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Tom,Here below is a URL to see the simple shields I constructed for the 2 EZ81s an 4 EL84s, .... to prevent the irons' fields from "talking" to them.
Jeff Medwin
Follow Ups:
Jeff
Did you have the major oxidation on those transformers that you didnt repaint because they look almost new? I wonder if I can do that to my eicos? The only eico I have where the power tranny has no oxidation is my Hf-87 which is the worst sounding power amp of all Eicos..I am putting fixed bias in it like the hf-89 and separate fil for the 12ax7.
Michael,There was major yellow-white crud on the PT endcover on top, however, the output trannies were OKish. Just buy a can of Bon Ami cleanser and give it a try. It doesn't scratch !!
Also, Bon Ami does the BEST job in restoring my Fulton FMI Kinetic Barrier turntable mat, unlike anything else.
Got stainless cookware, use "Bar Keepers Helper" cleanser, another ancient formula and the best possible for stainless. Regards.
Jeff
Jeff
I like the way you did that and the ingenuity you used to muster such an appeal to the eyes.Those transformers look brand new but I know they are repainted being every eico amp I have seen almost especially my own has oxidation on the EI type transformers..Good Job!!
Michael,NO sir, I did not paint the transformers. The Eico's P.T. in particular was nasty looking. I scrubbed it with a toothbrush, rags, and "Bon Ami" cleanser " No chlorine, Perfume, or Dye......Since 1886.....Hasn't scratched yet". Guess where that Bon Ami is made...Kansas City !!
The magnetic shields on the HF-81 will need to be redone Michael in 10 gauge steel, same exact configuration, because I think Mr. Dave "clip lead" Slagle is correct about materials choice, as usual. Thank you Dave!!
Michael, whenever tubes are too close to iron, including rectifier tubes, the shields HELP in a clearly audible way. And, I have found that rectifier tubes themselves will feed into small signal tubes if they are too close to each other.
Tre', mikeyb, Triode_Kingdom and arend-jan will gladly give us all the scientific explaination, as to "why",.... you see, its all gotta be clearly spelled out in RDH4. :-)
Jeff Medwin
... until Dave S had "questioned" you on your choice of materials, you had presented a solution that in fact was marginal at best in its execution and admitted by yourself to be untested in its current configuration. Your "listening" test being with completely different materials. It makes you think how many other of your presentations and test results are similarly flawed.Oh and don't worry about RDH4. It's all there on pages 784 to 786. I mean, all the stuff you and Dennis have been talking about - layout, screening (which is all your plates are).
I myself have done much experimentation in this regard but I found other materials better suited. I don't doubt you have heard what you report. It just isn't news and is well documented if you care to look for it.
Hi StephenR,Thats cool. In my very first post I mentioned 10 gauge steel as being my favorite in the past. And....I was VERY open in discussing my shortcomings on testing this shielding piece, so people would NOT get the wrong information and impression.
Frankly, I do not seek, or require, your vote of confidence.
I will listen, when I have time, to both materials none the less, each does something different, and needs to be heard for evaluation. Best wishes.
Jeff Medwin
sorry, I somehow missed your clarification post. I got tetchy when you were being sarcastic about a very good reference book where it does explain this sort of thing as it isn't voodoo but good engineering i.e. it is documented and an educated tech type would know where to look. It doesn't help slagging people off that have an education. It also doesn't stop people from experimenting; another claim you keep making.
what are they made of?dave
And 6/32 threaded rod to hold the copper spaced where desired. In the past I used heavy 10 gauge steel for field shields, my favorite implementation.This HF81 mod can be "lifted out" to A-B the shield's effect on the EL84s, except for the shield protecting the two EZ81 rectifier tubes from the power transformer's field. That requires use of a small wrench to remove the sheild, one nut ( not me ), but it can be done.
These steps were necessary, because the original HF81 layout is wrong, tubes too close to the iron !! It is, of course, audible.
Jeff Medwin
I do not doubt you heard a difference with the copper in place, but for the magnetic fields, the copper i essentially invisible. What you may be hearing is some sort of ES screening.dave
Dave,That materials information is helpful to me. Thanks. Would my aforementioned and prior favorite material, 10 gauge steel be a lot better?
You see, when I did my actual listening over this past month, I was using about 10 gauge cold rolled steel, all around the power trannie, which fed into the pair of EZ81s and EL84s.
I just made the copper pieces over this weekend, and I have as yet to hear them. I will test them, and I can sub in 10 gauge steel pieces and compare the two. I had actually intened to do this, but had not taken the time to do so.
In past years, after doing some reading on the subject, I had selected the 10 gauge steel. Your Forum-posted comment will remind me NOT to delay or overlook its use again. Thanks a whole lot.
It will be easy to do it in steel, the same way as I executed the copper. Thanks !
Jeff Medwin
Hi Jeff,
there has been a lot of dicussion about this at another site. The conclusion was to use 1/4" thick aluminum when you need to shield low frequency magnetic fields. This gives you somewhere between 5 to 10db attenuation at 60Hz, depending on what formulation of aluminum you use. Its pretty much purely resistivity based, the higher the conductivity of the material and the thicker the sheet the more attenuation you get.Copper would work fine, but its cheaper to go with the thick aluminum rather than the thick copper.
The experience was that using the aluminum sounds better than the steel. The theory being that the steel relies on the BH curve which is not all that linear but the aluminum is using eddy currents which are far more linear.
I tried this in my preamp, I was getting some hum from the filament transformers (the BDT tubes are quite sensitive to external magnetic fields). I used some .375 aluminum plate between the tubes and the transformers and it made a significant improvement. I haven't tried the steel sheet yet though.
John S.
Jeff, I'm wondering whether when you executed it with steel you actually surrounded the transformer or just had flat plates as you do with the copper. With the flat plates I would think the magnetic field would go around the plates, which would mean a longer path length thus decreasing the strength, but not nearly as good as a full shield, which would mean encasing the tranny. Of course decreasing the field strength might be "good enough", kind of like effectively moving the transformer a bit farther away. If you're intent on shielding mu metal might be even better since that is specifically designed to shield magnetic fields. Can't comment on its sonic qualities per se, but it's used a lot to block hum fields.
Very nice job on the Eico! :)
I like a lot your idea of a removable screen.
I have grounded copper screens on my tubes, and it does make an audible difference if you live in an RF-heavy region. (Cell phone towers, computer in household...) I think your copper screen has a similar effect on the tubes.For magnetic shielding, you could place an iron sheet parallel to each copper sheet. If you bolt them together tightly, or glue them together with conductive glue, the effectiveness of the metal shield will be even greater, as the generated eddy currents will be dissipated by the copper sheet.
Recently I saw in a textbook that when you have two magnets, and place an iron plate between the two, they stop seeing each other, and they see the metal plate instead. You could use one metal plate on each side of the copper sheet, and that would be a very effective magnetic shield between the iron / tube.
Also try ferrite plates. I've recently started using them for magnetic shielding, and they are terrific - and pretty cheap, around 1$ for a square in piece. BTW, that will be all you will need per copper plate. Place the ferrite plate at the height of the tube's anode structure, and it will prevent the magnetic lines from crossing to the other side that would most interfere with th tube.
I have also tried using transformer laminations from gutted iron for magnetic shielding, and is very effective, too. :)Long live DIY!
Janos
PS: you can also use ferrite plates to boost transformers! take off the endbells, and glue a plate on the coil! Will keep lots of magnetic lines inside the transformer, making it more efficient, and also will reduce emitted magnetic field greatly.
of some fine work. I allways enjoy your work! Good detial with a formal yet sturdy no nonsense aproach. T Willman
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