KT66 CV321 CV1075 VT75(RAF) OUTPUT BEAM TETRODE VARIOUS MANUFACTURERS

Kinkless beam tetrode output valve
Manufacturers: GEC Valves , Osram , Marconi
The KT66 beam tetrode was introduced in 1937 primarily for radar use. But soon became highly acclaimed as an audio output valve. 
 
The KT66 was essentially a 6L6 redesigned to have a larger cathode and a more robust wide section anode but more importantly, the electrodes were configured within the electrode cage in maximizing efficiency by positioning of the anode such that it was a specific multiple of the distance from the screen grid to cathode distance. A pair of beam deflection plates connected to the cathode were added to focus the electron beam into two discrete planes 180 degrees apart hence providing an elegant solution to the  limitation of space charge secondary emission without using an extra suppressor grid.
 
More importantly, this ‘focussed’ four electrode design mimicked the characteristics of Philips’  power pentode designs but neatly sidestepped their patents.
 
One  important performance benefit of the beam tetrode electrode configuration Is that the beam plates limit the negative resistance kink in the anode current vs anode voltage in the lower regions of the characteristic curves of a full tetrode which could serve to produce disturbing instability in Push Pull amplifiers using a full tetrode, having said that, the beam tetrode was not very forgiving of poor design which could reward the unwary designer with severe parasitic oscillations.
 
A further sonic benefit is that the beam tetrode electrode configuration also produces the lowest distortion of any output valve due to it’s inherant stability producing very low  third-harmonic distortion and  even lower intermodulation distortion, especially when used in ultralinear mode with appropriate levels of negative feedback.
 
The KT66 has been seen in many presentations over the years. Many prefer the early ‘fat boy’ baluster balloon devices which were used to great effect in the H2S radar during WW2. Although they have a sublime appearance and exquisite sonic performance, care must be taken as these early incarnations of the device are prone to heater-cathode shorts so must be chosen and tested accordingly – especially as they are very expensive and hard to obtain.
 
 More familiar is the classic ‘coke bottle’ shape, earlier devices having an internal charcoal mist screed applied and more typically a black micanol base, with later devices of the same shape having a clear glass envelope allayed to both black and brown micanol bases which show the copper grid supports and distinctive carburised grey electrode cage to good effect.  
 
I shall briefly discuss the Mullard EL37 here, an interesting device often touted as Mullard’s equivalent to the KT66 but the EL37 is actually a pentode device!!!   It does however make an interesting alternative and many guitar amplifier experts are staunch advocates in the use of EL37 even though they are not as robust to overloading, have a shorter life than the KT66 – conversely, they do offer a nominal mutual conductance gm of 11.0 mA/V – almost twice that of the KT66 and as a pentode needs only limited signal to drive them, so a unity gain triode gain stage could be de rigeur -  something you could never try with a KT66 which needs a good belt of power for healthy driving!!!!
 
The KT66 sees duty in so many classic hifi and guitar amplifiers it’s hard to know where to start the list, PYE HF91, HFS25, QUAD II, Williamson, the Marshall JTM45 but here’s a further list of exotica, old n new to lift your spirits:-
 
