The power tubes in my main gigging amp died recently. They were JJ E34L's around a year or two old. Some history leading up to the failure: at a gig, I left the amp on and idling after we soundchecked. About 5 minutes later the 2A slo blo mains fuse was blown. I replaced it with a 4A slo blo from a friend and made it through the gig. I replaced that with a 2A slo blo before playing it again. This time it made some humming noises and then started to drop in volume. Looking at the power tubes, one was redplating. I turned the standby switch immediately but not before the B+ fuse died. So I replaced the power tubes with a new set of E34L's. The B+ fuse dies within 30-60 seconds of powering up past standby and the amp now makes zero sound/ noise through the speakers.
The amp is basically a 2204/ ax84 Hi-Octane which usually runs at close to 490Va. 5500 primary. Metering around, the DC at the rectifier output is about 330V. I took out the power tubes, measured, then disconnected the OT primary CT, measured, then disconnected the screen supply lead and measured, each still showing 330V at the rectifier output.
Measuring resistance of the filter supply caps shows ~2M and falling. The diodes in forward bias show about 2Mohm each and read as open in reverse bias. The screen resistors measure about right at 1.1K or so, and the grid resistors similarly at 2.2K. With the screen supply lead reconnected, there's about 275K ohms between the PI side of the grid resistors and pin 4 for the screens, which made me consider arcing on the socket, but even with the screen lead disconnected the problem is still there. The OT primary measures about 49 ohms on one side and 53 ohms on the other, no connection to ground, and no reading from end to end although it does make a quiet popping noise when the meter is connected across the primary ends.
Any thoughts about what the issue(s) is?
(and sorry about the length of the post! )
The amp is basically a 2204/ ax84 Hi-Octane which usually runs at close to 490Va. 5500 primary. Metering around, the DC at the rectifier output is about 330V. I took out the power tubes, measured, then disconnected the OT primary CT, measured, then disconnected the screen supply lead and measured, each still showing 330V at the rectifier output.
Measuring resistance of the filter supply caps shows ~2M and falling. The diodes in forward bias show about 2Mohm each and read as open in reverse bias. The screen resistors measure about right at 1.1K or so, and the grid resistors similarly at 2.2K. With the screen supply lead reconnected, there's about 275K ohms between the PI side of the grid resistors and pin 4 for the screens, which made me consider arcing on the socket, but even with the screen lead disconnected the problem is still there. The OT primary measures about 49 ohms on one side and 53 ohms on the other, no connection to ground, and no reading from end to end although it does make a quiet popping noise when the meter is connected across the primary ends.
Any thoughts about what the issue(s) is?
(and sorry about the length of the post! )
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