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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old Yesterday, 10:52 PM   #661
Mr_Zenith is offline Mr_Zenith  United States
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Join Date: Aug 2009
Location: KC Metro
After a 14 year run, the TSE must DIE!
I've never heard of cathode stripping in pass tubes, but I wouldn't be surprised if it were true. It's definitely a phenomenon in high voltage transmitting tubes though, which is one of several reasons broadcast transmitters employ a substantial filament heating period when beginning from a cold start. I discarded the idea of cathode stripping in regular audio amps 20 years ago when I acquired a 1930's Zeniith tombstone radio with its original 6L6G outputs intact. Today they're 80+ years old and still going strong.

As a novice builder I did get swept up (briefly) in a similar phenomenon - babying tube heaters. 18 or so years ago I built two amps from scratch using schematics from a popular magazine. Both used a three switch scheme that placed power resistors in series with the heaters. The idea was that you'd "preheat" the heaters when the first switch was thrown. The second switch shorted out the "preheat" resistor, then when all the tubes were a' glowin' you'd apply the B+ with the third. It was all pretty Rube Goldberg-esque and completely unnecessary of course, but it did sell magazines and get me infecte- er, started in this hobby.

Parenthetically, the first amp (a hoary old PP 6V6 Hafler/Dynaco circuit) sounded stunning, probably because of the old Heathkit iron I'd scavenged for the outputs. The second, however, was a SE 6550 job that was an utter turd on toast. Not wanting to hold my nose and nibble around the edges, I parted it out and didn't build another SE amp for 15 years. To this day it remains the only full-up build I've ever dismantled.

Today I just throw a CL-60/90/140 in between the fuse and the transformer primary and call it good. I'm too old to be looking for monsters under my bed, much less be afraid of them.
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