Garey Barrell <[EMAIL PROTECTED]> made an utterance to the drakelist gang ---------------------------------------------------------------------- Jack -
Three "common" problems that show up in tubes as they age are "weakness", "leakage" and "noise".
Weakness is a result of the cathode coating just "runs out" of electrons! This is probably the least common failure mode, but the one that inexpensive tube checkers test. You can estimate how much life a "good" tube has left by reducing the filament voltage switch by one step and seeing how much the emission level drops. With a new tube it will drop less than 5 -10%, but a tube that is weak will drop as much as 50%.
Leakage refers to grid current flow, usually a result of gas ions "leaking" past the metal/glass seals around the pins. The result is a "drifting" plate current as the tube heats up and the grid voltage increases as a result of the grid current flow. In the 4 Line receivers that often shows up in the R4-B as drifting AVC / S-Meter voltages, usually from the 12BA6 IF tubes V4 and 5. Less often in the 6BZ6, V1. If the S-Meter drifts as the receiver warms up, this is usually the problem. Many tube testers have a leakage test, but most are very inaccurate.
Noise is just that, crackling, static type noises, sometimes from 12BE6 mixers, but usually from the 6EJ7's, most often the 3rd Mixer in the R-4C. Many of these are noisy when new, especially brands other than Sylvania, for some reason. Generally, once you find a quiet one it stays quiet for years. Most tube testers don't have a test for this, but you can thump the tube envelope with a pencil (if you can find one!), and watch for variations in meter readings. This will find noise caused by loose elements, but most noisy tubes are just noisy! Random gas ions floating around inside? I dunno.
Bottom line is that the only "real" test is substitution of a known good tube. Techs that service a lot of tube type gear have a "reference" set of tubes that they use just for substitution, because even in the good ol days not ALL new tubes were good.
73, Garey - K4OAH Atlanta Drake C-Line Service Manual <http://www.k4oah.com> Jack Wilson wrote:
"Jack Wilson" <[EMAIL PROTECTED]> made an utterance to the drakelist gang ---------------------------------------------------------------------- Hi, Can anyone help me with this question. I have owned Drakes since the early 70's, and on a couple of occasions back in the good ole days, sent my 4 Twins back to Drake for fine tuning and fixing up. When the invoice came back, it always had comments on the Tubes that were replaced such as weak, leaky, noisy etc. Now I have an old tube tester that I try to use these days, but it only tells me that the tubes are either bad or good. How do you find out, or tell, when tubes are leaky or noisy? I suppose I can know if they are weak just by the tester's reading, but these other two conditions have me stumped. Maybe I could just replace them all if they don't read in the upper 25% on the tester, but that gets expensive these days. Any comments? But please be gentle. Jack K1HW
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