Gain & Gm Matched. Gain & Gm Tested. The Tung-Sol 12AX7 is one of the very best new production 12AX7 models offered. These tubes are high gain, low microphonics and are VERY warm and detailed. You'll get true 3D sound with these. The filaments are almost completely encased within the plates so they don't light up much, but they are fantastically quiet. The tone is comparable to the best vintage 12AX7s like Mullard or Telefunken. Will work in place of: 12AX7, ECC83, 12AX7A, 7025, 12AX7WA, 12AX7WC, 12AX7WB, ECC803S, CV4004, CV492