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Placing the CD player under the preamp raised the preamp 5.5 inches above the shelf. I tried stacking books to the same height instead and it was just about as effective (no hum reading was made though). At this point I gave Emil a call to discuss strategy. He felt that it was important to locate the source of the magnetic field on the amp so that a shield could be designed to target it. I opined that it was likely the Tron’s power transformer since the amp produces 1000 volts for the 211 plates. I also asked him to loan me a gauss meter so I could measure the field levels.

Things got interesting when the gauss meter arrived. With everything powered up I took some readings for all of my components. A background reading out in the room showed approximately 1 milligauss. Going right to the Tron power transformer I measured a whopping 900 milligauss half way up the side of the transformer. Moving up to its top edge had the reading drop to 470 milligauss. Comparatively speaking there was no other sizeable source of magnetic field radiation on the amp or other components for that matter.

I decided to do a series of shielding experiments using the gauss meter. On the preamp I mainly measured the magnetic field around the tubes which stick out. Nick had earlier mentioned that he was trying to design a tube shield to protect the circuitry from RF and EMF fields. I suggested to Emil that it seemed best to make a mu metal box that would fit directly over the Tron’s power transformer (PT). I did this and then made a second box to fit over the first one. This time the magnetic field strength measured as follows.
  1. Amp on shelf below preamp, no shielding
    1. 5.1 milligauss at the line stage tubes
    2. 5.9 milligauss at the phono stage tubes
  2. Amp on shelf below preamp, one mu metal box on PT
    1. 3.2 milligauss at the line stage tubes
    2. 2.5 milligauss at the phono stage tubes
  3. Amp on lower left shelf, no shielding on PT
    1. 3.5 milligauss at the line stage tubes
    2. 2.5 milligauss at the phono stage tubes
  4. Amp on lower left shelf, one mu metal box on PT
    1. 1.4 milligauss at the line stage tubes
    2. 1.0 milligauss at the phono stage tubes
  5. Amp on shelf below preamp, two mu metal boxes on PT
    1. 1.8 milligauss at the line stage tubes
    2. 1.8 milligauss at the phono stage tubes

With the amp on the shelf below the preamp and one mu metal box over the PT, the magnetic field read 80 milligauss at the PT side center. This was a huge drop brought about by the mu metal box. I placed the second mu metal box over the transformer and the reading dropped to 45 milligauss. In this latter configuration the hum through the speakers on phono was greatly reduced but still not as low as when the amp was moved away to the lower left shelf. I did not take dB readings in this series of trials.

Given that the milligauss readings were at very low levels when measured at the tubes yet the hum level was still a bit too high, I concluded that the milligauss readings at the tubes were of limited value as a diagnostic tool. Why? I believe it is probably because the magnetic field is affecting the preamp circuitry by entering directly through the bottom of the chassis. Another possibility is that some of the hum is being carried into the preamp by the cables attached to the back of the preamp.