KT66 GEC NIB Original GEC box V5



    • King of the KT66 tubes
    • Old GEC UK production
    • NOS in original box
    • Best sonics

1 in stock

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Rare NIB GEC KT66 tube; manufactured by GEC in Hammersmith, UK.

Detailed description

Here is a very strong  KT66 tube. The tube has been produced by GEC in Hammersmith, UK. The tube is unused in original factory box.

The KT66 is an English beam tetrode from 1938. The characters “KT” stand for “Kinkless Tetrode”. The maximum anode voltage is 550 Volt. Two of these tubes in a class AB1 PP circuit deliver a power of 50 watts. The tube is a direct descendant of the American 6L6 tetrode. These tubes are used in the Hi-Fi amplifiers amongst others of the British designer Peter Walker (Quad). Also H.J. Leak (and many other) took advantage of this tube.

Most people have heard of the famous GEC KT66, which has become known as the best sounding KT66 ever made. It is known for a smooth but extended treble, warm but accurate mid-range, and thunderous bottom end. It’s fame has caused this tube to become nearly impossible to find.

This is the original vintage GEC KT66 (NOT a Russian or Chinese) reissue)! 

This KT66 valve has very nice test values. Seems back in the golden tube era they really understood how to make high(est) quality tubes.

The valve is supplied in original GEC box. It is a real jewel also, so please excuse the firm price.

Note: my tubes are often not the cheapest. But it is very hard to find better tubes !


The vintage GEC KT66 creates an open sound image with a natural midrange, silky trebles and a solid bass. The GEC KT66 is considered by many as the king of the KT66/6L6 tubes; better than any other! I refer to the many reviews on internet.

Test results

The measurement data below show that the tubes are nicely matched. 



Ia (plate current):

Transconductance (Gm):

Gas test, Shorts test:

74  mA

6,3  mA/V





Factory AVO Specification for new unused KT88 tubes

  • The rated emission value for a new KT66 is:  65 mA.
  • The rated transconductance value for a new KT66 is:  6,3 mA/V. 

As specified in AVO Valve Data Manual 23rd edition 1981. 

Test conditions & equipment:

Mutual transconductance, plate current, gas and shorts test equipment: AVO Mk IV (AVO Valve Data Manual 23rd edition 1981), RFT stabilised mains, Utracer3+, Philips PE 4832, 2x Korad KA3005P programmable power supply, 2x Keithley 2700 and 2x Fluke 145. Test conditions: Ua (plate voltage) = 250 Volts, Us (screen voltage) = 250 Volts, Ug = -15 Volts and Uf = 6,3 V (AVO manual). 

Note: Please read our test page.

Summarized description

Tube type KT66
Quantity of Tubes One piece
Brand GEC
Manufactured by GEC Hammersmith, UK
Date code behind the GEC label
Plate Construction Welded grey plate; see pictures
Getter Construction Bottom halo getters
Print Condition Excellent; see pictures
Box included Original GEC box
Equivalent tubes:  6L6, KT77
General test results  NOS
Please study photos carefully; what you see is what you get! Ask any questions before bidding. Thanks

Please read our test and shipping policy before buying !

Quad example