Dr. Z Route 66
 Marshall Bluesbreaker JTM45 MKIV 1966
 Marshall Bluesbreaker reissue 1964
 Marshall JTM45
 Metro Amp (George Metropoulos)
 Metropolis JTM-45 2009
 Pure Sixty-Four The "64" Head
 QUAD II
 Soultone 1986ps
 THD Bivalve
 Victoria Regal 2
Picture of OSRAM FAT BOY KT66 VT75 WARTIME ISSUE GREY GLASS
OSRAM FAT BOY KT66 VT75 WARTIME ISSUE GREY GLASS
The British Marconi-Osram GEC Valve Company of England introduced in 1938 the legendary KT66, K.T standing for kinkless tetrode. This designation was given as when the performance of the valve is measured from it's output curve it was found to be linear - hence it's great sound. The KT66s superlative sonic performance found audio fame in the Q.U.A.D II amplifier in the field of hi-fi and in 1965 Jim Marshall used the KT66 in the JTM 45 guitar amp to power those famous biting solo's to a crescendo in Eric Clapton's band!! Hence the KT66 joined the rock and roll valve legend in the first position where it has been ever since. The performance of the MOV originals is all due to the heavy grade glass and rigid mica spacers with stiff grid support specified by MOV engineers to eliminate microphonics and provide longevity of performance. Many replicas of this valve have been made over the years, mainly in China but these replicas don't 'cut the mustard' in comparison to the original GEC MOV devices in terms of sound, longevity and visual aesthetics. The problem is not just locating vintage devices but getting good ones which have been fully tested using an AVO VCM to determine stability, microphonics, cathode heater insulation hot & cold, internal shorts, anode current and mutual conductance. For this you need expertise and the requisite Valve Characteristic Meter - hard to find and jealously coveted these days - and so they should be as they cost the price of an average semi back in the 1950's The device offered here today is AVO VCM163 tested and as good as you can get. Its a GREY glass, black low loss micanol based unit from wartime production. The base is tight to the envelope with no sign of re-fixing. The markings on the front are in etched cartouche and show KT66 Osram and the rear of the envelope carries a perfect Osram ellipsoid yellow and blue decal. Tested using an Avo specification for 100% emission of anode current, Ia =63 mA; mutual conductance gm = 6.3 mA/V @ At Va = 250V Vg = -15V the results obtained were:- Ia = 65 mA. gm = 6.3 mA/V.
Stock Code: y42
Availability: 1 in stock
Delivery date: 1 week
€199.63
Picture of OSRAM FAT BOY KT66 VT75 WARTIME ISSUE GREY GLASS
OSRAM FAT BOY KT66 VT75 WARTIME ISSUE GREY GLASS
The British Marconi-Osram GEC Valve Company of England introduced in 1938 the legendary KT66, K.T standing for kinkless tetrode. This designation was given as when the performance of the valve is measured from it's output curve it was found to be linear - hence it's great sound. The KT66s superlative sonic performance found audio fame in the Q.U.A.D II amplifier in the field of hi-fi and in 1965 Jim Marshall used the KT66 in the JTM 45 guitar amp to power those famous biting solo's to a crescendo in Eric Clapton's band!! Hence the KT66 joined the rock and roll valve legend in the first position where it has been ever since. The performance of the MOV originals is all due to the heavy grade glass and rigid mica spacers with stiff grid support specified by MOV engineers to eliminate microphonics and provide longevity of performance. Many replicas of this valve have been made over the years, mainly in China but these replicas don't 'cut the mustard' in comparison to the original GEC MOV devices in terms of sound, longevity and visual aesthetics. The problem is not just locating vintage devices but getting good ones which have been fully tested using an AVO VCM to determine stability, microphonics, cathode heater insulation hot & cold, internal shorts, anode current and mutual conductance. For this you need expertise and the requisite Valve Characteristic Meter - hard to find and jealously coveted these days - and so they should be as they cost the price of an average semi back in the 1950's The device offered here today is AVO VCM163 tested and as good as you can get. Its a GREY glass, black low loss micanol based unit from wartime production. The base is tight to the envelope with no sign of re-fixing. The markings on the front are in etched cartouche and show KT66 Osram and the rear of the envelope carries a perfect Osram ellipsoid yellow and blue decal. Tested using an Avo specification for 100% emission of anode current, Ia =63 mA; mutual conductance gm = 6.3 mA/V @ At Va = 250V Vg = -15V the results obtained were:- Ia = 95 mA. gm = 8.0 mA/V.
Stock Code: A84
Availability: 1 in stock
Delivery date: 1 week
€222.45
Picture of GEC BROWN BASE GREY GLASS COKE BOTTLE KT66 FROM 1958
GEC BROWN BASE GREY GLASS COKE BOTTLE KT66 FROM 1958
Well, here's a tasty rarity, a grey glass "coke bottle' KT66. The device carries an excellent white ink classic cartouche with a date code of NC which equates to March 1958. The obverse side of the envelope carries a perfect elliptical two tone blue and white GEC decal. The brown low loss micanol base is tight to the envelope This device has been tested using our AVO VCM163 against the specification for 100% emission of anode current, Ia =63 mA; mutual conductance gm = 6.3 mA/V @ At Va = 250V Vg = -15V. The results obtained were:- Ia = 70 mA. gm = 6.3 mA/V. These test figures indicate a very strong device which meets nominal specification - not to be missed.
Stock Code: Y58
Availability: 1 in stock
Delivery date: 1 week
€125.48
Picture of KT66 CV1075 OUTPUT BEAM TETRODE MATCHED PAIR FROM FEB 1967
KT66 CV1075 OUTPUT BEAM TETRODE MATCHED PAIR FROM FEB 1967
Well, here's a tasty pair of device both clear glass "coke bottle' KT66 in GEC livery. The device carries markings in excellent white ink, a classic GEC/M-OV military cartouche with a date code of YB which equates to February 1967 and factory code Z which indicates Highgrove manufacture. The obverse sides carry perfect elliptical two tone blue and white GEC decals. The brown low loss micanol bases are both tight to the valve envelopes. This device has been tested using our AVO VCM163 against the specification for 100% emission of anode current, Ia =63 mA; mutual conductance gm = 6.3 mA/V @ At Va = 250V Vg = -15V. The results obtained were:- Ia1 = 90 mA. gm1 = 6.0 mA/V. Ia2 = 93 mA. gm2 = 6.1 mA/V. These test figures indicate not only a very strong pair of devices which EXCEED nominal specifications but also are a supremely matched pair - not to be missed.
Stock Code: a5 a4
Availability: 1 in stock
Delivery date: 1 week
€325.12
Picture of  GEC CLEAR GLASS BROWN BASE KT66 FROM 1973
GEC CLEAR GLASS BROWN BASE KT66 FROM 1973
Well, here's a tasty device a clear glass "coke bottle' KT66. The device carries the classic GEC cartouche with a date code of 7315 which equates to April 1973 and factory code Z which indicates Highgrove manufacture. The brown low loss micanol base is tight to the envelope This device has been tested using our AVO VCM163 against the specification for 100% emission of anode current, Ia =63 mA; mutual conductance gm = 6.3 mA/V @ At Va = 250V Vg = -15V. The results obtained were:- Ia = 80 mA. gm = 6.3 mA/V. These test figures indicate a very strong device which meets nominal specification - not to be missed.
Stock Code: y41
Availability: 1 in stock
Delivery date: 1 week
€125.48
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