7591 cathode resistor unstable - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 10th January 2011, 05:41 PM   #1
diyAudio Member
 
Join Date: Aug 2007
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????
__________________
  Reply With Quote
Old 10th January 2011, 08:24 PM   #2
diyAudio Member
 
indianajo's Avatar
 
Join Date: Jan 2010
Location: Jeffersonville, Indiana USA
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.
__________________
Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey CS800S,1.3K, SP2-XT's, T-300 HF Proj's, Steinway console, Herald RA88a mixer, Wurlitzer 4500, 4300
  Reply With Quote
Old 10th January 2011, 08:29 PM   #3
Speakerholic
diyAudio Moderator
 
Cal Weldon's Avatar
 
Join Date: Jan 2004
Location: Near Vancouver
Thread moved to Tube section.
__________________
Next stop: Margaritaville
Some of Cal's stuff | Cal Weldon Consulting
  Reply With Quote
Old 10th January 2011, 09:58 PM   #4
diyAudio Member
 
Join Date: Aug 2010
Forgive me, but are measuring the resistor with the amp on?
  Reply With Quote
Old 11th January 2011, 01:29 AM   #5
diyAudio Member
 
Join Date: Aug 2009
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.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
unbypassed cathode resistor rongon Tubes / Valves 8 18th September 2010 02:11 PM
newbie question about 7591 cathode bias joshuahhhhhhh Tubes / Valves 3 9th December 2007 04:33 PM
Wirewound cathode resistor Klimon Tubes / Valves 12 30th August 2006 10:19 PM
Common Cathode Resistor D0Hbert Tubes / Valves 1 4th March 2005 11:42 AM


New To Site? Need Help?

All times are GMT. The time now is 04:20 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2