Hi all, hoping someone will be able to bail me out on a guitar amp repair.
The amp is a 2009 Fender Blues JR - schematic is attached.
When the amp was brought to me, one of the EL84s had lost its vacuum so I took that set of tubes out and I put a brand new set of tubes in.
However, as soon as I turn the amp on, I can hear some mechanical hum from the PTX. There's no standby switch on this amp so, when the "on" switch is flipped, the tubes start warming up. When I flip the "on" switch, I can hear the mechanical hum from the PTX and, as the tubes warm up, I hear a hum from the speaker begin to fade in. The mechanical hum and the hum from the speaker are both at the same frequency (I'm guessing both 60ish hz?). Once the tubes warm up, the speaker hum is pretty significant with quite a bit of low-end. This hum is present even with all the knobs turned to 0.
One other thing I noticed was that, whenever I put the positive probe of my DMM on either the plate or G1 of the left power tube (facing the back of the amp), almost all of the hum goes away. If I do the same on the other power tube, the hum gets slightly louder.
Any diagnoses or words of advice are appreciated. Thanks, Reo
The amp is a 2009 Fender Blues JR - schematic is attached.
When the amp was brought to me, one of the EL84s had lost its vacuum so I took that set of tubes out and I put a brand new set of tubes in.
However, as soon as I turn the amp on, I can hear some mechanical hum from the PTX. There's no standby switch on this amp so, when the "on" switch is flipped, the tubes start warming up. When I flip the "on" switch, I can hear the mechanical hum from the PTX and, as the tubes warm up, I hear a hum from the speaker begin to fade in. The mechanical hum and the hum from the speaker are both at the same frequency (I'm guessing both 60ish hz?). Once the tubes warm up, the speaker hum is pretty significant with quite a bit of low-end. This hum is present even with all the knobs turned to 0.
One other thing I noticed was that, whenever I put the positive probe of my DMM on either the plate or G1 of the left power tube (facing the back of the amp), almost all of the hum goes away. If I do the same on the other power tube, the hum gets slightly louder.
Any diagnoses or words of advice are appreciated. Thanks, Reo
