Shielding Preamp Tubes

[Adji]

Hey guys just wondering something that I cannot find the answer to.

My amp has 4 12ax7 tubes for the preamp section but only one of the tubes, (the one on the right) is shielded with a metal cover. What is the reason for this? I am guessing it is to prevent the tube from causing excess hum, like a kind of shielding or something? Not sure why only one tube would be covered though.

[jimbeaux]

I believe they're for securing tubes in their sockets (they should have springs in the top of the metal cans)

My Marshall has metal ones for the preamp tubes & wire clips with asbestos nets for the power tubes (it's an old Marshall).

[ubersam]

maybe it is the 1st stage/input tube and the shielding is an attempt to block any possible RFI...

[bancika]

Shielding tubes is just like shielding wires, prevents noise and helps keep tube in place. shielding is the most important at input stages of the amp, that's why you have it only on the first tube...

[Adji]

Thanks for the information guys. I was just curious that is all.

[candidate]

The shields also retain heat, which is why you only want to use them if necessary.

[petemoore]

  Warms 'em up faster.
  Also has to do with 'preamp-tube reverb' and microphonics.

[bancika]

The shields also retain heat, which is why you only want to use them if necessary.

true, but it shouldn't matter for preamp tubes, they can take much more heat and it may be good for any components that are near tubes because it keeps them cooler, see my latest build...

[alfafalfa]

That looks very nice Bancika, what kind of preamp is it ?
What is the topology ( or schem ) ?

[jimbeaux]

Shielding tubes is just like shielding wires, prevents noise and helps keep tube in place. shielding is the most important at input stages of the amp, that's why you have it only on the first tube...

I think bancika has it right.

http://mcintoshlaboratory.tripod.com/aa/mc275.htm

[bancika]

Thanks,

It's basically a lead channel of SLO-100 amp, or x88r preamp with all switching and the tone stack stripped (using graphic EQ pedal instead).
I kept both cathode followers which turned out to be a cool feature since I can use volume control to overdrive the output stage.

Cheers

[Paul Marossy]

The shields also retain heat, which is why you only want to use them if necessary.

true, but it shouldn't matter for preamp tubes, they can take much more heat and it may be good for any components that are near tubes because it keeps them cooler, see my latest build...

Wow, nice build. I've never seen blue preamp tube sockets & shields before. And I dig the toroidial transformer, too.

[bancika]

Tnx man! Banzai has shields in a few different colors, other sellers probably too..