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.
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.
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.
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.
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.
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? 😕
when you mounted the sockets on the opposite side of the PCB from where it was intended, did you alter the corresponding connections? 😕
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?
He's using a 2 amp fuse, which should be more than enough.
http://www.diyaudio.com/forums/tubes-valves/135110-kaidees-simple-se-build-thread.html#post1878868
http://www.diyaudio.com/forums/tubes-valves/135110-kaidees-simple-se-build-thread.html#post1878868
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!!
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!!
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.
@pchw .. my chassis is plexiglass, non conducting so I dont think anything is shorting, its basically like a wood panel that george uses on his website, I haven't made a proper enclosure or anything
whoa.. it just hit me.. would running 1 channel with load (speaker) and the other channel without load cause the fuse to blow? cuz I've been testing with a single speaker...
In general, you don't want to leave the secondary open without any load because the primary will see an extremely high impedance (infinite) when there is is no load on the secondary. I am not sure it can be the cause of blowing the fuse, but it will harm in the long run for sure. If you only have one test speaker, you can use a 4 or 8 ohm 20W resistor on the other side. Otherwise, you can test one channel at a time by unplugging the KT88 that without speaker.
...is a bad idea. Just don't do it. If you are testing an amp, always keep an 8 ohm dummy load or a junk set of speakers connected to both channels. Running the amp with no load can result in a shorted output transformer - permanent damage. 🙁
i just cleaned up the wiring, made sure everything is connected good, checked solder points AGAIN and threw in a new 5ar4 by a different factory. Will test again tonight. still have 2 fuses left =)
So, out of curiosity, what is the voltage across R17 and R27 while the amp is running? Is that voltage stable, or does it wander around a lot? I'm guessing if you are running KT88, you probably see about 40 volts across a 560 ohm resistor?
might be my nickname is a clue, but what I'd be tempted to do, is go bigger on the fuse, see what burns and then fix it!! kidding, well kind of...
hey sparky.. i actually considered that.. just getting rid of the fuse and connecting an ammeter to it with my hands on the plug.. lol
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