Amp is a 2002 AC30TB.
I have a 60HZ hum/buzz that I am having trouble tracking down.
Confirmed 60HZ by measuring frequency on a scope.
With the volume at half or noon, and a guitar plugged in, there is 330mV AC of hum at speaker out with me close to the amp. If I move the guitar about 1.5m away, this quiets down to 13mV but it is still audible and annoying.
The hum is largely controlled by the normal channel volume control, but also by the brilliant control to a slightly lesser degree. The Vib channel volume does not effect the hum. If the cut control is all the way clockwise it can filter out most of it. It can become more apparent with the treble at 10(max).
Removing all preamp tubes except for the PI tube, the hum is still present. When the PI tube is pulled, the hum disappears. I have tried swapping a fresh matched quartet of EL84's with no difference.
Scoping the PI tube, hum is present on the plate of both sides, pins 6 and 1. I don't see it on the grid of either side. There is no AC signal there).
At C45 reservoir cap I have 12.5VAC, is this a bit high?
At the junction of the choke and C46 filter cap I have average 200mvAC, it is fluctuating between 150 and 400mv.
B+ around 350vDC, all power tubes have correct voltages.
I got this amp in non-functioning and neglected condition. After bringing it back to stock condition, it was very noisy. After some troubleshooting and research I realized it was just the way these amps were from the factory, but it was still not an acceptable level of noise.
I performed a couple noise/reliability modifications developed by Lyle Caldwell (shown in this video) that essentially involved:
I've attached the schematic.
Any help in tracking this down is appreciated.
I have a 60HZ hum/buzz that I am having trouble tracking down.
Confirmed 60HZ by measuring frequency on a scope.
With the volume at half or noon, and a guitar plugged in, there is 330mV AC of hum at speaker out with me close to the amp. If I move the guitar about 1.5m away, this quiets down to 13mV but it is still audible and annoying.
The hum is largely controlled by the normal channel volume control, but also by the brilliant control to a slightly lesser degree. The Vib channel volume does not effect the hum. If the cut control is all the way clockwise it can filter out most of it. It can become more apparent with the treble at 10(max).
Removing all preamp tubes except for the PI tube, the hum is still present. When the PI tube is pulled, the hum disappears. I have tried swapping a fresh matched quartet of EL84's with no difference.
Scoping the PI tube, hum is present on the plate of both sides, pins 6 and 1. I don't see it on the grid of either side. There is no AC signal there).
At C45 reservoir cap I have 12.5VAC, is this a bit high?
At the junction of the choke and C46 filter cap I have average 200mvAC, it is fluctuating between 150 and 400mv.
B+ around 350vDC, all power tubes have correct voltages.
I got this amp in non-functioning and neglected condition. After bringing it back to stock condition, it was very noisy. After some troubleshooting and research I realized it was just the way these amps were from the factory, but it was still not an acceptable level of noise.
I performed a couple noise/reliability modifications developed by Lyle Caldwell (shown in this video) that essentially involved:
- Cutting the ground trace on the PCB and modifying into 3 separate ground sections (1 output + filter caps, 1 for input section (connected to chassis near inputs) and 1 for reservoir cap.
- Deleting power tube heater trace on PCB, running standalone power tube heater wiring, as well as new preamp tube heater wiring with tighter twists
- Disconnecting heater centre tap from ground, and installing an elevated artificial centre tap referencing output cathode (I later tried reversing this mod to see if grounding the stock transformer CT helped get rid of buzz - it didn't)
- Running shielded cable from input to grid of first preamp tube
I've attached the schematic.
Any help in tracking this down is appreciated.
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