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.
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.
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!!
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.
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