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Quality of Blue-print and Green-print 6111WA Tubes Compared

by Phil Taylor

For sometime I’ve been utilising subminiature pencil tubes to build compact vacuum tube effects pedal designs such as the Effectrode PC-2A Compressor, Mercury fuzz, Fire Bottle boost and my new tube buffer pedal, the Glass-A. After testing and hand-selecting thousands of these tubes over the last five years or so I’ve become convinced that some batches aren’t quite as good as others in terms of electrical noise and microphony. Without proper acoustic/vibration test equipment, a rigorous test methodology and a good sample of hundreds of N.O.S. tubes from different manufacturers and batches from every year a it’s difficult to scientifically confirm what the statistical variation might be or even if there are any significant statistical variations and if you are just purchasing one or two of these tubes at time it’s impossible to make any assessment of trends. However, I’ve been working with Philips ECG pretty much exclusively and believe that I can shed at least a little insight into the quality of these tubes.

Whilst testing a box 100 ‘green-print’ 6111WA tubes (batch from October 1985) I was impressed at how electrically quiet and how low the microphony was. These were what I consider “excellent” tubes. They were very consistent too and even the worse tubes within this batch would be considered “good”. After completing testing I decided to test a batch of ‘blue print’ tubes (batch from February 1987). I began testing but stopped before I had tested no more than 20 of these tubes. Progress was painstakingly slow because they were considerably more microphonic. My curiosity was roused.

I don’t have a calibrated rig for testing susceptibility to microphonic pickup from constant or impact excitation, however my own qualative ad-hoc test methodology of tapping the tubes with a pencil whilst listening seem to indicate me that there were variations of statistical significance. Evidence indicates higher mean value of microphony with greater deviation around this mean in the ‘blue-print’ tubes. Grey plate tubes were manufactured from 1987, almost end of production. Was the tooling wearing out? Was less care taken with manual assembly on these last production runs of tubes? Indicating the end of the road for tube manufacture?

To do this properly I really should build a proper rig that taps the tube with a known mass to excite it, measure the signal induced and undertake more thorough statistical analysis of larger batches of tubes, say in the hundred from each year. Trouble is these tubes are running out, there will soon be not enough to test and soon not even any to use in designs. This really is the end of an era of golden tone.

Comparision of Microphony in blueprint and greenprint 6111WA pencil tubes

Graph showing estimated probability distributions for Microphony in ‘blue-print’ and ‘green-print’ 6111WA pencil tubes.

Black plate and grey plate compared

Black plate (green-print) and grey plate (blue-print) 6111WA tubes appear to have been manufactured with the same machine tooling.