Hi-Fi Classic
15WWilliamson
D.T.N. Williamson's landmark 1947 design. Triode-connected push-pull with global NFB. Set the standard for high-fidelity amplification.
Wireless World, April 1947Ampera
KT66 Tetrode Vacuum Tube
Tetrode6.3V / 1.3AOctalSPICE
Beam power tetrode rated at 30W for high-power output stages
Datasheet ↗SPICE
22.5kΩ
7.0mA/V
14.5pF
1.0pF
10.50pF
GAIN0%
POWER75%
DRIVE78%
LINEARITY75%
SPECIFICATIONS
Heater6.3 V · 1.3 A
Max Plate Voltage550 V
Pa max30 W
Gain Factor—
gm7 mA/V
rp22.5 kΩ
Inter-electrode capacitances
Cgk14.5 pF
Cgp1 pF
Cpk10.5 pF
PinoutOctal
Era1937
RaritySought-after
Source datasheets
3 manufacturer datasheets cross-checked for KT66
Triode-connectedMarconi's Wireless Telegraph CJUNE 1956
Specs when the screen grid (g2) is tied to the anode — the tube acts as a triode.
Va
400 V
According to Marconi's Wireless Telegraph Company Limited (JUNE 1956): « Triode Connection »
EXECUTIVE SUMMARY
The KT66 is the predecessor to the KT88, also developed by GEC/MOV, with a lower plate dissipation (~25W) and a warmer, more romantic sonic character. It was the output tube of the original Marshall JTM45 and the Williamson amplifier, both landmark designs in their respective domains. The KT66 has a sound often described as more triode-like than the KT88, with a lush midrange and gentle top end that flatters acoustic music and vocals. Original GEC KT66s with the clear-glass bottle are extremely rare and expensive; Gold Lion reissues are the most common current production option.
APPLICATIONS
High-power output
Marshall JTM45Leak Stereo 20Quad II
• Guitar amp• PA/public address
HISTORY
Introduced1937ManufacturerGEC·Marconi
Pre-WarMETRIX TESTER
Test this tube on a Metrix
CLASSIC CIRCUITS
Hi-Fi Classic
12WLeak TL12+
Harold Leak's improved Point One amplifier. Triode-connected KT66 with very low distortion. Refined, transparent British sound.
Leak TL12+ SchematicHi-Fi Classic
15WQuad II
Peter Walker's iconic 1953 power amplifier. Cathode-coupled phase splitter with KT66 output. Legendary musical presentation.
Quad II Service ManualGuitar Amp
30WMarshall JTM45
Jim Marshall's first amplifier (1962). Based on the Bassman circuit with KT66 output tubes. Raw, dynamic British crunch. The birth of rock tone.
Marshall JTM45 Schematic