Tube Shields

[Willie]

I was wondering if anyone knew why the metal tube shields are not usually used in Trainwreck, Allessandro, Komet amps on the 12ax7s....most amps I see from Fenders , Marshalls,older Boogies, HiWatt, Two Rock, Budda, some Vox...do use them..both in heads and combos...now I have never been sure whether the "shielding" was electric or physical...and have been told by different sources that it was either...the Trainwreck etc. are always heads so it might be superfluous if the "shielding" was physical....very curious, Thanks

[VaughnC]

The purpose of preamp tube shields is to keep noise, radio signals, etc. from getting into amp so they would be more important in higher gain amps. However, they also act as a shunt capacitor and could subjectively affect the tone of the amp. So, while each manufacturer probably has their own reasons whether or not to use tube shields, some reasons might be related to how well the amp cab itself is shielded or just the economics.

[Jon Silberman]

http://63.151.115.106/board/showthre...threadid=23369

[Willie]

Thanks for the info...this place is so great....and makes sense since I had a Soldano Lucky 13 head with 6 pre amp tubes including reverb, all were un-shielded and the hummm on that amp was very loud (for me)......

[John Phillips]

I reviewed an Avenger recently and found the same thing - IMO slightly excessive background noise, mostly hum. It was the only thing I didn't like about the amp, actually... I spoke to Soldano about it, and they said that the amp design was basically so 'open' that a certain amount of noise is inevitable when there's that much gain - you can filter it out, but they think it adversely affects the tone. Given how great it sounded (even with the noise), they're probably right.

I did try shielding the first tube with a spare shield and a clip lead to the chassis, and I even tried grounding the metal front panel (which I think is only on the Avenger), but neither made any difference.

[Fuchsaudio]

I guess depending on your hearing, as well as the circuit in question, they can have a slight affect on tone. It's basically the capacitance surrounding the tube itself. It would be the equal of putting lots of small capacitors (a few pf each) from each tube lead to ground. Depending on the tube type and brand, circuit configuration (a cascode for example, would be more sensitive to a shield than a traditional cascade voltage amp), the impact will vary from amp to amp. Paul Smith swears his ODS sounds better without a shield on the OD tube (a Ruby 12AX7). Some hear it, some dont.

As far as absolute real shielding, I doubt it's significant unless the shield is made of mu-metal (like used in old test gear, in which case it could potentially shield magnetically induced noise). If an amp is correctly designed and laid-out, magnetic noise should not be induced by a transformer into a tube. I see no difference in an amp with or without shields, when my cellphone rings near an amp, so I doubt any RF is blocked by their shielding.

They do an excellent job of holding tubes in place, and look pretty swift when polished and/or plated however.

[thesedaze]

alluminum shields definately have a purpose. Take off the shield...run the amp without a guitar plugged in...put your hand around the tube (make sure the amp hasn't been getting hot for an hour of course). You should notice it get quieter when your hand is on the tube.

The alluminum grounds this.

Just had a tube shield fabricated for my 6sl7 octal preamp tube (no tube shields are really made for octal preamp tubes since octal preamp tubes were commonly used in low wattage amps).

[Fuchsaudio]

Originally posted by thesedaze
alluminum shields definately have a purpose. Take off the shield...run the amp without a guitar plugged in...put your hand around the tube (make sure the amp hasn't been getting hot for an hour of course). You should notice it get quieter when your hand is on the tube.

The alluminum grounds this.

Just had a tube shield fabricated for my 6sl7 octal preamp tube (no tube shields are really made for octal preamp tubes since octal preamp tubes were commonly used in low wattage amps).

And exactly how many times have you played your guitar, while holding your hand around the tubes in your amp ? LOL. Honestly, unless there's some serious outside noise influence (transformer magnetic field, radio station, neon light) there isn't much benefit (if any) to the shield. Copper, steel or Mu-metal would have better shielding properties than aluminum anyway.

[thesedaze]

It's noticable to me. call me wonder ears if you will, but I notice it. I must say I didn't really notice or a give a damn about these small noise issues until playing through one of Chris Siegmund's amps. I was used to fender amps, and had just taken the noise thing for granted, especially playing through single coil pickups. Even with my Z90 pickups through Chris's amps, I hear absolutely no buzz. Before shielding the octal tube, there was a slight buzz noticible that would have easily slipped by if it were a Fender amp, but I noticed it right away through the Siegmund. Transformed an old oil filter can into an octal shield...

[Fuchsaudio]

You have singlehandedly come up with a use for all those dryed-up filter caps people yank out of their old tube amps !

It's a heck of an idea, and I'm glad it worked out for you.

[thesedaze]

Wasn't my idea...Chris Siegmund did the great work.

[Robal]

Originally posted by Willie
I was wondering if anyone knew why the metal tube shields are not usually used in Trainwreck, Allessandro, Komet amps on the 12ax7s....most amps I see from Fenders , Marshalls,older Boogies, HiWatt, Two Rock, Budda, some Vox...do use them..both in heads and combos...now I have never been sure whether the "shielding" was electric or physical...and have been told by different sources that it was either...the Trainwreck etc. are always heads so it might be superfluous if the "shielding" was physical....very curious, Thanks

I asked Ken Fischer about this years ago and he said he simply preferred the sound of his Trainwreck amp without the tube shields. He suggested I try it both ways on my amps, and let my ears decide. I assume Holger and Ken took the same approach with the Komet amps.

[Willie]

Well that makes sense, I assumed that KF did not use them for tonal reasons , and that the Komets continued in that vein.....

[jamison162]

I love this stuff!!!!

[titovanburen]

Aspen Pittman says that the shields, being metal, can potentially disturb the flow of electrons within the tube.