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Old 24th September 2010, 08:42 PM   #21
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My guess about impurities in the tube still is the most probable. What if to bake tubes for several hours in 250 degrees oven, then experiment again?
If it is caused by impurities some of them will be absorbed by getter flash, the rest by cathode when they start working. I don't know what kind of molecules getter and cathode absorb, though... I do not know if metal evaporated by hot cathode can be absorbed such a way. But it definitely leaves traces on mica, I know that aging tubes have higher leakage because of this mechanism.
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Old 24th September 2010, 09:39 PM   #22
M Gregg is offline M Gregg  United Kingdom
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Wavebourn,
"My guess about impurities in the tube still is the most probable."

I think you may be correct I can not think of any other reason that fits!

If anyone else can explain it I would be very interested!

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Old 24th September 2010, 10:43 PM   #23
gcwills is offline gcwills  Australia
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Quote:
Originally Posted by Ian444 View Post
I've never grounded pin 9 on 6N1 or 6N2 and never had any problems.
Thanks for your feedback Ian - I guess you made the same assumption as me that it shouldn't matter if it is grounded or not. I am coming to the conclusion that I may have had a batch of these valves that are prone to this problem.

EDIT: I just saw your update Ian - yes 6N2P-EV valves do not demonstrate this phenomenon - it has only been the 6N2P valves that have this effect

EDIT: You can ignore the "images" in the original post - they are smilies from another forum post

Last edited by gcwills; 24th September 2010 at 10:54 PM.
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Old 24th September 2010, 11:31 PM   #24
gcwills is offline gcwills  Australia
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Originally Posted by DF96 View Post
To investigate what is happening, connect a DVM from pin9 to ground and see what happens to the voltage on the screen when it is left floating (apart from the DVM input impedance 10M?). If it is collecting electrons (from anode holes?) then it will go negative. On the other hand, it could go positive if each electron arriving kicks out more than one secondary electron - it could get very positive!

If it can receive electrons from the triode structure then it can affect the triode structure. Does the valve have anode holes facing the screen? This could be the difference between those which show the effect and those which don't. Or it could be the exact metal or coating of the screen affecting secondary electron emission.
An update - I have just done some testing with the pin 9 shield disconnected and measuring for any voltage on this pin. The results are inconclusive as a) it is very high impedance and my multimeter has an input impedance of 12Mohm and b) the readings were variable. I read a fluctuating negative voltage of about -300mv DC. I attempted to get an ac reading, but the reading of approx 1v ac maybe due to stray capacitances. I must admit, I was hoping for a high DC voltage which would support Wavebourne's theory of leakage from the plates across the mica supports but no
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Old 24th September 2010, 11:41 PM   #25
gcwills is offline gcwills  Australia
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DF96: "If it can receive electrons from the triode structure then it can affect the triode structure. Does the valve have anode holes facing the screen? This could be the difference between those which show the effect and those which don't. Or it could be the exact metal or coating of the screen affecting secondary electron emission."

I think you may be on to something - I have compared the 6N2P with the 6N2P-EV - the 6N2P has a round hole in the plate facing the shield whereas the EV version does not have a hole - could this be it? That the shield collects electrons and acts as an extra electrode in the triode??
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Old 24th September 2010, 11:48 PM   #26
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Quote:
Originally Posted by gcwills View Post
I must admit, I was hoping for a high DC voltage which would support Wavebourne's theory of leakage from the plates across the mica supports but no
Wavebourne's theory is, if the shield does exist between anodes, it helps. I never ever experienced similar problems like you describe, most probably because I ground a shield between anodes when it exists, and I don't hesitate to use grid stoppers.

Edit: I wonder, if 12AX7 that used in almost all guitar amps would work well in your amp without grid stoppers, like on your schematic...
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Old 25th September 2010, 08:06 AM   #27
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Quote:
Originally Posted by gcwills View Post
I think you may be on to something - I have compared the 6N2P with the 6N2P-EV - the 6N2P has a round hole in the plate facing the shield whereas the EV version does not have a hole - could this be it? That the shield collects electrons and acts as an extra electrode in the triode??
There you have it. It is very well possible that the shield potential (which you might not be able to measure with a ~10Megohm DMM) changes the tube curves.

In an ironic twist, the best samples (those having the least leakage between the shield and other electrodes) would probably suffer most from this effect
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Old 25th September 2010, 08:12 AM   #28
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by gcwills View Post
I have compared the 6N2P with the 6N2P-EV - the 6N2P has a round hole in the plate facing the shield whereas the EV version does not have a hole - could this be it? That the shield collects electrons and acts as an extra electrode in the triode??
Could you post some photos if possible?

Why does it not come back after disconnecting the shield from earth?

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Old 25th September 2010, 02:07 PM   #29
DF96 is offline DF96  England
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It may come back but only slowly as the screen charges up again. You must have very high quality valveholders, as any surface moisture would make the charge leak away. Are you in a hot dry region?

I am pleased that my prediction of anode holes seems to be correct. After suggesting measuring it with a DVM I realised that the impedance would be so high that even a DVM would just discharge it. Try running it for a while with no pin9 connection and see if the problem comes back. Also try putting some AC on pin9 (plus DC bias?) and see what effect it has on the triode.

My guess is that if the screen is much below the anode potential it will have little effect. If it is near or above the anode potential then it will attract electrons, and modify the triode behaviour. You could try just charging the screen up to anode potential then isolating it; this would happen naturally but slowly via the mica. My guess is that for higher screen potentials secondary emission would push it even higher. This may be why you get a binary response: low screen potential collects just a few low energy electrons so goes more negative and has even less effect, while high screen potential gets lots of fast electrons and loses even more through secondaries so goes more positive and has greater effect.
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Old 25th September 2010, 09:51 PM   #30
gcwills is offline gcwills  Australia
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Quote:
Originally Posted by DF96 View Post
It may come back but only slowly as the screen charges up again. You must have very high quality valveholders, as any surface moisture would make the charge leak away. Are you in a hot dry region?

I am pleased that my prediction of anode holes seems to be correct. After suggesting measuring it with a DVM I realised that the impedance would be so high that even a DVM would just discharge it. Try running it for a while with no pin9 connection and see if the problem comes back. Also try putting some AC on pin9 (plus DC bias?) and see what effect it has on the triode.

My guess is that if the screen is much below the anode potential it will have little effect. If it is near or above the anode potential then it will attract electrons, and modify the triode behaviour. You could try just charging the screen up to anode potential then isolating it; this would happen naturally but slowly via the mica. My guess is that for higher screen potentials secondary emission would push it even higher. This may be why you get a binary response: low screen potential collects just a few low energy electrons so goes more negative and has even less effect, while high screen potential gets lots of fast electrons and loses even more through secondaries so goes more positive and has greater effect.
Yes, the valveholders are high quality ceramic and humidity is fairly low here at this time of year. I have yet to test it, but I think you are correct - that if left for some time, the distortion will return due to electrons collecting on the shield. Thanks for your insight!
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