Graphite ring on sweep tubes

M Gregg · 02:03 PM

Just a quick question..

What was the graphite ring (looked like aquadag..a line that went around the tube base about half an inch up the bottle) that was put on the outside of glass on some sweep tubes for?

I have seen them arc across it..I can't find any pictures and I am sure it was connected to one of the pins on the bottom of the tube..

Curiostiy has the beter of me again

I think this is close:
http://www.google.co.uk/imgres?q=pl5...1t:429,r:9,s:0

Regards
M. Gregg

GloBug · 28th November 2011, 02:07 PM

RF shielding?

Doz · 28th November 2011, 02:40 PM

It's to prevent the "Barkhausen effect" I think. My Pye LV20 has one such tube.

mississippi · 28th November 2011, 03:16 PM

When a high voltage is placed on the anode the glass insite will be charged.
The other pins of the bottom in the tube will "see" that charge and a very small current wil flow. This current can reach the G1 and the cathode. Now your settings are not correct anymore.
This ring is a barrier for this effect.

radiostar · 28th November 2011, 03:54 PM

Quote:

Originally Posted by mississippi

When a high voltage is placed on the anode the glass insite will be charged.
The other pins of the bottom in the tube will "see" that charge and a very small current wil flow. This current can reach the G1 and the cathode. Now your settings are not correct anymore.
This ring is a barrier for this effect.

Thanks missisippi!!!

Dimitris AR · 28th November 2011, 10:10 PM

In the EL502 ( which is similar to EL504 ) this ring is connected to pin 9 .

trobbins · 29th November 2011, 08:35 AM

So that makes it a guard ring then I guess, to minimise dV/dt induced capacitive coupling to the exposed grid leads.

mississippi · 29th November 2011, 11:11 AM

Not only dV/dt, dc charges are also not allowed.

trobbins · 29th November 2011, 08:11 PM

mississippi, I don't quite understand the dc aspect you are referring to?

The characteristic of a sweep application that stands out from the crowd is the short high voltage flyback transient - which to my mind would be the reason for extra shielding, rather than the high duty cycle low voltage conduction period (which is perhaps the DC aspect you are referring to?)

Ciao, Tim

Hi-Q · 29th November 2011, 11:11 PM

Barkhausen oscillation or BK as we called it, manifested itself on some Televisions as a thin vertical stripe of white dots. We used to wrap a single shorted turn of tinned copper wire on the outside of the sweep output tube, take the loop down to a ground point, then adjust the wire loop up and down the tube for minimum interference. Later tubes came with the graphite ring already located inside it as it was becoming a common problem in its day with certain makes of Televisions. I doubt if BK would be a problem with normal audio usage though.

28th November 2011, 01:58 PM	#1
M Gregg diyAudio Member Join Date: Jun 2010 Location: UK	Graphite ring on sweep tubes Just a quick question.. What was the graphite ring (looked like aquadag..a line that went around the tube base about half an inch up the bottle) that was put on the outside of glass on some sweep tubes for? I have seen them arc across it..I can't find any pictures and I am sure it was connected to one of the pins on the bottom of the tube.. Curiostiy has the beter of me again I think this is close: http://www.google.co.uk/imgres?q=pl5...1t:429,r:9,s:0 Regards M. Gregg __________________ What is the sound of one hand clapping? Last edited by M Gregg; 28th November 2011 at 02:03 PM.

28th November 2011, 02:40 PM	#3
Doz diyAudio Member Join Date: Jul 2010 Location: Sat Down	It's to prevent the "Barkhausen effect" I think. My Pye LV20 has one such tube. __________________ "To err is human.. to make a real balls-up requires a computer"

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28th November 2011, 02:07 PM	#2
GloBug Banned Join Date: Jun 2011	RF shielding?

28th November 2011, 03:16 PM	#4
mississippi diyAudio Member Join Date: Apr 2006 Location: holland	When a high voltage is placed on the anode the glass insite will be charged. The other pins of the bottom in the tube will "see" that charge and a very small current wil flow. This current can reach the G1 and the cathode. Now your settings are not correct anymore. This ring is a barrier for this effect.

29th November 2011, 08:35 AM	#7
trobbins diyAudio Member Join Date: Feb 2009 Location: Melbourne, Oz	So that makes it a guard ring then I guess, to minimise dV/dt induced capacitive coupling to the exposed grid leads.

29th November 2011, 11:11 AM	#8
mississippi diyAudio Member Join Date: Apr 2006 Location: holland	Not only dV/dt, dc charges are also not allowed.

29th November 2011, 08:11 PM	#9
trobbins diyAudio Member Join Date: Feb 2009 Location: Melbourne, Oz	mississippi, I don't quite understand the dc aspect you are referring to? The characteristic of a sweep application that stands out from the crowd is the short high voltage flyback transient - which to my mind would be the reason for extra shielding, rather than the high duty cycle low voltage conduction period (which is perhaps the DC aspect you are referring to?) Ciao, Tim

29th November 2011, 11:11 PM	#10
Hi-Q diyAudio Member Join Date: Oct 2007	Barkhausen oscillation or BK as we called it, manifested itself on some Televisions as a thin vertical stripe of white dots. We used to wrap a single shorted turn of tinned copper wire on the outside of the sweep output tube, take the loop down to a ground point, then adjust the wire loop up and down the tube for minimum interference. Later tubes came with the graphite ring already located inside it as it was becoming a common problem in its day with certain makes of Televisions. I doubt if BK would be a problem with normal audio usage though.

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