hum problem in swr basic black

[black_labb]

i'm troubleshooting a hum problem for someone in a basic black amp. schematics can be found here http://music-electronics-forum.com/t16858/

now that i've finally got my oscilloscope working i've found that the hum is first appearing after the first 12ax7 gain stage (checked after the coupling cap). i cant see any hum on the input, and pulling the tube removes the hum so it's not an issue with the circuitry around it (meaning the tuner out circuit and such). the cathode resistor and plate resistor both measure correct resistance. i've changed to a few different tubes so that isnt the issue.

the hum signal looks like a 50hz sin wave with a section inverted below the centreline. i'll try to give a better description.

it could be a groundloop problem, but i dont know how that could develop.

[pecorporation]

Has the 1M resistor drifted off-spec at the input jack?

[black_labb]

its sitting nicely at 988k. thanks for the tip though.

[black_labb]

does this issue seem like a ground loop issue?
any common faults causing grounding issues?

[gomez63]

make sure that the filter caps are good, intact and have proper connection to the trace.

[JR.junior]

i'm troubleshooting a hum problem for someone in a basic black amp. schematics can be found here http://music-electronics-forum.com/t16858/

now that i've finally got my oscilloscope working i've found that the hum is first appearing after the first 12ax7 gain stage (checked after the coupling cap). i cant see any hum on the input, and pulling the tube removes the hum so it's not an issue with the circuitry around it (meaning the tuner out circuit and such). the cathode resistor and plate resistor both measure correct resistance. i've changed to a few different tubes so that isnt the issue.

the hum signal looks like a 50hz sin wave with a section inverted below the centreline. i'll try to give a better description.

it could be a groundloop problem, but i dont know how that could develop.

hello,

did you find the source of the problem? I have exactly the same problem.

Thanks.

Kind Rgards,
JR.