samgraysound
Well-known member
Seems like I'm always here asking about hum. Here we go again.
Supro Sahara tube Amp. It's a Zinky design based on a 1958 Supro design (so I have read on random forums). I don't have a schematic. I contacted Supro and Zinky Electronics for one and waiting to hear back. Two 12ax7 and two EL84s.
It has a significant 60hz hum with a source plugged into it (in this case my oscillator). No hum without a source. This hum is entirely controlled by the volume pot.
I have:
Swapped all tubes
Checked for ripple on the B+. Negligible ripple, all filter caps are good.
Checked the ground reference for the filament supply. It is referenced through a CT. Continuity and voltage checks are good.
Checked the ground connection on the input jack.
Checked the ground connection from the AC power cord, and transformer chassis connection.
Checked all ground connections on the preamp PCB
The internal signal routing is all in shielded cable and I've checked the ground connections for all the shields.
Suggestions?
Supro Sahara tube Amp. It's a Zinky design based on a 1958 Supro design (so I have read on random forums). I don't have a schematic. I contacted Supro and Zinky Electronics for one and waiting to hear back. Two 12ax7 and two EL84s.
It has a significant 60hz hum with a source plugged into it (in this case my oscillator). No hum without a source. This hum is entirely controlled by the volume pot.
I have:
Swapped all tubes
Checked for ripple on the B+. Negligible ripple, all filter caps are good.
Checked the ground reference for the filament supply. It is referenced through a CT. Continuity and voltage checks are good.
Checked the ground connection on the input jack.
Checked the ground connection from the AC power cord, and transformer chassis connection.
Checked all ground connections on the preamp PCB
The internal signal routing is all in shielded cable and I've checked the ground connections for all the shields.
Suggestions?