007 15:59:47 GMT, John Byrns
wrote:

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.

That's fine, but the question is would enough charge buildup to prevent
the tube from saturating? I would think the situation would be similar
to the so called "contact bias" scheme used in the first audio stage of
old US built tube radios?


I can't be absolutely definitive on that, because I don't have a valve
to try it with. I'm dimly aware of it being caused by a space charge
rather than actual travelling electrons headed for the anode, but when
you have a few hundred volts sucking hard a couple of mm away, how
much space charge can you build up? It is a phenomenon I only really
associate with the spaces between electrodes when they tend towards
the same potential.

How close to saturation would you consider close enough?

d

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

Grid Resistors
 

John Byrns wrote:


That's fine, but the question is would enough charge buildup to prevent
the tube from saturating? I would think the situation would be similar
to the so called "contact bias" scheme used in the first audio stage of
old US built tube radios?


Yep, its the same as grid bias, you sometimes see it in use, cathode to
0v, 1M or so resistor between the grid and 0v, it will bias itself up as
you suggest.

TL did it on the d3a in the second version of his LCR phono stage.

But in this case, the anode load won't have been selected for this sort
of bias, so you will still get more current than you expect.

--
Nick

Grid Resistors
 

On Thu, 15 Nov 2007 16:25:19 +0000, Nick Gorham
wrote:

John Byrns wrote:


That's fine, but the question is would enough charge buildup to prevent
the tube from saturating? I would think the situation would be similar
to the so called "contact bias" scheme used in the first audio stage of
old US built tube radios?


Yep, its the same as grid bias, you sometimes see it in use, cathode to
0v, 1M or so resistor between the grid and 0v, it will bias itself up as
you suggest.


Like grid leak biasing - where the coupling cap from the previous
stage gets charged up and provides negative grid bias?

TL did it on the d3a in the second version of his LCR phono stage.

But in this case, the anode load won't have been selected for this sort
of bias, so you will still get more current than you expect.

Isn't this something you'd expect to find only on low voltage tubes.

d

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

Grid Resistors
 

"Don Pearce" wrote in message
...
On Thu, 15 Nov 2007 15:59:47 GMT, John Byrns
wrote:

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.

That's fine, but the question is would enough charge buildup to prevent
the tube from saturating? I would think the situation would be similar
to the so called "contact bias" scheme used in the first audio stage of
old US built tube radios?


I can't be absolutely definitive on that, because I don't have a valve
to try it with. I'm dimly aware of it being caused by a space charge
rather than actual travelling electrons headed for the anode, but when
you have a few hundred volts sucking hard a couple of mm away, how
much space charge can you build up? It is a phenomenon I only really
associate with the spaces between electrodes when they tend towards
the same potential.

How close to saturation would you consider close enough?

As a quick experiment I've just tried disconnecting the grid of the second
half of the ECC83 in a Quad QC2 pre-amp that happens to be on my bench at
the moment. With 280V HT the anode voltage is normally 235V (50K anode load,
2.2K cathode bias resistor), with the grid disconnected it drops to 160V.
Trying the same trick with the first half (470K anode load, 4.7K cathode
resistor) only results in the anode voltage dropping from 114V to 105V.

David.

Grid Resistors
 

Don Pearce wrote:
On Thu, 15 Nov 2007 16:25:19 +0000, Nick Gorham
wrote:


John Byrns wrote:


That's fine, but the question is would enough charge buildup to prevent
the tube from saturating? I would think the situation would be similar
to the so called "contact bias" scheme used in the first audio stage of
old US built tube radios?


Yep, its the same as grid bias, you sometimes see it in use, cathode to
0v, 1M or so resistor between the grid and 0v, it will bias itself up as
you suggest.



Like grid leak biasing - where the coupling cap from the previous
stage gets charged up and provides negative grid bias?

Yes, except, you don't actually need a cap, as long as grid current
generated across the grid resistor generates the required voltage.


TL did it on the d3a in the second version of his LCR phono stage.

But in this case, the anode load won't have been selected for this sort
of bias, so you will still get more current than you expect.


Isn't this something you'd expect to find only on low voltage tubes.


