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

  1. HomeKnowledge BaseMullard ECC83 (12AX7) Reissue vs Original: An Electrical Comparison

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

I must confess Matt Lachesky’s detailed autopsy of New Sensor’s “Reissue” Mullard 12AX7 and a genuine Mullard Blackburn ECC83 stirred my interest [see ‘Mullard ECC83 (12AX7) Reissue vs Original – A Physical Comparison’]. His dissection revealed substantial anatomical differences between the two tubes. In fact, the internals of the Reissue bore little resemblance to the genuine article. This got me thinking: in what other ways might the two tubes differ?

Had any effort been made to replicate Mullard’s metallurgy and chemistry? Were the base nickel alloys used for the cathodes different? And the oxide-coatings? Was the Reissue made from the right stuff? I wanted to know more, but there was nothing to be found on materials quality in New Sensor’s sales literature or datasheets.

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

A deeper investigation was needed. This report will test and compare the gain, section matching, microphony and electrical noise of both tubes.

Gain

A “Tube Imp” tube tester was used 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 ECC83, two of the ‘reissue’ 12AX7 tubes and a JJ 12AX7 for reference. The results are shown in the table below:

ManufacturerSection 1Section 2
Mullard95.095.4
Reissue “A”94.095.1
Reissue “B”93.993.3
JJ106.3103.5

The two triode sections were well matched in the Mullard ECC83 and the Reissue; but keep in mind that the Mullard Blackburn tube is a vintage tube pulled from used equipment that was over 40 years old—it’s possible its performance may have declined over the decades. We need to recognise that the test results lower than when it was first manufactured and some kind of adjustment may need to be made for aging.

Matching

The graphs below show plots of plate 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 is good in the Mullard and Reissue, however the plate currents track each other more closely in the Mullard ECC83 tube. Closely matched sections aren’t essential for the correct operation of a resistance-coupled amplifier, or a phase-splitter circuit in a guitar amp, but it’s vital in a vacuum tube differential (balanced) amplifier circuit, if a high common mode rejection ratio is to be attained. And matching is critical in a stereo vari-mu tube limiter/compressor if the compression ratios of each channel are expected to track each other well.

This high correlation between the triode sections of the Mullard ECC83 gives us an indication of the gap in the quality of tubes made back in the 1960s and today.

Noise and Microphony

For this test a ‘Tube-Drive’ pedal was used 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 a computer. The input was grounded and the tubes were tested by inserting them into the top right-hand tube socket. 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 small screwdriver to test for microphony.

This listening test was more revealing of the “character” of the noise and microphony than a simple meter level measurement. Immunity to external vibration pick up of the Mullard and the reissue were both subjectively “good”. My “knock” test revealed the Mullard tube to be acoustically “dead”. There was no audible sustained, bell-like “ringing”, resonance or rattles after the tube had been struck. It was exceptionally well damped—an indication that the “cage” had been assembled and inserted properly into the glass envelope during manufacture; and that the component parts of the tube were accurately machined.

In contrast, the Reissue was noticeably more sensitive to vibration. Further, the microphony had like a bell-like quality with a significantly longer decay time [demonstrated in sound clips above]. This was despite the fact the Reissue has a “U” shaped getter fixed at both ends to the cage assembly. The “U” getter acts as a brace to stiffen and support the cage—this restricts vibrational movement. In contrast, the Mullard ECC83 only has a single getter mount. Now, all things being equal, the Mullard should have been more sensitive, not less sensitive, to the knock test.

But all things are not equal. The Reissue is more microphonic, indicating the cage assembly might be looser. A loose cage allows the plate, grid and cathode to move relative to one another. The tube literally becomes a microphone. Inter-electrode movement can be due to sloppy manufacturing tolerances, but also differences in construction. The Reissue is fitted with square-shaped micas that are typical of Russian tubes. This type of mica is inferior to Philips/Mullard “star” micas, as it only makes contact the internal walls of the glass envelope at four points.

Remedial remedies, such as fitting tube dampers or cryo-treatment will do 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; however they’re not top-notch, like the Mullard ECC83.

Discussion

From visual inspection it can be seen that the Reissue is not manufactured with Mullard tooling. Its internal construction appears to be identical to the ‘Sovtek’ 12AX7LPS. This isn’t surprising, since both tubes originate from New Sensor’s “Экспопул”( “Expopul”) factory (formerly Reflector) in Saratov, Russia. So is the Reissue simply a rebranded ‘Sovtek’? A hand-selected ‘Sovtek’ tube—possibly tested and graded for microphony and noise, and stamped with the Mullard logo?

Maybe this doesn’t bother you, but I’m the kinda guy who likes to know what he’s buying. And, as an engineer it bothers me, a lot. You cannot test quality into a product. Quality has to be engineered into the design from the ground up. For instance, choosing high quality cathode alloys, micas and glass. And—if you really are serious about meeting Mullard’s stringent high quality standards—using the same construction techniques and assembly practices. Given this lack of heritage, wouldn’t it be more fitting to call this tube by it’s real name: a ‘Sovtek’ 12AX7LPS “platinum” grade; or, better still, a Reflector 12AX7, because that’s what it is.

