Tube Shields

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Willie, Jan 28, 2004.

  1. Willie

    Willie Member

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    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
     
  2. VaughnC

    VaughnC Supporting Member

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    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.
     
  3. Jon Silberman

    Jon Silberman 10Q Jerry & Dickey Gold Supporting Member

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  4. Willie

    Willie Member

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    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)......
     
  5. John Phillips

    John Phillips Member

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    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.
     
  6. Fuchsaudio

    Fuchsaudio Member

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    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. :dude
     
  7. thesedaze

    thesedaze Member

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    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).
     
  8. Fuchsaudio

    Fuchsaudio Member

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    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.
     
  9. thesedaze

    thesedaze Member

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    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...

    [​IMG]
    [​IMG]
    [​IMG]
     
  10. Fuchsaudio

    Fuchsaudio Member

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    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.
     
  11. thesedaze

    thesedaze Member

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

    Robal Supporting Member

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    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.
     
  13. Willie

    Willie Member

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    Well that makes sense, I assumed that KF did not use them for tonal reasons , and that the Komets continued in that vein.....
     
  14. jamison162

    jamison162 Supporting Member

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    :) I love this stuff!!!!
     
  15. titovanburen

    titovanburen Member

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    Aspen Pittman says that the shields, being metal, can potentially disturb the flow of electrons within the tube.
     
  16. gldtp99

    gldtp99 Member

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    This is pretty much what i've come to believe despite all the claims/threads i've read on forums for years.
    I tell those who ask that if the tubes aren't mounted upside down like a BF Fender than the preamp tube shields are mostly for looks.
    I use shields in most of my amps and they don't seem to filter out Cellphone-ring noise at all.
    I do like the way they look-like in this DR103 clone i did with internally mounted cap cans and Edcor trannies
    [​IMG]
     
  17. VacuumVoodoo

    VacuumVoodoo Member

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    That's disinformation. Electrons flow from cathode to anode, that's "plate" for you, which basically is a metal cage surrounding cathode and grids.
    Shield isolates the tube from external interfering electric fields that can "potentially disturb flow of electrons", if anything, the shield prevents this disturbance.
     
  18. donnyjaguar

    donnyjaguar Member

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    Aspen Pittman is also the same person who says low-vacuum tubes sound more "airy" 'cuz there's more air in 'em. Then he goes on to contradict himself elsewhere in his book. I was not surprised to learn he used to be a sales rep. :) I do give him credit for the idea of selling matched sets of tubes though.
     
  19. mrface2112

    mrface2112 Member

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    :AOK :agree

    absolutely. take anything AP says with some salt and maybe some tequila. however, that doesn't mean it's not an interesting book, has lots of great pics of some purty old amps and guitars, and isn't chock full of loads of great schematics and mods, collected all in one place. it's quite useful, as long as you know what not to believe. :stir


    cheers,
    wade
     
  20. thesedaze

    thesedaze Member

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    +1 It's almost an issue of 'taste' for many people...Certain amps & playing conditions lend to certain interferences or lack thereof, and it's really up to the player to determine what's best for them...

    Aspen is definitely a salesman first...was that even a question?
     

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