Yes, but again thats the sort of tube you would expect to have a pot
connected to the grid.

--
Nick

Grid Resistors
 

On Thu, 15 Nov 2007 16:48:13 -0000, "David Looser"
wrote:

"Don Pearce" wrote in message
...
On Thu, 15 Nov 2007 15:59:47 GMT, John Byrns
wrote:

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.

That's fine, but the question is would enough charge buildup to prevent
the tube from saturating? I would think the situation would be similar
to the so called "contact bias" scheme used in the first audio stage of
old US built tube radios?


I can't be absolutely definitive on that, because I don't have a valve
to try it with. I'm dimly aware of it being caused by a space charge
rather than actual travelling electrons headed for the anode, but when
you have a few hundred volts sucking hard a couple of mm away, how
much space charge can you build up? It is a phenomenon I only really
associate with the spaces between electrodes when they tend towards
the same potential.

How close to saturation would you consider close enough?

As a quick experiment I've just tried disconnecting the grid of the second
half of the ECC83 in a Quad QC2 pre-amp that happens to be on my bench at
the moment. With 280V HT the anode voltage is normally 235V (50K anode load,
2.2K cathode bias resistor), with the grid disconnected it drops to 160V.
Trying the same trick with the first half (470K anode load, 4.7K cathode
resistor) only results in the anode voltage dropping from 114V to 105V.

David.


OK - that is about what I would expect. The ECC83 has an internal
resistance of 80kohm at 100V, so that is about as close to saturated
as you could expect it to get. I suspect that if you could actually
remove the grid wire, the voltages would not have come out far
different.

d

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

Grid Resistors
 

On Thu, 15 Nov 2007 16:54:41 +0000, Nick Gorham
wrote:

Don Pearce wrote:
On Thu, 15 Nov 2007 16:25:19 +0000, Nick Gorham
wrote:


John Byrns wrote:


That's fine, but the question is would enough charge buildup to prevent
the tube from saturating? I would think the situation would be similar
to the so called "contact bias" scheme used in the first audio stage of
old US built tube radios?


Yep, its the same as grid bias, you sometimes see it in use, cathode to
0v, 1M or so resistor between the grid and 0v, it will bias itself up as
you suggest.



Like grid leak biasing - where the coupling cap from the previous
stage gets charged up and provides negative grid bias?

Yes, except, you don't actually need a cap, as long as grid current
generated across the grid resistor generates the required voltage.

Except that in the case we are looking at, there is no grid resistor;
it has been disconnected.


TL did it on the d3a in the second version of his LCR phono stage.

But in this case, the anode load won't have been selected for this sort
of bias, so you will still get more current than you expect.


Isn't this something you'd expect to find only on low voltage tubes.


Yes, but again thats the sort of tube you would expect to have a pot
connected to the grid.

Always expect the unexpected!

d

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

Grid Resistors
 

Don Pearce wrote:


As a quick experiment I've just tried disconnecting the grid of the second
half of the ECC83 in a Quad QC2 pre-amp that happens to be on my bench at
the moment. With 280V HT the anode voltage is normally 235V (50K anode load,
2.2K cathode bias resistor), with the grid disconnected it drops to 160V.
Trying the same trick with the first half (470K anode load, 4.7K cathode
resistor) only results in the anode voltage dropping from 114V to 105V.

David.



OK - that is about what I would expect. The ECC83 has an internal
resistance of 80kohm at 100V, so that is about as close to saturated
as you could expect it to get. I suspect that if you could actually
remove the grid wire, the voltages would not have come out far
different.

d


I think you are confusing the rp with Rp, looking at the diode line

http://www.mif.pg.gda.pl/homepages/f...37/1/12AX7.pdf

I think its more like 46k, but the numbers still make sense,

160v across the diode line would expect about 3.4ma, you are getting a
bit less (280-160)/50000 = 3.2ma, so again near enough, maybe its
biasing itself off a little.

--
Nick

Grid Resistors
 

Don Pearce wrote:


Except that in the case we are looking at, there is no grid resistor;
it has been disconnected.

== very high value resistor.

--
Nick