Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: astronomicum on March 11, 2026, 04:28:55 pm
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Just finished a restoration of an Omega (Japanese) guitar amp from the late 50s. The amp is equipped with a shunt jack prior to the output driver circuit to redirect the preamp out (called Output). I had a persistent 9mV hum traced to this circuit. Yes, not bad, but annoying. And while rewiring the jack with shielded cable did not get rid of the hum, bypassing it did. Feeling the issue was the jack itself, or maybe more accurately, the jack position, and since removing or moving the jack was not an option, I went looking for a shielded shunted jack. Well, you probably know how that turned out.
So I decided to see if I could put one together. I found some good quality shunted square body jacks on E-bay, and out of aluminum flashing, razor blade, Dremel, and micrometers, constructed a shield. To my delight, after installing the jack, the amp now has only 3mVs of hum.
Something to keep in mind for the toolbox.
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Very nice work. :icon_biggrin:
Switchcraft used to make/sell a shielded 1/4" jack.
It was in their catalog.
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Very nice work. :icon_biggrin:
Switchcraft used to make/sell a shielded 1/4" jack.
It was in their catalog.
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So have you concluded that it was the location of the jack that was too close to some component that was causing the hum, or some other reason? If it was the location, do you have any idea what was going on so as to create the hum?
Thanks. Pictures of before/after would be great if possible.
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So have you concluded that it was the location of the jack that was too close to some component that was causing the hum, or some other reason? If it was the location, do you have any idea what was going on so as to create the hum?
Thanks. Pictures of before/after would be great if possible.
The jack location is the likely cause. It is most likely due to its close proximity to the main power supply capacitor. I did not do full power up testing before restoration so it is unknown if there was hum from the original layout.
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Yea, maybe so. It looks like the main can filter cap is over on the left side right under what I assume are the input jacks. Strange place for it. I have seen them normally near the power switch and PT, not all the way over to the other side by the input jacks and pre-amp circuit.
Glad it's working better for you. Your "remodel" looks really well done.
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In the original layout, the main B+ cap is on the right just above the rectifier. The B2 screen cap is just visible in the middle behind the black 200 ohm resistor. The caps on the left are for the B3 and B4 nodes.
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It looks like the main can filter cap is over on the left side right under what I assume are the input jacks.
What you thought might be the Cap Can is actually the OT.