A deeper investigation was needed. This report will test and compare the gain, section matching, microphony and electrical noise of both tubes.
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:
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.
The graphs below show plots of plate current verses grid voltage.
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.
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.