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Old 31st December 2009, 04:27 PM   #61
rknize is offline rknize  United States
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I didn't read the rest of the thread carefully so i don't know if this has been mentioned, but it could also be a tube that is shorting internally after it reaches a certain temp. I would expect that to happen sooner than 1/2 hour, but it's possible.
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Old 31st December 2009, 04:38 PM   #62
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Quote:
Originally Posted by rknize View Post
...but it could also be a tube that is shorting internally after it reaches a certain temp.
I considered that also. Obviously, one possible way to try to troubleshoot this would be to swap in different power tubes. Of course, if you have no spare power tubes that can be kinda difficult.

Another option might be to try running the amp for an hour with only one of the power tubes installed. I would recommend carefully monitoring the voltage at the C1 cap, since the B+ is going to rise when the power supply is only half loaded. If it goes over the rated working voltage of the cap (500VDC?) then abort the test. He should also carefully monitor the current draw / plate dissipation of the power tube remaining in the circuit. At the higher B+ voltage, it is going to draw more current and dissipate more watts.
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Old 31st December 2009, 06:17 PM   #63
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power tubes as in the 88's? I've swapped the 88's and the 5ar4. only one i haven't tried is the 12at7.
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Old 31st December 2009, 07:58 PM   #64
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Pretty unlikely to be the 12AT7, but not impossible.

Like Tyler said, run the amp for an hour or so without B+. It would be like having it run on standby. It's not great for the cathodes, but an hour won't hurt it. The simplest way would yank the 5AR4. A better way would be to disconnect the HV winding from the PCB so that the load of the 5AR4 heater remains. If nothing happens then you haven't learned much. If the fuse blows, then you know it's a heater or possibly the power transformer.
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Old 1st January 2010, 03:58 AM   #65
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This might be a really dumb question, and I don't mean to be insulting, but
when you mounted the sockets on the opposite side of the PCB from where it was intended, did you alter the corresponding connections?
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Old 1st January 2010, 04:35 AM   #66
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yes sir.. made sure they were on the proper pins
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Old 1st January 2010, 03:56 PM   #67
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fuse fatigue? I had that experience with a gainclone, had to go oversize on the fuses. Maybe you are just on the edge, can you put an ammeter in line. What size fuse are you using?
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Old 1st January 2010, 04:13 PM   #68
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He's using a 2 amp fuse, which should be more than enough.

http://www.diyaudio.com/forums/tubes...ml#post1878868
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Old 1st January 2010, 08:09 PM   #69
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There are many good suggestions. I will extend the ideas and hope will narrow the
cause:
1. If everything is mounted to the chassis, I will remove them from the chassis.
This is just to eliminate one factor and to ease trouble shooting.
2. If you have 3 DMM's,
a) hook 1 across the dropping resistor between the input cap and big filtering cap.
b) hook 2 across the cathode resistors.
While the amp is warming up, monitor the voltage on the DMM to see any of these
will start going up.
Notice that (i) the current drawn should be relatively constant after 2-3 mins, (ii) the
voltage across the 2 cathode resistors should be very close. If you don't see this to
be the case, you may have narrow the trouble area.

Good luck!!
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Old 4th January 2010, 06:04 PM   #70
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Thanks for all your input guys, I'll give these a shot in the next few days, in the meanwhile I've also got a new set of tubes coming in, just incase its a faulty tube.
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