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OK, so I lend out my 69 SVT to a friend. I trust him, he treats my stuff decent and good.
But on his last gig this head was accidentally put in 8ohm when the unit needs 4. He played 1/2 the gig perfectly until he saw flames and smoke from the unit and no sound.
Upon first inspection today I noticed that one of the plate resistors (5ohm, 5W) was burned a hole into. Further I noticed some burned wires that were mostly heaters.
I replaced all 6 plate resistors with a decent replacement (5,6ohm, 5W) and replaced the heater wiring. I further noticed that the ground connection of the HUM pot was floating 22ohms higher so I disconnected the HUM pot and made a new grounding point at the chassis.
I replaced the output tubes with some test tubes I have laying around.
Now when I turn on the unit it'll make noise, the output tubes light up blue and the current over the test points rise to >200mA. I turn off the power by this.
I tested the BIAS and the BIAS follows the voltages as given on the schematic. I set the BIAS now at the lowest point and still the problem stays. When I remove the 12BH7 tubes the above does not happen. When I replace em with 12AT7's this also is kept away BUT it'll happen when I turn up the volume.
I checked most resistors and they are all fine. Diodes check out also good. I noticed some resistance on caps but I have to unsolder em first to have em checked out.
Any thoughts? I am currently at work so I can't check it now but I would love to hear some opinions on this problem. First thought was the BIAS (set or circuit) but that turns out to be fully functional. Maybe some positive feedback?
But on his last gig this head was accidentally put in 8ohm when the unit needs 4. He played 1/2 the gig perfectly until he saw flames and smoke from the unit and no sound.
Upon first inspection today I noticed that one of the plate resistors (5ohm, 5W) was burned a hole into. Further I noticed some burned wires that were mostly heaters.
I replaced all 6 plate resistors with a decent replacement (5,6ohm, 5W) and replaced the heater wiring. I further noticed that the ground connection of the HUM pot was floating 22ohms higher so I disconnected the HUM pot and made a new grounding point at the chassis.
I replaced the output tubes with some test tubes I have laying around.
Now when I turn on the unit it'll make noise, the output tubes light up blue and the current over the test points rise to >200mA. I turn off the power by this.
I tested the BIAS and the BIAS follows the voltages as given on the schematic. I set the BIAS now at the lowest point and still the problem stays. When I remove the 12BH7 tubes the above does not happen. When I replace em with 12AT7's this also is kept away BUT it'll happen when I turn up the volume.
I checked most resistors and they are all fine. Diodes check out also good. I noticed some resistance on caps but I have to unsolder em first to have em checked out.
Any thoughts? I am currently at work so I can't check it now but I would love to hear some opinions on this problem. First thought was the BIAS (set or circuit) but that turns out to be fully functional. Maybe some positive feedback?
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