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7591 cathode resistor unstable
7591 cathode resistor unstable
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Old 10th January 2011, 05:41 PM   #1
glassandlight is offline glassandlight  United States
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Question 7591 cathode resistor unstable

Strange and unexplained event on a cathode resistor of 10 ohms, 1/2 watt, Xicon brand suddenly racing up to 250 to 350 ohms. They are supposed to be rated to 350V. This has happen three times now, different tubes, different sockets, different times. I caught the event twice after ruining one tube. The tubes are NOT shorting, the resistor is NOT popping. The tube which was damaged, now draws considerable more plate current than before, the other two are OK and still the same. Tubes tested on a restored and calibrated Triplett 3444.

The amp is a restored Sherwood 5500 II, hot-rodded with outstanding parts, except these Xicon resistors. Plate is 405, screen is 397, grid is pot adjustable to get 22ma across the 10 ohm resistor so the tubes run quite cool. The amp is never run more than 2/3 open driving 8 ohm speakers.

Everything checks out as OK circuit wise so I am stumped what is making these resistors fail unless it is simply the brand.

Any ideas????
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Old 10th January 2011, 08:24 PM   #2
indianajo is offline indianajo  United States
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A lot of different brands have a bad day, and doubt if they do much of a sample test at 400V. I've been using generic indian manufactured "multicomp" metal film resistors from newark.com in the 1 and 2 watt sizes in my tube preamp. They are rated 400 volts up. Nothing exciting happened. I put some salvage RN65 dale resistors in my tube organ power supply to drop main B+, and nothing unusual happened. I've got a Triplett 3444 I found in the trash in 1987, for cause because with new electrolytic caps, everything is still a shorted tube. ? I do have a schematic but it is hard to understand with all those switches.I Would expect this question to be on tubes thread, since you've been here since 2007. Good luck.
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Old 10th January 2011, 08:29 PM   #3
Cal Weldon is offline Cal Weldon  Canada
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7591 cathode resistor unstable
Thread moved to Tube section.
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Old 10th January 2011, 09:58 PM   #4
MattyAve is offline MattyAve  United States
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Forgive me, but are measuring the resistor with the amp on?
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Old 11th January 2011, 01:29 AM   #5
Capt Grogg is offline Capt Grogg  United States
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Much more likely that the tube failed, overloading the resistor and causing rise in resistor value,have seen it many times, the resistor rising in value by itself would not damage the tube.
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