Mullard 12AX7 / CV4004 dual triode preamp tube. Made in Russa.

The CV4004/12AX7 is a high-gain, low-noise valve with a short plate structure for reduced microphonics. Excellent for use in high-gain Marshall pre-amplifier circuits where noise and microphonics can be a problem. Great for guitar amps.
Short plate structure for reduced microphonics
Best for guitar amp circuits
12AX7 / ECC83 valve equivalent