I've ordered a fender tweed princeton kit from mojotone and I've finished building it. But after my first completion, I've found that the amp started to hum loud 10 or so seconds after turning it on. It was a kind of swelling hum that persisted. I later found out that I haven't properly installed the preamp tube and miswired the preamp tube socket. However, even after fixing the preamp tube section, the hum didn't go away but got even worse. It started to make a crazy squeaking sound as if it were to explode right away. I've checked my wiring and it seems normal but I still get the sound. So I recorded it and found that the hum was a 120hz one. Other than that I can't get a grasp of what the problem is. One thing that gets in my mind is that I've seen comments on the mojotne website saying that the provided diagram has the output transformer wire in the opposite direction and before fixing it themselves, they had positve feedback from the amp. But I'm unsure what the problem with my amp could be. I'll post a recording of the symptoms, and if anyone knows what the problem might be, please leave a comment. Thank you.
Sincerely
newly built amp humming and squealing like crazy
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newly built amp humming and squealing like crazy
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Stevem
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Re: newly built amp humming and squealing like crazy
First you need to narrow down where it's coming from, I.E. what stage of the amp.
Remove all the preamp tubes, is the issue still there?
If not then reinstall the preamp tubes one at a time starting with the one nearest the output tubes until the issue starts again.
Report back with your findings.
Remove all the preamp tubes, is the issue still there?
If not then reinstall the preamp tubes one at a time starting with the one nearest the output tubes until the issue starts again.
Report back with your findings.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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martin manning
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Re: newly built amp humming and squealing like crazy
The quickest way to test this theory is to temporarily disconnect the feedback wire, which is the one going from the speaker output to the 22k resistor on the board. If the squeal stops, then you should reverse the blue and red output transformer primary leads.
Re: newly built amp humming and squealing like crazy
Wow that's a good idea! I've discussed with a few other people and I think I narrowed it down to the NFB circuit - output transformer wiring issue. I'm not sure but I will definitely check it out.
Re: newly built amp humming and squealing like crazy
UPDATE
Hello folks,
So I reversed the OT wires and stopped the oscillation that happened. However, even after getting rid of the feedback I still got a loud hum 10 seconds after turning on the amp. Along with the hum, I got essentially no guitar sound until I literally had to crank the volume knob to get a barely audible guitar signal. The volume of the hum varied with the sweep of the volume control, but in a funny way. When the volume knob is at 0, the hum is moderately loud. But if I turn it up slowly, the hum would suddenly decrease around 5 or 6. Then if I push it more, it would go so loud at about 9 or 10. To fix this issue, I got my multimeter and inspected the voltages on each components. Weird thing is while touching the circuit with the probe, it made crackling noise and gave a sudden change in volume as i nudged the eyelet board a little. First the resistors near the filter caps (circled red) showed a healthy 334 volts. But the issue is the other resistors (circled blue) showed 0V. Because I don't know much about inspecting an amp, I just decided to measure the preamp and power tube socket pins. One of the power tube pins gave me a 20~24V reading. The real issue is the preamp tube pins gave me no voltages then it suddenly made a huge spark and the amp went totally silent. I immediately turned the amp off. Does anyone no what the issue might be? I'm really frustrated and honestly after experiencing the spark I'm scared. I assume it's the preamp side that's the issue but I have no idea.
Hello folks,
So I reversed the OT wires and stopped the oscillation that happened. However, even after getting rid of the feedback I still got a loud hum 10 seconds after turning on the amp. Along with the hum, I got essentially no guitar sound until I literally had to crank the volume knob to get a barely audible guitar signal. The volume of the hum varied with the sweep of the volume control, but in a funny way. When the volume knob is at 0, the hum is moderately loud. But if I turn it up slowly, the hum would suddenly decrease around 5 or 6. Then if I push it more, it would go so loud at about 9 or 10. To fix this issue, I got my multimeter and inspected the voltages on each components. Weird thing is while touching the circuit with the probe, it made crackling noise and gave a sudden change in volume as i nudged the eyelet board a little. First the resistors near the filter caps (circled red) showed a healthy 334 volts. But the issue is the other resistors (circled blue) showed 0V. Because I don't know much about inspecting an amp, I just decided to measure the preamp and power tube socket pins. One of the power tube pins gave me a 20~24V reading. The real issue is the preamp tube pins gave me no voltages then it suddenly made a huge spark and the amp went totally silent. I immediately turned the amp off. Does anyone no what the issue might be? I'm really frustrated and honestly after experiencing the spark I'm scared. I assume it's the preamp side that's the issue but I have no idea.
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Re: newly built amp humming and squealing like crazy
Do you have the ground on the back of the Volume control ? Check all the points on the eyelet board that are supposed to be grounded measure as close to zero Ohms to the chassis as you meter will measure. Check any wires that are supposed to go under the eyelet board are going to the correct eyelet with you Ohm meter.