Why shielded tubes

[off1]

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]

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]

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]

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

 

[off1]

Thanks, that makes sense!

 

[Soonerorlater]

May also provide some acoustic isolation

 

[Hi_Q]

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]

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.