by EWBrown » Wed Mar 06, 2013 8:42 pm
In this case, the 12AX7 is handling relatively high (approx 1 volt rms input) audio signal voltages, so the shielding isn't necessary, as it would be in a high-gain phono stage or guitar amp input stage, which is dealing with low-millivolt audio signals, and which would be much more susceptible to being influenced by external EM fields..
/edB
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