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Mullard ECC83 (12AX7) Reissue vs Original – An Electrical Comparison

Following on from Matt Lachesky’s article ‘Mullard ECC83 (12AX7) Reissue vs Original – A Physical Comparison’ in which he performed a dissection of a ‘Reissue’ New Sensor Mullard 12AX7 (American for ECC83) and an authentic Mullard Blackburn ECC83 tube, I decided to undertake some additional electrical performance measurements of these two tubes to supplement his findings. Matt’s dissection revealed that there were substantial physical differences between the ‘reissue’ and a real Mullard tube, that the ‘reissue’ was obviously not manufactured with the Mullard tooling but with the tooling used for the Sovtek 12AX7LPS which is made in the Reflector plant in Sarato, Russia. Despite these physical differences I was curious to find out as to whether New Sensor had made improvements to their plant’s manufacturing process or materials to improve their 12AX7 tube so that it matched the electrical specifications of the Mullard ECC83.

This report compares the gain, inter-stage matching, microphony and electrical noise of both tubes.

Original-Mullard-ECC83 — Original Mullard ECC83 (12AX7) pictured on the left and the ‘reissue’ tube on the right.

Gain

A Tube Imp Mini Tube Tester was utilised to make a spot measurement voltage gain. Voltage gain is also known as amplification factor (mu) and is typically stated in the ECC83 tube datasheet as having a value of 100. Measurements were made of an original Mullard Blackburn ECC83, two of the ‘reissue’ 12AX7 tubes and a JJ 12AX7 for reference. The results are shown in the table below:

Manufacturer	Section 1	Section 2
Mullard	95.0	95.4
‘Reissue’ A	94.0	95.1
‘Reissue’ B	93.9	93.3
JJ	106.3	103.5

The stages were well matched in both the original and ‘reissue’, however bear in mind that the Mullard Blackburn tube is 40 years old and was pulled from a used item of tube equipment – it’s performance is certainly lower than when it was first manufactured indicating that the mu of the ‘reissue’ is possibly a little lower than it should be.

Matching

The graphs below show plots of anode current verses grid voltage.

Mullard-ECC83-Graph — Plot of Ia against Vg for a Mullard ECC83

Mullard-‘Reissue’-12AX7-Graph — Plot of Ia against Vg for a Mullard ‘Reissue’ 12AX7

Mullard-‘Reissue’-12AX7-Graph-2 — Plot of Ia against Vg for another Mullard ‘Reissue’ 12AX7

The matching also seems good in the original and ‘reissue’ tubes with the anode currents tracking each other exceptionally well in the Mullard tube. This close matching probably isn’t of major importance in a tube amp clipping stage or even a phase-splitter section for a push-pull amplifier, however if you own a vari-mu tube compressor then it might be that Mullard tubes will give tighter matching on the compression ratio for stereo channels. This high correlation between the sections in the Mullard ECC83 gives us a clue as to the gulf in quality between tube manufacture back in the 1960s and today. Mullard were building tubes to tighter specifications than any manufacturer does today because in audio applications tight matching of sections is not so critical, however in laboratory instrumentaion such as a D.C. amplifier for precise physiological measurements it is.

Noise and Microphony

For this test I used a Tube-Drive pedal as a test jig. All the controls were set at 12 o’clock and the pedal output was connected to the sound capture device of my computer. The input was grounded and the tubes were tested by inserting them into the top right-hand tube socket – pretty much a real life situation. The tests were run for 20 seconds. During the first ten seconds of the test electrical self-noise from the tubes can be heard. For the last 10 seconds the glass envelope of tube was being tapped with a screwdriver to test for microphony.

This audio test was more revealing of the ‘character’ of the noise and microphony than a simple level measurement. Susceptibility to external vibration pickup of the Mullard and the ‘reissue’ were what I’d consider to be good, however the Mullard tube was exceptionally well damped. There was none of the audible bell-like ringing that occurs after the tube has been struck. It sounds acoustically ‘dead’ because the boffins at Mullard had really ‘nailed’ their manufacturing process and packed those electrodes very tightly within the glass envelope of the tube to minimise movement and maximise damping, therefore improving immunity to external vibration pickup. This contrasts with the ‘reissue’ in which – despite having a getter fixed at both ends to the electrode assembly – the microphony exhibited a significantly longer decay and higher peaks in amplitude. The electrodes are looser, possibly because of differences in design or manufacturing tolerances and, in case you’re wondering, the dubious process of fitting tube dampers or cryo-treatment will do absolutely nothing improve the situation. Microphony can only be ‘engineered out’ during the development and manufacture of the tube. There are no remedial fixes for a poorly designed or constructed tube. That said, the microphonics of the ‘reissue’ aren’t terrible, they’re just not as ‘stellar’ as the Mullard ECC83.

Conclusion

The noise figures, matching and gain of the New Sensor ‘reissue’ 12AX7 are certainly adequate for purpose, that is being used in a guitar amp, it’s a good tube, but it isn’t a Mullard ECC83. The Mullard is a superior device from a bygone era when tubes were manufactured not only for audio but for use in other, more demanding applications such as ruggedised military gear and laboratory instrumentation. From the test results it seems reasonable to conclude that the ‘reissue’ is just a graded Sovtek 12AX7LPS, however a full and detailed dissection of both tubes would be required to absolutely confirm this. There’s nothing unethical with grading and paying additional coin for what appears to be a premium Sovtek 12AX7LPS. In fact, I’d think more highly of New Sensor if they stood by their own brand name and marketed this tube as a premium Sovtek 12AX7LPS rather than using the Mullard’s logo to distance themselves from their Russian roots – the association with Mullard is just plain wrong in my opinion. There’s no heritage, no link at all with the engineering expertise and no use of the original tooling or materials.

It unfortunate that tube manufacturers don’t target their efforts on making a sublime, genuine contribution to the state of the art of tube design and construction rather than just rebranding their tubes under the name of once great companies. As a builder of tube gear I like to know what I’m buying – I buy quite a few tubes and it’s really important for me to be able to obtain good quality parts to continue building Effectrode effects pedals. I’m sure I’m not the only one who would like to see some more options on the table as far as tube quality goes. How I wish Mullard were still around or for a modern manufacturer to step up to the plate and take on the challenge of constructing tubes that meet or even surpass the specs of N.O.S. tubes. There is a growing demand for true quality tubes today as N.O.S. supplies continue to dwindle and dry-up. Who knows, perhaps this article might in some small way help catalyse tube manufacturers into doing something magnificient. But until then, I’ll ‘Specify and Insist on Mullard Tubes’ or at least N.O.S. Philips tubes while I can still find them.

If this technical report interested you, then you might enjoy reading something of Mullard’s history and the people who worked there in this reproduction of a fascinating article that was originally published in 1954, Speed, Efficiency & Perfection – Aims That Have Built a Mammoth Factory in 16 Years.

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Mullard ECC83 (12AX7) Reissue vs Original – An Electrical Comparison