KT66 — Beam Tetrode, Octal | Tubeofile

KT66

Beam Tetrode — Octal

GEC

Known Equivalents & Substitutes

CV1075CV321VR47

Similar Tubes (Not Drop-In Compatible)

5881

Sams lists as substitute — ruggedised 6L6 variant with similar ratings

6L6GC

Sams lists as substitute — RCA beam tetrode with same pinout and similar characteristics but different internal construction

Sams lists KT66 as substitute for 7581 — improved 6L6-family beam tetrode with higher ratings

Philips interchange lists EL37 as replacement — Mullard Octal power pentode with similar application but different internal design

Electrical Specifications

Section 1 — Beam Tetrode

Absolute Maximum

Anode Voltage (max)Anode Voltage (max, design)500V

Screen Voltage (max)Screen Voltage (max, design)500V

Anode Dissipation (max)Anode Dissipation (max, design)25W

Screen Dissipation (max)Screen Dissipation (max, design)3.5W

Notes

Pa_max has two ratings: absolute maximum 30 W, design maximum 25 W. Ps_max: absolute 4.5 W, design 3.5 W. Combined Pa+g2 absolute max is 32 W (design 27 W).
The KT66 is a commercial version of the CV1075. CV321 and VR47 are additional British military designations (Brans Vademecum 1963).
Triode-strapped characteristics at Va = Vg2 = 250 V, Vg1 = -15 V: gm = 7.3 mA/V, ra = 1.3 kΩ. Schlangen reference gives mu = 8, gm = 4.7 mA/V, ra = 1.7 kΩ at a different operating point.
Pin 1 is an internal shield connection. Pin 6 has no physical pin on the Octal base.
Rg_max for cathode bias: 1.0 MΩ (Pa+g2 ≤ 27 W), 500 kΩ (Pa+g2 > 27 W). For fixed bias: 250 kΩ (Pa+g2 ≤ 27 W), 100 kΩ (Pa+g2 > 27 W).
In push-pull operation it is essential to use two separate cathode bias resistors — one per tube.
Average life expectancy at absolute maximum ratings is at least 8000 hours. At Pa+g2 = 21 W, at least 10,000 hours.

Pin Layout — Octal

1Internal Shield / Beam Plates

2Heater

3Anode

Socket-Compatible Tubes ⚠ Not electrically compatible

References