6N2P - WHAT THE????????????? - diyAudio
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Old 24th September 2010, 12:36 AM   #1
gcwills is offline gcwills  Australia
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Default 6N2P - WHAT THE?????????????

Hi Brains Trust,

I have recently made a discovery that has really surprised me, and wanted to find out if others have had a similar experience.

I have been supplying 6N2P valves for an Amp School that I run here in Sydney and numbers of these amps have been built successfully this year. However, there has been a niggling issue that until now I have been unable to lay to rest. The issue concerns the 6N2P valves used in the amps that students build. For those who are not familiar with this valve, it is effectively a 12AX7 with a 6.3v heater.

A few of the constructed amps have had a weird distortion when the preamp is overdriven - the preamp sounds great when run clean, but when overdriven has had a really nasty sounding distortion that I was able to "fix" by changing a few preamp valves. However, I haven't been really satisfied with this situation, and have sine wave tested a few sample amps and the ugly distortion is clearly evident when the preamp is overloaded. The odd thing is that the valves bias perfectly, and when a 6N2P-EV is substituted, the distortion seems to go away. So I have had a growing pile of "faulty" 6N2P valves that stare at me in the workshop and stir me to think about this issue on a regular basis.

As mentioned, the valves bias fine, however when driven by a sine wave, the signal triangulates (looks like slew rate limiting) and when driven further there will be a breakthrough on the peaks of the signal - no wonder it sounds so nasty

Yesterday, I thought maybe I had a bunch of valves that had low emission so I cranked up the heater volts to get an indication of heater current and performance at higher heater volts - made no difference

Anyway, in desperation, I remembered that these valves have a shield between the two triodes - in fact this in common with a heap of similar twin triodes (ECC88, E88CC, 6DJ8 etc) and this shield is brought out to pin 9. Many valves have internal connections brought out to valve pins and normal practice is to leave them unconnected which is what we have done in the Amp School amps. Surely this shield could not have anything to do with this issue?????????????????

Well to cut a very long story short, to my amazement when I shorted the unconnected pin 9 to ground this nasty distortion disappeared!!!!!!!!!!!!!!! And it gets weirder!! When I removed the ground from pin 9 the distortion is still gone! The only way to get the distortion back is to replace the valve with another "faulty" valve from the bin and sine wave drive it again - distortion. Ground pin 9 - clean full sine waves!!

So, has anyone here had similar experiences?? And who is brave to suggest the mechanism by which this could occur?? The best idea that I can come up with is that with all the electrons flying around inside the valve envelope, the shield develops a static voltage buildup and that this somehow affects things. But how on earth can that affect the electron flow within the triode envelope itself???

Your thoughts???
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Old 24th September 2010, 12:51 AM   #2
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Most probably some tiny residue between anodes on mica insulator caused unwanted feedback.
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Old 24th September 2010, 02:51 AM   #3
taj is offline taj
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Just curious, what is the typical procedure for the shield pin? I'm a newbie, and I assumed it would normally be earthed. Is it usually left unconnected?

..Todd
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Old 24th September 2010, 03:02 AM   #4
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Quote:
Originally Posted by taj View Post
Is it usually left unconnected?
No, it is made to be earthed. I earth it routinely, that's why probably never faced such problems.
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Old 24th September 2010, 03:19 AM   #5
gcwills is offline gcwills  Australia
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Thanks Wavebourn for your suggestion about leakage, but this doesn't explain the fact that the valve then works fine after the ground on pin 9 is removed. If it were a feedback issue, the distortion should return when the ground is removed.
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Old 24th September 2010, 03:28 AM   #6
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Quote:
Originally Posted by gcwills View Post
If it were a feedback issue, the distortion should return when the ground is removed.
Not necessary. It it was a trace of some residue between anodes that are on approximately equal potential it could evaporate after applying the full voltage in the middle.

What you described was looking like some kind of parasitic feedback. However, I could not open your images, though...

Edit: oh, never mind: that images were kind of residue from Australian guitar forum from which you copied the message...
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Last edited by Wavebourn; 24th September 2010 at 03:31 AM.
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Old 24th September 2010, 04:38 AM   #7
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Quote:
Originally Posted by gcwills View Post
Thanks Wavebourn for your suggestion about leakage, but this doesn't explain the fact that the valve then works fine after the ground on pin 9 is removed. If it were a feedback issue, the distortion should return when the ground is removed.
If the pin is left unconnected, the distortion might return after hours, or maybe days -- the time it takes for that static charge to rebuild. Something to try out.
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Old 24th September 2010, 08:48 AM   #8
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Interesting! I always grounded pin 9 just because I could - I did not realise I needed to
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Old 24th September 2010, 01:42 PM   #9
Ian444 is offline Ian444  Australia
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I've never grounded pin 9 on 6N1 or 6N2 and never had any problems.
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Old 24th September 2010, 01:50 PM   #10
M Gregg is offline M Gregg  United Kingdom
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Interesting,

In the Aikido pre circuit it talkes about AC grounding with a capacitor. Other audio dual triodes like ECC... do not have a shield. I assume due to the operating frequency. Even stranger that you have batches of "problem" tubes.

gcwills can you please repost the images we can't get to see them! Thank's

Regards
M. Gregg

Last edited by M Gregg; 24th September 2010 at 01:55 PM.
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