Sunday, January 28, 2007
About those tubes
Few electronic devices have developed a lore about them greater than the mysterious vacuum tube. Since the first DeForest Audions, vacuum tubes (or valves as the British call them) have had a certain appeal, an observable life to them that was and still is fascinating. After all, how boring it is to gaze upon stone cold FETs.?
It is Winter here in the Northern Hemisphere, and so there is time to test tubes. Shown here is a store-type tube checker, not really a tester, that I keep filled with tubes. Actually, it isn't big enough. Not nearly. I have a lot of tubes here. Octals, loctals, minis, Nuvistors, ceramic tetrodes, power pentodes, glass, metal, ceramic, transmitting, recieving, regulators, ballast, gas discharge, thyratrons, common, obscure, NOS, rectifiers, and more. Names like Sylvania, Tung-Sol, RCA, Mullard, Philips, Telefunken, Raytheon, GE, Western Electric, and even a couple of Svetlanas, Gyorgy, Cunningham and many others reside in boxes, bins and bags in my basement, over by the furnace. I get them in bulk these days. People call and say "Come get these tubes", so I do. And I ask no questions, even if the numbers don't look very promising, those beginning with oddball filament voltages like 18 for instance, which mean old TV tubes that are generally not wanted. I take them all and horde them like acorns.
Real tube testers measure transconductance and can check for leakage, shorts, and are able to quantify emission. If you know what you are doing you can "run the curves" and attempt to match up sets of tubes, devices that rarely match, for a number of manufacturing reasons. I like my Weston portable (Navy type) tester and the Hickok. Both have extensive numbers of rotary switches which must be set to the exact position according to the roll chart or book of tube settings that also contains the good vs bad limits, the numbers that say when to pitch or keep.
Like anything vintage, tubes have their myths that I won't get into here, but I will say that names should not be confused with quality and performance, at least not initially, when evaluating tubes. All but the most persnickety makers relabeled tubes and made various changes throughout the production lifetime, for instance. Two seemingly identical 12AX7s will have different noise, gain and perceptible sound differences, if you have the ear. JAN (joint Army-Navy) tubes were produced by so many different manufacturers that it is often impossible to identify any specific maker, or brand. I sometimes see "JAN" referred to like it was one maker or specification. Often JAN and other mil tubes from WW2 were made in a big hurry and have wide performance variations.
It is always nice to have enough tubes, as you never know when that next Mesa MKII will come through the door, or that Collins 75A4 will show up.
Posted by Bob Crowley at 10:47 AM