JJman
01-19-2008, 01:49 PM
http://www.schematicheaven.com/ampegamps/m12_6v6.pdf
I thought I was done working on this amp but it develops 60hz hum after a few minutes. This hum is present with the volumes all the way down, with the inputs shorted and with the preamp tube out. With the PI tube out there is zero hum. The preamps and PI input grid now have shielded wires on them. The pots are grounded and functioning well.
If I ground the input grid wire of the PI, on either end of the wire, the hum disappears. I tried rotating the 3 6SL7s and some also have some “buzz” but they all have the 60hz hum (confirmed with scope as 60hz.)
Grounding the volume pot wipers does nothing to reduce the hum. (They are already grounded when turned down and functioning correctly of course.) I’m doing all this testing with the preamp tube out to help isolate the hum. Same problem when it’s in. The grid wire is not near nor parellel to the new twisted heater wires.
So are all 3 6SL7s bad? Why does grounding the PI input grid stop the PI tube from sending out 60hz hum to the 6v6s? It’s already referenced to ground thru the 2-270k channel junction resistors (confirmed as ~140k measured to ground.) Do all 6SL7s do this? The hum is not huge but it’s too much. I think the specs on these are similar to 12ax7s but in the large size and 6v heaters. Is this all about the non-HBing type filaments? They are Sovteks and about a year old.
I thought I was done working on this amp but it develops 60hz hum after a few minutes. This hum is present with the volumes all the way down, with the inputs shorted and with the preamp tube out. With the PI tube out there is zero hum. The preamps and PI input grid now have shielded wires on them. The pots are grounded and functioning well.
If I ground the input grid wire of the PI, on either end of the wire, the hum disappears. I tried rotating the 3 6SL7s and some also have some “buzz” but they all have the 60hz hum (confirmed with scope as 60hz.)
Grounding the volume pot wipers does nothing to reduce the hum. (They are already grounded when turned down and functioning correctly of course.) I’m doing all this testing with the preamp tube out to help isolate the hum. Same problem when it’s in. The grid wire is not near nor parellel to the new twisted heater wires.
So are all 3 6SL7s bad? Why does grounding the PI input grid stop the PI tube from sending out 60hz hum to the 6v6s? It’s already referenced to ground thru the 2-270k channel junction resistors (confirmed as ~140k measured to ground.) Do all 6SL7s do this? The hum is not huge but it’s too much. I think the specs on these are similar to 12ax7s but in the large size and 6v heaters. Is this all about the non-HBing type filaments? They are Sovteks and about a year old.