used 6N2P-EV tubes instead of the much more common 12AX7, prompting user modifications of the amplifier to 12AX7, because the original Russian 6N2P-EV was scarcely available outside of Russia. References 6n2p-ev rugged version of 6n2p Data sheet 6n2p amplification factor Beijing 6N4 datasheet 5751 data 5751WA data Using 6N2P in 12AX7 circuit Vacuum tubes