Very rare 70's NOS RTC Philips Mullard 12AT7WA special quality ecc81 12at7 like M8137 best pro audio tube preamp...

 Here is a very rare matched pair of RTC - Philips-Mullard 12at7WA pair, which is actually a Mullard CV4024 same as M8137 K/B a special military low noise, low microphony military version of ecc81 12at7. Made in the 70's.

Test results 

tube 1 124 %  121 %

tube 2 128 % 128 %

Only one factory is known to make those very unique version of special construction plate ecc81  12at7s. It's the special quality Mitcham factory in Uk. Where the major part of the most demanding tube production, like military ones, was made.
It seems that a few part of this production was also made in France in the Suresnes factory with Uk  Mullard Mitcham parts in partnership with Mullard UK.

The Philips Suresnes factory assembled the tubes with Mullard parts and created vacuum to make this awesome tube.

That's the very few informations I could rather about these tubes even if most people say they are just Mullard. But I respect others views. And let each person judge by himself.
This tube having no Mullard factory code was either built in France with UK Mullard part from Mitcham or came directly all ready from Mullard Mitcham. In both case a very rare and great tube.

Ruggedized and superb unique construction, especially the plates with some copper foil parts inside the plate. It has halo getter copper getter post and some extra copper construction.

Highly sought after by the most regarding audiophiles and professional recording studios.

Extended frequency range and very detailed and upfront 3D sound with the legendary British Mullard smoothness and incredible mids.

Highly recommended for high end audio, phono stage, headphone amp, tube microphone or any pro audio preamp.

For some application probably one the best .

Also built to reduce microphony.

It has been fully tested and revealed very strong and nicely balanced, within 7 % !    So absolutly fine for regarding stereo purpose.

Auditioning tests for this pair were excellent, great sound.

Also ok for microphony in most application and super quiet tubes.

Those military tubes are made to last very very long.

Perfect top condition, sold as new or very few use.
                       
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These tubes has been tested for

Heater filament test: Short circuit
Heater filament test: Open circuit
Heater filament test: Tolerance check
Heater cathode insulation: Leakage
Heater cathode insulation: Short Circuit
Tests for heater current abnormalities
Amplification factor
Voltage gain
Power gain
Screen grid test
Mutual conductance test
Dual test for double triodes
Emission
Inter electrode leakage
Inter electrode short circuit
Flash-over (arc detection, high voltage breakdown)
Gas ionisation test

with a first tester

and then with a second tester ( Jackson 648S ) for

Leakage

Filament

Circuit

Shorts

Plate

Life test...

The Jackson 648. It is definitely one of my favorite tube testers. While it does not employ a mutual conductance test circuit, it does use an advanced emission circuit that applies separate element voltages to each tube element, and separate load circuits are also used. “These voltages and loads have been carefully selected for each tube to meet most ideally the normal operating condition of the tube.” (Source: 1950 Jackson catalog.)

In my experience, the Jackson circuitry is very effective, and often will surprise you with accuracy that approaches Hickok’s coveted “mutual conductance” test method. ( Source tubesound web site.)

I do my best to test and describe the tubes the best I can.

Note that test results may vary from one tester to an other so this must be taken as an indication, although my Jackson is regularly tested for calibration.

 Photos are part of the description you get what you see.

Thank you for your interest.