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 Post subject: Grid leakage
PostPosted: Sep Mon 17, 2007 9:47 pm 
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Can someone explain what grid leakage of a vacuum tube is, in layman terms if possible.
How would it effect the emission of the tube also?


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 Post subject:
PostPosted: Sep Mon 17, 2007 10:21 pm 
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Grid leakage is caused by contamination of the grid structure by emissive material from the cathode. Small pieces of the oxide coating can come off and migrate to the grid due to attraction by the plate. When they get stuck on the grid they emit electrons and that current appears to originate in the grid.

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 Post subject:
PostPosted: Sep Mon 17, 2007 10:46 pm 
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My understanding of grid leakage is that too much gas (not a good vacuum) in the tube envelope creates positive ions which travel in the opposite direction of the electrons emitted from the cathode.

These positive ions are attracted to both the control grid and the cathode. This ion current makes the grid more positive and increases the cathode to plate current even more. If the grid current is excessive the tube will be detroyed. The amount of grid current in a power tube limits the maximum value of grid resistance that can be use for grid bias.

Another term for tubes with excessive grid current are "gassy tubes". Sometimes these gassy tubes will glow with an nice pink or blue color.

I've got a few gassy western electric tubes that have a very interesting glow when checking in a tube tester.

Chuck D.


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 Post subject:
PostPosted: Sep Mon 17, 2007 10:48 pm 
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Thanks Leigh
The reason I'm curious is I've been playing with my first-ever tube tester that I just aquired.
Tubes that test weak for emissions when doing the "life test" (dropping down the filament voltage), still test well on the grid leakage test.
Can or is this normal?, I'm just wondering if the tester is working ok when on the grid leak function.


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 Post subject:
PostPosted: Sep Tue 18, 2007 12:09 am 
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Yes, what you are seeing is very common.

The control grid of a tube is generally biased negatively to set the operating parameters of the tube in the circuit. What this grid leakage or gas does, is it negates the bias, which is the same as making the grid positive. And with a positive control grid, the plate current increases drastically causing the tube to heat up more, which negates even more bias so the grid goes even further positive, and more plate current. After a while the domino affect takes over and the tube runs away and destroys itself, and maybe the output transformer and power supply components as well.
Curt

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 Post subject:
PostPosted: Sep Tue 18, 2007 12:13 am 
Silent Key

Joined: Jan Tue 16, 2007 11:48 pm
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Leigh, I'm glad we egree on something. :D :D I have a theory about gas vs contamination as both cause the same problem, the grid goes positive and tube current becomes excessive. Seems to me gas would show up rather rapidly, just how rapidly I have no clue. Contamination, in my opinion, may take a while to take over due to the grid having to heat up in order to cause the grid to emit, say like 15 or 20 minutes, maybe a bit less. Any thoughts on this??

Common problem on AA5 output tubes, seems more prevalent on miniature tubes as they run hotter. Can happen on other tubes in radios and TVs. Philco is notorious in that I have often (way back) had to replace all IF amp tubes, 3 or 4 6CB6s, in order for the AGC bus to regain control. They used high value grid returns which just added to the problem.


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 Post subject:
PostPosted: Sep Tue 18, 2007 12:22 am 
Silent Key

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The way I look at it, contamination or gas causes the tube to run hot and get even hotter as it runs. This will eventually get the grid so hot that it starts emitting electrons from the contamination it receives from the cathode, which is shedding more and more material with the increased heat which collects on the grid, making it even more emissive.

When operated in Class C where there is considerable grid current normally flowing, you got to be careful with some tubes as a bit too much grid current will soon cause runaway. The 6146 with its fine wire used for a control grid is notorious for this. Run one at 5mA grid current for a while and see what happens. At least the 807/1625 tubes are a lot more stout in this regards.
Curt

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 Post subject:
PostPosted: Sep Tue 18, 2007 1:23 am 
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jkaetzjr wrote:
Leigh, I'm glad we egree on something. :D :D I have a theory about gas vs contamination as both cause the same problem, the grid goes positive and tube current becomes excessive. Seems to me gas would show up rather rapidly, just how rapidly I have no clue. Contamination, in my opinion, may take a while to take over due to the grid having to heat up in order to cause the grid to emit, say like 15 or 20 minutes, maybe a bit less. Any thoughts on this??

Agreement is good for the sole (as in feet or fish, whichever you prefer :D).

Unwanted stuff occupying part of the vacuum can cause all sorts of problems inside a tube. I'm not sure to what extent the effects can be categorized, since all such conditions are outside the design parameters of the tube.

Certainly some symptoms of contamination will increase with grid temperature. I've always thought this to be related to the migrated cathodic material deposited on the grid, but I've never researched it in detail.

The presence of gas within the envelope is a much more complicated subject since different gases would react very differently to current flow and temperature.

Very interesting subject.

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 Post subject:
PostPosted: Sep Tue 18, 2007 1:35 am 
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As long as we are on this topic, I have a question in regard to grid leakage too.

Is it possible to have a NOS tube and still have grid leakage register on the tube testor?


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 Post subject:
PostPosted: Sep Tue 18, 2007 2:13 am 
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The authority on grid current is Robert Tomer's book "Getting the Most out of Vacuum Tubes".

Grid current can be caused by three things: grid emission, gas, or leakage. Tomer explains how to tell the differences. Gas current goes away if the tube is biased beyond cutoff. Leakage is there all the time (even cold, sometimes). Grid emission does require time for the grid to heat up, as noted.


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 Post subject:
PostPosted: Sep Tue 18, 2007 3:15 am 
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If you want to see a good example of grid emission, find a pair of well used and abused 7591's. They are one type that frequently suffers from that problem. I have seen some that will start out OK but within a minute or so start to run away, and eventually the G1 structure will actually glow bright orange just like the cathode does. This is caused by cathode material becoming deposited on the grid, and getting very hot in operation. Of course the tubes are useless once that happens.

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 Post subject:
PostPosted: Sep Tue 18, 2007 4:04 am 
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The 7591 has a very close grid spacing to give it its high transconductance. And the designers of the 7591 pushed the spec for maximum grid resistance too, to make it easier to drive - they allowed up to 300K (6L6 and 6V6 are 100K). With such a high grid resistance, even a little grid leakage is too much. Good thing new 7591s only cost $2.30... or have they gone up since 1970?


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 Post subject:
PostPosted: Sep Tue 18, 2007 5:46 am 
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In a normal tube, a very tiny current flows from grid to ground. This is caused by the occasional electron striking the grid and returning to ground through the grid resistor. In the absence of a return path, the grid will accumulate electrons and eventually cut off.

With grid contamination or gas, the electron flow is in the other direction from ground to grid either to replace those lost by grid emission or those consumed in discharging positive gas ions.

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