Why shielded tubes

off1 · 13th November 2010, 06:02 PM

I've noticed that every amp has one or two shielded tubes (in the preamp stage). Is there a reason for that? Why only those specific tubes and not all of them?

Thanks.

johansen · 13th November 2010, 06:06 PM

gross-over simplification to follow..

if -70Db noise is injected into every un shielded node, wouldn't it make sense to shield only the circuitry that has to process signals lower than -40Db?

tomchr · 13th November 2010, 06:27 PM

The first stages of an amp generally have the highest gain, hence, any noise or interference introduced into these stages will have the greatest impact on system performance. The shields shield against electrostatic fields and keep them from interfering with the electron flow inside the tubes.

~Tom

6L6 · 13th November 2010, 06:36 PM

The first stages of an amp are the most sensitive to picking up hum. The shields help keep the circuit quiet.

off1 · 13th November 2010, 06:44 PM

Thanks, that makes sense!

Soonerorlater · 13th November 2010, 09:46 PM

May also provide some acoustic isolation

Hi-Q · 13th November 2010, 11:28 PM

When I designed my RIAA Phono amp I used skirted tube holders from the start due to the fact that the signal levels from the magnetic cartridge would be very low. I was glad I did because in use, if you lift any of the screening cans, hum and noise levels increase very noticeably. For well laid out amps with lower sensitivity and which require much higher input voltages, the need to screen is not so important.

DF96 · 14th November 2010, 10:55 AM

Valves inside a screened metal case may not need individual screens. As well as removing electrostatic fields, the screen may have some effect on magnetic fields, and light. I read somewhere that photoemission can sometimes increase valve noise, although I guess it might depend on the extent to which the grids are visible.

13th November 2010, 06:02 PM	#1
off1 diyAudio Member Join Date: Oct 2010 Location: Alpine, UT	Why shielded tubes I've noticed that every amp has one or two shielded tubes (in the preamp stage). Is there a reason for that? Why only those specific tubes and not all of them? Thanks.

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13th November 2010, 06:06 PM	#2
johansen diyAudio Member Join Date: Apr 2010 Location: seattle, wa	gross-over simplification to follow.. if -70Db noise is injected into every un shielded node, wouldn't it make sense to shield only the circuitry that has to process signals lower than -40Db?

13th November 2010, 06:27 PM	#3
tomchr diyAudio Member Join Date: Feb 2009 Location: Greater Seattle Area	The first stages of an amp generally have the highest gain, hence, any noise or interference introduced into these stages will have the greatest impact on system performance. The shields shield against electrostatic fields and keep them from interfering with the electron flow inside the tubes. ~Tom

13th November 2010, 06:36 PM	#4
6L6 diyAudio Member Join Date: Oct 2010 Location: Denver, Colorado	The first stages of an amp are the most sensitive to picking up hum. The shields help keep the circuit quiet.

13th November 2010, 06:44 PM	#5
off1 diyAudio Member Join Date: Oct 2010 Location: Alpine, UT	Thanks, that makes sense!

13th November 2010, 09:46 PM	#6
Soonerorlater diyAudio Member Join Date: Jul 2008 Location: Belfast	May also provide some acoustic isolation

14th November 2010, 10:55 AM	#8
DF96 diyAudio Member Join Date: May 2007	Valves inside a screened metal case may not need individual screens. As well as removing electrostatic fields, the screen may have some effect on magnetic fields, and light. I read somewhere that photoemission can sometimes increase valve noise, although I guess it might depend on the extent to which the grids are visible.