Replace 12AX7 / ECC83 with inexpensive and reliable miliarty spec Russian NOS 6n2P-EV (6H2П-EB) with this simple adapter.  Suitable for 6.3V heater wiring only (most amps). Small diameter means it can fit into deep recessed sockets.

Offer is for one military spec 6n2P-EV and one adapter.