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Old 24th September 2010, 02:11 PM   #11
M Gregg is offline M Gregg  United Kingdom
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Hi,

I am reading this information:

Wapedia - Wiki: Vacuum tube

It has some information that anyone interested in tubes will find interesting! If anyone finds something in this info please comment!

Regards
M. Gregg

Last edited by M Gregg; 24th September 2010 at 02:38 PM.
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Old 24th September 2010, 02:34 PM   #12
Ian444 is offline Ian444  Australia
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Originally Posted by Ian444 View Post
I've never grounded pin 9 on 6N1 or 6N2 and never had any problems.
Sorry, should have mentioned I only ever used -EV versions. But then, the hi-fi people don't normally push any stage of the amp into hearable distortion either, cause then we back off. If it was a guitar amp, then I guess turn it up to 11 and then some more.
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Old 24th September 2010, 02:52 PM   #13
DF96 is offline DF96  England
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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.
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Old 24th September 2010, 03:07 PM   #14
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Quote:
Originally Posted by M Gregg View Post
gcwills can you please repost the images we can't get to see them! Thank's
The "images" are just smilies from the Australian forum where Grant first posted his conundrum. They appear correctly on my screen - probably because I frequent that particular forum, and the smilies will be in my temporary internet files.

Looking at Grant's schematic, there are no grid stoppers on the first two triode sections. Perhaps one or both triode sections are on the brink of oscillation, and a large input signal tips the balance. Earthing the shield may be just enough to keep them stable. This doesn't really explain the ongoing stability once the shield is again disconnected though.

If the valve manufacturers went to the extra trouble to include a shield between the triodes, perhaps we should just make use of it.
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Old 24th September 2010, 03:14 PM   #15
M Gregg is offline M Gregg  United Kingdom
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It would be "nice" to know if it is linked to Miller effect!
The problem not showing after removing the earth is a mystery!
What can remain "magnetic effects" "static/ electrostatic" would discharge after a time!

Or as Wavebourne says contamination being "burnt off". Interesting it dosen't burn off until the shield is connected to earth!

So the shield was not connected to earth at testing?

Last edited by M Gregg; 24th September 2010 at 03:20 PM.
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Old 24th September 2010, 05:23 PM   #16
DF96 is offline DF96  England
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Miller effect?

I think it is an electrostatic effect. Mica and glass are good insulators.
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Old 24th September 2010, 05:32 PM   #17
M Gregg is offline M Gregg  United Kingdom
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Just for interest!

http://web.mit.edu/klund/www/papers/jmiller.pdf


Regards
M. Gregg
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Old 24th September 2010, 06:55 PM   #18
DF96 is offline DF96  England
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Just to clarify, I wasn't asking what the Miller effect is, but why you think it might be relevant.
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Old 24th September 2010, 08:07 PM   #19
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by DF96 View Post
Just to clarify, I wasn't asking what the Miller effect is, but why you think it might be relevant.
Well if we ground the shield we are changing the capacitance within the tube. Its just a thought!

The Valve Wizard

Things like this do make the hobby interesting!
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Old 24th September 2010, 08:29 PM   #20
DF96 is offline DF96  England
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Grounding the screen will mainly affect the capacitance between the two anodes. Mller effect arises from the capacitance between grid and anode. However, if the two halves of the triode are cascaded then one anode will be coupled to the other grid, so there could be an effect but all it would do is change the HF frequency response a little. Unlikely to introduce serious distortion, and it would be repeatable i.e. remove the ground and the problem comes back straight away.
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