The EL12spez has a completely different pinout from the standard EL12 despite sharing the Y8A base. The anode is brought out via a top cap for improved high-voltage isolation, and the socket pin assignments differ on every functional pin. The two are NOT interchangeable.
Separate cathode resistors required per tube in push-pull configurations.
The RFT datasheet specifies operation exclusively in push-pull Class AB at Va=425V. No single-ended Class A operating point is published for this variant.
Stopper resistors recommended: at least 1000 Ω before control grid and 100 Ω before screen grid to prevent VHF parasitic oscillation (noted in the 1954 RFT datasheet).
Fixed bias permitted (unlike the standard EL12 which is cathode-bias only), but anode voltage must not drop significantly below screen voltage to avoid excessive screen dissipation. If the loudspeaker is disconnected, a dummy load of equal value must be substituted.
The EL12N (RFT, Mühlhausen) is the direct successor with identical electrical characteristics but brings the anode to a socket pin (pin 8), eliminating the top cap. It is not pin-compatible due to rearranged pin functions.
The EL12sp is the Tesla (Czechoslovakia) designation for the same tube with identical specifications and pinout.
Triode-strapped data (mu=18, ra=1.1kΩ, gm=15 mA/V) sourced from diyAudio forum discussions on the EL12N (https://www.diyaudio.com/community/threads/el12n-in-hifi.346273/page-2), not from the manufacturer datasheet. Values are consistent across the EL12/EL12spez/EL12N family.