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August 18, 2019, 06:51:50 PM

Author Topic: 6F8G grounding  (Read 175 times)

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Offline L0rdGwyn

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6F8G grounding
« on: February 13, 2019, 05:19:31 AM »
Hi - hoping someone can shed some light on a minor issue I'm having.

In my Crackatwoa, I rotate through 12AU7, 6SN7, and 6F8G driver tubes with adapters.

To me, the 6F8G sound best, but they are picking up some 60Hz noise in the right channel only, presumably from the top cap, so I end up not using them.

As I move my finger closer to the cap, the noise increases.  It also increases with increasing volume.  The if I cover the tube with a metal cup and make contact with one of metal volume knobs, the noise completely goes away.

I'm no engineer, and I know the amp isn't designed for this tube, but seems like the cap is picking up power supply noise and isn't properly grounded, any way to fix that so I can use these great tubes?  Thanks.
Keenan McKnight

Offline Paul Birkeland

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Re: 6F8G grounding
« Reply #1 on: February 13, 2019, 05:25:20 AM »
Do you have well grounded outlets where you're using the C2A? When you see big noise changes based on when you touch the amp, that tends to suggest poor grounding from the outlet.

For the occasional grid wire that I have poking up through the chassis, I will sometimes shield them if the signal level isn't super strong.  With a plug in adapter, this will become pretty difficult!
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline L0rdGwyn

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Re: 6F8G grounding
« Reply #2 on: February 13, 2019, 05:43:21 AM »
The sad truth is, I don't.  I live in an old house and rely on GFCI outlets to make sure it doesn't burn down.  If that's the issue, I'm assuming there isn't much I can do about it, is there? (sigh).
Keenan McKnight

Offline Paul Birkeland

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Re: 6F8G grounding
« Reply #3 on: February 13, 2019, 06:01:55 AM »
If said outlet is on an exterior wall, you can add a grounding rod close by and properly ground the outlet. 
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Online Doc B.

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Re: 6F8G grounding
« Reply #4 on: February 13, 2019, 09:10:39 AM »
There are actually purpose made shields that fit over old shouldered tubes, but they are pretty hard to find. They are shaped like the tube, split into two halves either vertically or horizontally. You can see them here -

https://www.nutsvolts.com/uploads/wygwam/NV_0515_Koebel_Figure09.jpg
Dan "Doc B." Schmalle
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Offline L0rdGwyn

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Re: 6F8G grounding
« Reply #5 on: February 13, 2019, 10:01:19 AM »
Thanks Doc, I'll scrounge around the internet and see what I can find.  And thanks for your suggestions PB, it is not on an exterior wall, but I am going to look into what can be done i.e., grounding the particular outlet.
Keenan McKnight