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Old 26th September 2010, 02:29 PM   #31
M Gregg is offline M Gregg  United Kingdom
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DF96,

So is the court out

Its the position of the holes on the Anode!

I must admit its realy interesting, I would not have expected a build up on the shield.

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M. Gregg
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Old 26th September 2010, 02:48 PM   #32
DF96 is offline DF96  England
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The jury is waiting for more evidence.
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Old 26th September 2010, 03:15 PM   #33
Merlinb is offline Merlinb  United Kingdom
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I am looking at a 6N2P now, and something else I see is that the shield is also attached to the getter, and it hangs right over the grid support rods of one of the triodes where they poke through the mica. This might not be the case for all samples though (I only have one).
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Old 26th September 2010, 08:46 PM   #34
DF96 is offline DF96  England
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It could have an effect if the grid input is high impedance.
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Old 27th September 2010, 05:49 AM   #35
gcwills is offline gcwills  Australia
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As others have suggested, I have done further tests with applying several hundred volts positive to the shield. This immediately induces the distortion effect and the distortion remains until the shield is grounded again. So it seems that there are two mechanisms happening - firstly some leakage as has been suggested between the plates and the shield which builds up a positive charge on the shield. Secondly, because there is a hole in the plate this voltage on the shield has a deleterious effect on electron flow causing this distortion.

So evidently in the valves that I regarded as "faulty", there was an unhappy congruence between these two issues causing the distortion. Either no leakage - or no hole in the plate, and this phenomenon would not occur.

Of course this is all now academic as all shields will be grounded in future, nonetheless, I have been very keen to understand why this originally happened.

Last edited by gcwills; 27th September 2010 at 05:58 AM.
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Old 27th September 2010, 01:50 PM   #36
Merlinb is offline Merlinb  United Kingdom
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A final point I would be interested to know:
If you take one of the valves that shows this distortion and momentarily ground pin-9, it disappears, right? But after removing the connection again, does the distortion return after a period of continuous running?
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Old 27th September 2010, 07:52 PM   #37
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Quote:
Originally Posted by gcwills View Post
So it seems that there are two mechanisms happening - firstly some leakage as has been suggested between the plates and the shield which builds up a positive charge on the shield. Secondly, because there is a hole in the plate this voltage on the shield has a deleterious effect on electron flow causing this distortion.
I suppose there is no chance that a few electrons miss the anode and go straight through the hole, building the shield a negative charge?
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Old 27th September 2010, 08:19 PM   #38
TheGimp is offline TheGimp  United States
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I wonder if a FET input op amp rigged as a unity gain inverting buffer could be used to measure which direction the charge is biasing the shield?
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Old 27th September 2010, 09:02 PM   #39
DF96 is offline DF96  England
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That would be interesting to know. I expect that a zero voltage screen might stay there, or slowly rise due to mica leakage. A middle voltage screen would go lower as it collects electrons through the hole, so the two effects might cancel. A high voltage screen would go higher due to secondary emission. I am not sure where the boundary is between these regions.
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Old 1st October 2010, 10:38 PM   #40
gcwills is offline gcwills  Australia
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Have retested the 6N2P with a range of voltages on the shield - as mentioned before, when the shield is taken to +250V DC, the distortion is evident. Ground the shield and distortion gone. I have just done further tests with -300V applied to the shield (via limiting resistor) and no distortion. I then decided to check for any AC component on the floating shield. Turns out that there is a large AC component (probably larger but for the input impedance of my CRO) on the shield. This AC component is a distorted version of the input sine wave (see attached photo) and is probably in the hundreds of volts peak to peak. No wonder this had such an adverse effect on the triode operation - when the triode was driven a distorted version of the signal develops on the shield. This then distorts the normal operation of the triode. So it appears that the distortion issue was related in part to DC conditions on the shield, but also the presence of a large distorted AC signal on the shield is the primary reason for the distortion.

So it seems that my initial theory (electrons building up a negative charge on the plate) was not the mechanism for the distortion. Rather it seems that minute leakage in the valve across the mica support or capacitance between the plates and the shield builds up a POSITIVE voltage on the shield. This positive potential (with a large distorted AC signal superimposed) then has an adverse effect on electron flow in the valve due to the hole in the plate facing the shield.
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Last edited by gcwills; 1st October 2010 at 10:40 PM.
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