I've bought a set of NOS JAN Philips 5751 tubes to replace JJ 12AX7 in my preamp. After the change, the sound has become worse. The soundstage depth is flat, and just less involving overall. Do the tubes need break in, and if so how long do I need to burn them in?
12AX7 has a gain factor of 100, while a 5751 has a gain factor of 70, which may be the reason that your preamp sounds so poorly. Try using some NOS 12AX7's instead of the 5751's.
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