Preamp tube covers

[doveman]

What are the pros and cons for using metal covers on the preamp tubes? Anyone not use them in their builds?

[M Fowler]

It helps shield against RFI reducing noise in the preamp section. I would think it would also help in vibration.

Mark

[Richie]

alot matters depending on the amp or circuit,and tube used.. Some amps may whistle or have a slight squeal with the shield on the tube.or mot having it on the tube..yeah crazy i know, but thats how it is..Some may cure the same problem by putting the shield on the tube. Sometimes with the shield off, the sound or tone may be a very subtle, more deep sounding,again,depending on amp and circuit.
The best tube shields in my opinion are the mil spec, IERC..
here you can read about them. They have different sizes for different tubes. Little cooling fins inside the shield,that also help dampen any rattles.
And they are black. If you look in some of the old tube books they talk about useing black shields..

Code: Select all

http://www.amwindow.org/tech/htm/irec.htm

[Structo]

Cool, I hadn't heard of those before.
I just checked ebay and there is one seller with some.
But they are assorted sizes.

http://cgi.ebay.com.my/19-TUBE-SHIELDS- ... 7C294%3A50

[Richie]

for 12ax7,look for the 6020B..
and here are the Mil Spec numbers.

Med 9 pin 6020B MS24233-5 S0967*V00 M24251/6-5


You can also sometimes find the mounting base.

[DonMoose]

In a tubes-down configuration, they also help keep the tubes seated in their sockets.

(I know, duh)

[doveman]

So far nobody suggesting don't use them at all. I thought there might be some.

[Structo]

Eric Johnson says they mess with the tone.
But they have pills for that.

[Cygnus X1]

I'd guess they are useful for keeping the tubes secure and maybe to help guard against microphonics.

RF?
I don't see why there would be any difference unless the tube is grounded to the base? Not sure about that.

Tone?
Highly doubtful.

[Firestorm]

Among those who claim a tonal difference, the argument goes that since the shield is grounded, the tube sees a huge negative potential (relatively) on the other side of the plate. Dubious. The shield is a few orders of magnitude farther away from the plate than the cathode. Inverse square law and all that, it can't make much difference.

[ER]

I had trouble with some howling/squealing at high volumes in one of my "hanging tubes" amps, the tube covers had little compression springs in them that were acting like little microphonic reverb tanks. Sounds much better without them even at low volume.

Something to watch for if they have the springs at the top...

-ER

[greiswig]

On my Bad Cat amp, I swear I can hear a difference between V1 with and without a cover: crisper sound without. On my D'Lite, I can't tell a difference at all.