Hi all,
I am working on an early 70’s Ampeg SVT. This is the 6550 version. The amp has a very loud 60 cycle hum. I confirmed this with a tone generator and the 60 Hz tone was exactly the same as what the amp is producing. I have attached the service manual for the amp.
I have narrowed the source of the hum to the Power amp. I used a jumper across the molex connector pins 9 and 12 to serve as the power switch and have the preamp completely separated from the power amp. The hum is still there and quite loud. The owner said the amp hummed from the day he got it but when played loud he put up with the hum.
I installed a ¼” jack into the patch in (External amp) jack and grounded the tip to the shell and the hum does go down quite a bit but is still there. I read a thread where Enzo said doing this grounds the grid of the power amp V1 12DW7 tube and basically takes anything before it out of the system. Because of this I am suspecting that I have a ground loop issue possibly with the Patch in and out external jack connectors as I have read a few threads with this identified as the issue.
I am trying to figure out the grounding scheme for this amp. This amp does have a three prong grounded cord and the green ground wire is mounted to one of the power transformer bolts. When I look in the service manual page 4 for the power amp, it shows a resistor R53 10 ohm 0.5 watt for the patch in external amp jack. I cannot find this resistor in my amp. I loosened off the external amp jacks and let them hang loosely in the amp to see if there would be any change but this did not reduce the hum. Anyone have any thoughts or experience with this issue?
SVTHD Service Manual.pdf
I am working on an early 70’s Ampeg SVT. This is the 6550 version. The amp has a very loud 60 cycle hum. I confirmed this with a tone generator and the 60 Hz tone was exactly the same as what the amp is producing. I have attached the service manual for the amp.
I have narrowed the source of the hum to the Power amp. I used a jumper across the molex connector pins 9 and 12 to serve as the power switch and have the preamp completely separated from the power amp. The hum is still there and quite loud. The owner said the amp hummed from the day he got it but when played loud he put up with the hum.
I installed a ¼” jack into the patch in (External amp) jack and grounded the tip to the shell and the hum does go down quite a bit but is still there. I read a thread where Enzo said doing this grounds the grid of the power amp V1 12DW7 tube and basically takes anything before it out of the system. Because of this I am suspecting that I have a ground loop issue possibly with the Patch in and out external jack connectors as I have read a few threads with this identified as the issue.
I am trying to figure out the grounding scheme for this amp. This amp does have a three prong grounded cord and the green ground wire is mounted to one of the power transformer bolts. When I look in the service manual page 4 for the power amp, it shows a resistor R53 10 ohm 0.5 watt for the patch in external amp jack. I cannot find this resistor in my amp. I loosened off the external amp jacks and let them hang loosely in the amp to see if there would be any change but this did not reduce the hum. Anyone have any thoughts or experience with this issue?
SVTHD Service Manual.pdf
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