The lone preamp tube

[hareek]

Why is it that my marshall has 4 12AX7 preamp tubes with three just sitting there plugged in and the fourth having this cylindrical metal casing around it? What function does this metal casing serve? Is it a different tube completely and I'm just an idiot for not knowing that? The curiousity is killing me inside :yell:

 

[shredaholic]

Re: The lone preamp tube

I'd think it'd be for shielding against RFI - the first stage in an amp (first tube) is the most at danger of picking up stray frequencies, and a metal shield around the tube sends them to ground before they hit the tube.

 

[muttonchopsrule]

Re: The lone preamp tube

I'd think it'd be for shielding against RFI - the first stage in an amp (first tube) is the most at danger of picking up stray frequencies, and a metal shield around the tube sends them to ground before they hit the tube.

bingo. the first preamp tube in the signal chain is the most susceptible to picking up noise, which is why it is shielded. my amp has the same deal going on...just the one tube is shielded and the rest are naked. i've also seen amps where they're all shielded, just depends on the amp.

 

[hareek]

Re: The lone preamp tube

Oh cool. Thanks for the feedback guys.

 

[Jeff_H]

Re: The lone preamp tube

Actually, all of them are supposed to have the metal covers. If you bought it used, 3 of them likely were taken off or lost by previous owners.

 

[shredaholic]

Re: The lone preamp tube

Actually, all of them are supposed to have the metal covers. If you bought it used, 3 of them likely were taken off or lost by previous owners.

Tru dat