I'm having trouble tracking down a noise in my 68 JMP50.
When I got this amp a couple of years ago it was in need of some repair - all 1M pots were completely shot and it had the wrong value bass pot and the tone stack wrongly wired. Power transformer is a Marstran replacement, but everything else except the 1M pots and the power transformer is original. I added 5.6k output grid stoppers and 1k screen grid resistors.
After getting it back to stock it has sounded just glorious, until a weird static noise started to ride on the trails of the notes. It *may* have been there all the while, but I don't think so. I've added a video clip below which hopefully demonstrates it, but it's not easy to hear clearly in the recording as the note itself overpowers the buzz, but it's very clearly there. There's a more low pitched "murmur" at the attack of the note, followed by a more buzzing, static sort of sound on the trail and sustain of the note.
What's interesting is this;
- if I have an attenuator in line between the amp and the cab the noise goes away
- if I disconnect the purple feedback wire from the output jack it goes away. Doesn't matter which output (4,8 or 16 ohm) it's connected to.
- the noise is most evident with presence at 0 and almost entirely gone with presence at 10.
From this I deduce that there is something present on the output which feeds back into the phase inverter. That "something" only presents itself when it is being fed into the phase inverter again. If an attenuator is present, that "something" gets shunted to ground. Same thing when it passes through the presence control - shunting high frequencies from the negative feedback loop to ground by turning up the presence control prevents them from entering into the phase inverter to be amplified again.
I have eliminated the usual suspects; tried known good preamp and power tubes. I have subbed in known good and working potentiometers (all except the middle one, but I am looking harder at the output stage now). I have also eliminated the preamp filter cap and the phase inverter/screens filter cap by subbing in new ones. I also removed the output tube grid stoppers just to be sure. No effect whatsoever.
What next? I am running out of suggestions. I probably need to also try subbing in a different mains filter cap, but it tests just fine and there's no hum or any other problems. I would sort of expect a problem with a mains filter cap to also present itself with the negative feedback loop disengaged.
I would love to hear your suggestions guys. Hope you can get something out of this video:
When I got this amp a couple of years ago it was in need of some repair - all 1M pots were completely shot and it had the wrong value bass pot and the tone stack wrongly wired. Power transformer is a Marstran replacement, but everything else except the 1M pots and the power transformer is original. I added 5.6k output grid stoppers and 1k screen grid resistors.
After getting it back to stock it has sounded just glorious, until a weird static noise started to ride on the trails of the notes. It *may* have been there all the while, but I don't think so. I've added a video clip below which hopefully demonstrates it, but it's not easy to hear clearly in the recording as the note itself overpowers the buzz, but it's very clearly there. There's a more low pitched "murmur" at the attack of the note, followed by a more buzzing, static sort of sound on the trail and sustain of the note.
What's interesting is this;
- if I have an attenuator in line between the amp and the cab the noise goes away
- if I disconnect the purple feedback wire from the output jack it goes away. Doesn't matter which output (4,8 or 16 ohm) it's connected to.
- the noise is most evident with presence at 0 and almost entirely gone with presence at 10.
From this I deduce that there is something present on the output which feeds back into the phase inverter. That "something" only presents itself when it is being fed into the phase inverter again. If an attenuator is present, that "something" gets shunted to ground. Same thing when it passes through the presence control - shunting high frequencies from the negative feedback loop to ground by turning up the presence control prevents them from entering into the phase inverter to be amplified again.
I have eliminated the usual suspects; tried known good preamp and power tubes. I have subbed in known good and working potentiometers (all except the middle one, but I am looking harder at the output stage now). I have also eliminated the preamp filter cap and the phase inverter/screens filter cap by subbing in new ones. I also removed the output tube grid stoppers just to be sure. No effect whatsoever.
What next? I am running out of suggestions. I probably need to also try subbing in a different mains filter cap, but it tests just fine and there's no hum or any other problems. I would sort of expect a problem with a mains filter cap to also present itself with the negative feedback loop disengaged.
I would love to hear your suggestions guys. Hope you can get something out of this video:
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