Same Octal base beam tetrode but KT120 has much higher ratings (Pa_max 60W vs 35W). Same pin functions but different operating points and heater current.
Same Octal base beam tetrode family but KT120 has significantly higher ratings (Pa_max 60W vs 42W, different bias requirements). Not a drop-in — requires circuit adjustment for higher heater current and different operating points.
Electrical Specifications
Section 1 — Beam Tetrode
Absolute Maximum
Anode Voltage (max)Anode Voltage (max, tetrode)850V
Screen Voltage (max)Screen Voltage (max, pentode)650V
Pin 1 is the outer metal screen (internal shield), not a beam plate connection. Pin 8 is cathode tied to the beam-forming screen internally.
The KT120 is physically larger than a KT88 (52 mm diameter, 130 mm seated height) — verify clearance before substitution.
Triode-strapped data sourced from Turner Audio measured Ra curves (https://www.turneraudio.com.au/loadmatch-1-SE-triodes.html), not from the manufacturer datasheet. The Tung-Sol datasheet provides triode max ratings but no triode operating point or small-signal parameters.
Pin Layout — Octal
1Outer Metal Screen (internal shield)
2Heater
3Anode (Plate)
4
Socket-Compatible Tubes ⚠ Not electrically compatible