Tweet
The 70 SF Twin reverb I'm slowly restoring has a high amount of white noise and a little hum on the vibrato chan. I'm having trouble fixing it. To isolate it, I pulled the normal chan tube and grounded pin 7 of the vib chan. No Noise at all. I took that gnd off and grounded pin 2 - lots of noise... What's in between is the 1st half's cathode circuit which I checked and re-soldered and the plate circuit which includes the tone stack. It certainly seems like poor grounding but nothing I've tried has helped. The pre-amps power supply cap is tied to the brass plate at the normal chan soldered gnd. The 1st pwr supply cap goes to a chassis lug near the pt. The vib chan gnd including vol pot and mid pot are tied to the cathode gnd of the vib chan and a separate wire goes to the normal chan gnd. All the pots are secured to the brass plate with the grip/tooth washers. I separated the vol and mid pot gnds which were tied together at the vol pot into separate wires - no change. I did not try soldering the pot gnds to the pot caseing. I'm also thinking about taking all the gnds (1 wire each) back to the pwr supply gnd. Any thoughts or advice would be much appreciated.