Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: shortfuse on December 20, 2010, 12:54:19 pm
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The plexi build I just completed http://www.el34world.com/Forum/index.php?topic=10466.0 (http://www.el34world.com/Forum/index.php?topic=10466.0) was working great until i was able to really start turning the volumes up higher. We had if cranked up pretty good Thursday night playing on Chanel #1 then plugged a jumper from #1 to #2 when we first insert the jumper in the #2 channel (not yet jumped to #1) it made a high pitch squeal / hiss like a pick up with a break in the winding. Pulled that cable and used a different one then it stopped and we continued to play. Then after about 5 min it just sorta shut down. Checked HT fuse blown. Changed it out the next day while looking things over turned it on all seemed well and started to play and blew again with in 30 seconds. I did notice after if shut down this time 1 tube was hotter than the other. I am going to check voltages when I get home tonight and compare them to the ones I did initially in the previous post and compare. Whats my best starting point? It should be in the power section correct?
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Remove all the tubes and see if it blows.
I think one of your power tubes is bad and caused the problem.
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LC you are correct. Pulled tubes (KT88's) out turned the bias pot all the way back and installed a set of KT77's from another amp adjusted bias out to about 37ma and played for 2hours at fairly low volumes. Let it cool down and turned the bias pot all the way back again and reinstalled the KT88's, I have 1 tube that idles with the trim pot backed all the way off at about 56ma (my plate is 498 on both sockets) and the other will sometimes stay at 59ma then sometimes start to run to 75ma before I hit the standby switch. Why would one tube all of a sudden run away intermittently? Also when I first installed them (the 88's) I had to adjust the trim pot to get them to 45ma. And the range resistor is still at 218K I have several hours on it at lower volumes and it started this the 2nd time I really started making it work the 1st time had no issues at all and sounded great. Runs fine with the KT77's so I guess I need new tubes. KT88's with re bias are drop in for a EL34 amp are they not?
Scratching my head.
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Yeah, you just have a bad tube. Failed after working for a bit. Are they new? What brand?
All those octal tubes are interchangeable with a simple (sometimes not so simple) bias adjustment.
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Yeah, you just have a bad tube. Failed after working for a bit. Are they new? What brand?
Yes they are new about 20hrs on them. They are JJ's. I called Jay this morning and they are sending me replacements. This is the 2nd time I had a problem with their tubes 1st ones were a matched set 9ma apart. Again they sent me another set 1.5ma apart at no cost to me. So at least for their customer service rating I give them a 10.
I am really green when it comes to knowing how each of the different tube brands differ in performance and tone. I have always used their stuff because they have a lot of info and the prices are reasonable. I usually order direct from their web site. I have also used some EH tubes and really have no opinion one way or the other. Tubes are kinda like pick ups though you have to buy them to try them. The amp sound good with the KT77's but I think is sounds better with the 88's.
And thank you for the suggestion on where to start you helped save the lives of at least 4 fuses.
Steve
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Steve,
You may want to give ~C~ (old Svetlanas) a try, they sound really, really nice. I've used them for years now in my Major and never had a problem - and the Major is not kind to its tubes. It's architecture is more true to design of the old GEC, not just a relabeled 6550 (EH, Gold Lion, etc...). But hey, keep getting replacements until they say no! But the next batch, you may want to try something different. I've never had a problem with JJ's 5881 and they sound great, but this is not the first time I've heard about problems with JJ's KT88.
Good luck!
Jim
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Yes I've had back luck with JJ6L6. (But their 6V6S is fine tho' - no problems to-date).
Get Sovtek WXT 6L6s or the Svet Winged-C. The Russkie 6P3Se (with the wafer-thin black plastic base) is quite a good '5881' sub too.