JTM 45 - First two preamp tubes won't light up

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lespaulplayer
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JTM 45 - First two preamp tubes won't light up

Post by lespaulplayer »

I have a problem with my JTM 45 build. It only happens once in a while and other times the amp works just fine. Once in a while I power it up and the first two preamp tubes won't light up. Obviously I get no sound from the amp when this happens. The third one closest to the power tubes will light up and so will the power tubes, but the first two preamp tubes won't light up. I've replaced the tubes with new ones and the amp does exactly the same thing. This isn't a new build, I built the amp about 3 years ago, and so far haven't had any problems until now. Just wondering if this sounds like a cold solder joint or what? Why does the amp only do this once in a while, and then other times fires up just fine? And also when it does work, it doesn't cut out or a anything. It will stay on without any issues. This only happens during start up sometimes. It's like the first two tubes aren't getting power for some reason. Any ideas or advice?
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M Fowler
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Re: JTM 45 - First two preamp tubes won't light up

Post by M Fowler »

Sounds like intermittent problem so cold solder joint or broken filament wires.

When we trim our wires we sometimes nick the wire which eventually breaks.

Mark
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Re: JTM 45 - First two preamp tubes won't light up

Post by tubeswell »

Pull out all three pre-amp tubes, and check each heater wire (between the socket pins) for DC continuity with your R-meter. You've probably got a crack/open in one of the wires, like Mark suggested.
He who dies with the most tubes... wins
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johnnyreece
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Re: JTM 45 - First two preamp tubes won't light up

Post by johnnyreece »

Agree with the other two. At least you should have it narrowed down pretty well. Parallel wiring makes it nice and easy; it should be broken between where it's working and where it isn't.
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lespaulplayer
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Re: JTM 45 - First two preamp tubes won't light up

Post by lespaulplayer »

Thanks for the replies and suggestions. I will check it out and see what I come up with. Will post an update once I get it fixed. Thanks again.
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Re: JTM 45 - First two preamp tubes won't light up

Post by SoulFetish »

what kind of tube sockets are you using for the preamp tubes?
you might be loosing tension on some of your tube pins. This would cause intermittent connections. If you have Belton style sockets, you can take a fine point tool (ie. solder helper tool, heavy duty leather pin, etc) and retention the sockets.
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lespaulplayer
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Re: JTM 45 - First two preamp tubes won't light up

Post by lespaulplayer »

I re-hit the filament wires with some solder on the tube sockets and now it seems to be working great. Tubes light up with no issues so far. Amp sounds great. No issues. Thinking it was a cold solder joint on the second preamp tube that was causing it. When I first opened up the amp it was not working. That made it easy to track down where the problem was. Wiggled the black wire on the tube socket of the second preamp tube and it powered up. So re-soldered and it stayed on for good. Wiggled the wiring all around afterwords and everything is staying on. No shorts. Thanks again, everyone.
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Re: JTM 45 - First two preamp tubes won't light up

Post by Stevem »

You can't always take things for granted!
Just last week here at work I was trouble shooting a new compressor that would not power up.
At first it looked like the PT primanry was open , but 10 minutes later after checking out all the other possible things that made sence the culprit turned out to be a 8" lenght of stranded wire that was open near its center, yes stranded wire!!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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lespaulplayer
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Re: JTM 45 - First two preamp tubes won't light up

Post by lespaulplayer »

Been testing it out every day and so far all is good!
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