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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: what are the first signs of preamp tubes death? posted by paco on January 11, 2004 at 15:00:55:
My career was in the Hollywood broadcast and recording studios, beginning in the '50s. "In the early days" there were hundreds of tubes in most installations.Of course Hickock testers were a necessity. We routinely checked each amplifying tube for mutual conductance (the main parameter relating to performance) every few months, and most pro equipment is left on 24/7.
The main thing that determines tube life is how hard it is used. That is, how much plate dissipation is used compared with the rated value of the tube. In general, that translates to a tube that runs very hot will have a short life. What poops out is cathode emission; the tube just gets "soft." Usually no audible effect if they are caught before mutual conductance falls to 1/2 normal value. Amperex and some Telefunkens used to worry me because the heaters were SO bright it looked unhealthy. But they seemed to last as well as others.
Once in a broadcast station they had ordered a large stock of "6L6," not specifying the glass version, 6L6GC, for the last version. Well, a "6L6" is a metal version, used in RF applications where controlling RF radiation is important. Problem is, the metal tubes didn't get rid of their heat as efficiently as the glass, and those 6L6's had about half the life of the glass ones in audio amplfier ouputs. The devil is in the details.
I have been talking about normal lifespan. Then there were the abnormal problems - microphonics (preamp tubes only), gas in power tubes (that blue glow), sudden shorts etc. Some are not going to like hearing this, but it was my experience that RCA tubes usually lived out their lives quietly, eventually just pooping out when cathode emission fell too low. On the other hand, GE tubes usually went out with a bang of some kind. Rarely did I find a GE tube with a long, peaceful life.
I don't know how you can check tubes for emission other than with a tester, except by measuring the DC voltage across the cathode resistor in self-biased tubes. Some pro equipment had that feature built-in.
I turn off tube equipment if it is not going to be used again for several hours. But I don't leave it on 24/7; a tube is using up cathode coating (which is the source of the emission) every minute it is operating. The strain (which is only on the heaters) of firing them up has to be balanced against "emission time."
I hope this is useful information for those who didn't grow up with tubes by the hundreds in pro installations.
Pat Tobin
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Follow Ups
- Re: what are the first signs of preamp tubes death? - AudCon 00:25:55 01/14/04 (0)
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