отражатель (Reflector) company logo
отражатель (Reflector) company logo
'Sovtek' 12AX7
The 'Sovtek' 12AX7 is manufactured using Reflector tooling and expertise.

The noise figures, matching and gain of the Russian reissue are fit for purpose, that is, to use in a guitar amp. The Reissue isn’t a bad tube. In fact it’s a pretty good tube, but a Mullard tube it is not. The Mullard ECC83 is in a different league: it’s less sensitive to external vibration pickup, well damped, the sections are more closely matched and, it’s 40 years old! A superior device from time when excellence really did mean something, and when manufacturers stood by their brand name.

So why don’t New Sensor stand by their brand? Rather than trying to associate themselves with reputable American and European tube manufacturers. New Sensor did not, “…grow to absorb the brands Tung-Sol, Mullard, and Sovtek,” as Paul Kobylensky describes in his recent article “Don’t Sell Your Tube Amps!“. What they did was register the Mullard trademark in 2004 when Philips didn’t renew in it the US. But what’s in a name? There’s more to Mullard than a name or a logo—it’s the expertise, materials and people that made Mullard what it was.

New Sensor cannot legally sell their Mullard branded Russian tubes within Britain, or any other European country, because Philips still own  (since 1927) the Mullard trademark.

It seems New Sensor want to distance themselves from their Russian roots; that they have no strong belief in their own brand, or in Reflector’s heritage. That’s a great shame, because they’re missing a real opportunity here. Instead of riding on Mullard’s reputation they could share their own story. The story about the Reflector factory, the people that work there, and the engineering behind their tubes—Russian engineering. Like these beautiful 6N23 Reflector tubes in the picture below. And look at their retro-cool boxes proudly displaying Reflector’s space age logo. These tubes were made with a silver electrostatic shield between the two triode sections and silver grids. Magnificent stuff, eh? Now that’s what New Sensor should have brought back from Russia with love, to share with the music community. Ah, but hey… at least they’re still making tubes. And tube amp owners will be thankful for that.

Reflector 6N23-EV vacuum tubes
Photo of the Reflector 6N23-EV vacuum tubes taken by "Viva Tubes".

The Reflector factory is situated on Molodezhnyy Proyezd, Saratov, Russia in a retail park, about 5Km north of Yuri Gagarin State Technical University. According to Reflector’s long-standing director Vladimir Chinchikov the factory was erected in 1953. At the time Reflector manufactured high quality vacuum tubes for the military and tubes for domestic use in radios and televisions. The entire facility covers an area of approximately 100,000 square feet.

In 1998 the factory and tooling was acquired by an American import/export agent (New Sensor Corp., Long Island City, NY) and the name changed to Expopul. Expopul is a “closed joint stock company”, which means the company stock is owned jointly by the shareholders. There are presently about 900 people employed at the factory, many of which are skilled workers specialising in vacuum tube manufacture. Expopul is one of the last two remaining tube manufacturers in the world that can make preamp tubes (12AX7, 12AU7, etc) for guitar amps; the other being JJ Electronic in Slovakia. This short video made by Irusha Bitukova (general Director at Sovtek) contains some behind the scenes film footage of tubes being assembled in the factory.

In This Section

  • Black Plate Tubes
  • Chemical Highlights of Tube Manufacturing
  • Cryogenic Treatment of Tubes: An Engineer’s Perspective
  • Developments in Trustworthy-Valve Techniques
  • Evolution of the Tube
  • Foil Those Tube Forgers
  • Microphonics
  • Mullard ECC83 (12AX7) Reissue vs Original – A Physical Comparison
  • Mullard ECC83 (12AX7) Reissue vs Original: An Electrical Comparison
  • Noise
  • Oxide Cathode Life: Investigations into the Causes of Loss of Emission
  • Secrets of the Tube Alchemists
  • Signal Tubes
  • Speed, Efficiency & Perfection – Aims That Have Built a Mammoth Factory in 16 Years
  • Subminiature Tubes: The Future of Audio!
  • That’s a Sylvania tube, the print is green, no, it’s blue
  • The ‘Magic Eye’
  • The ’12AT7′ Tube
  • The ’12AU7′ Tube
  • The ’12AX7′ Tube
  • The 12AX7 Tube: The Cornerstone of Guitar Tone
  • The 6SN7GT: The Best General-Purpose Dual Triode?
  • The Accurate BSPICE Tube Models
  • The Cool Sound of Tubes
  • The Inner Workings of Vacuum Tube Buffers
  • The Tube Family Tree – Part 1
  • The Tube Family Tree – Part 2
  • The Tube Family Tree – Part 3
  • Tube Vendors
  • Tubes: The Old Verses the New
  • Vacuum Tubes and Transistors Compared
  • Valve Microphony Part 1: Production of Microphony and Methods of Investigation
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