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This Aguilar DB751 Bass Amp came to my shop from rental inventory, with explanation that it was making ‘woofy’ sounds thru the speakers. I put it up on the bench, removed the top cover for inspection, while powering it up. Connected it to my Ampeg 15” test cabinet, and switched out of Standby. Turning up the Master Volume, with the tone circuits at midpoint, and Gain not turned up, I was hearing what sounded like aggressive tube noise, and turned the master back down, switching it off.
I swapped out the output tube & the middle tube with a fresh J/J ECC83S and tried again, as that appeared to be the trouble tube. Still hearing hum, so swapped out the output tube. Hum now gone, but with the master turned up, engaging the Deep Bass switch emits explosive LF sounds that wasn’t there before. Switched back to S/B, disconnected the speaker and powered up the scope and Amber 3501a Audio Analyzer to watch at this point, and not listen thru my speaker.
Powered back up, and tapping on the middle tube yielded a lot of LF noise, as did the output tube, so powered down to see how to disassemble this. I found after the harness connectors were unplugged, front panel handles and screws removed, there were three more bottom panel screws that threaded into right angle bracket of the front panel Assembly, so disconnecting the AC mains wires from the power switch, I could remove this entire panel/PCB assy from the chassis.
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I de-soldered the Right Angle header connector pins one by one on the back side of the Preamp PCB assy, re-soldering each as I went thru the two connectors for the middle and output tubes. Cleaned the flux residue and put it back together to see if I’ve made any progress. The solder joints to begin with basically looked fine, with a lot of normal wicking of solder on the component side around each pin. Nothing really looked bad, but it seemed like a place to start. I also de-soldered and re-soldered the Master Volume pot.
I put it back into the chassis to give a listen. That seemed to have improved matters, but I was now seeing substantial noise that could be induced by touching/wiggling the input preamp tube, so I had that to do as well. Powered back down, removed the front panel assy and de-soldered/re-soldered the input tube’s right angle header. Then de-soldered and re-soldered the actual tube socket as well as the mating connections from the right-angle header to the tube socket PCB, and cleaned up all the flux from that task.
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Put it back together to give a listen. THAT now seems to have tamed the beast, no longer having the vibration-sensitivity on the input preamp tube. I had swapped out the input tube earlier, using a Mesa 12AX7A tube. I restored the Electro-Harmonix tube to the output stage, while using a J/J ECC83S in place of the EH tube for the middle tube, which was also misbehaving.
Listened to the results as well as watching them on the scope, and all that LF noise problems seems to have been cured by de-soldering/re-soldering the tube PCB headers and that of the Input tube Preamp tube socket. Amp is again quiet and well-behaved, while visually, I didn’t see anything obvious in the appearance of all those connections.
Lead-Free Solder. I did use Lead-Free solder in restoring the solder joints on the input tube, though used lead-tin solder on the three right angle headers to the main preamp tube PCB..
I swapped out the output tube & the middle tube with a fresh J/J ECC83S and tried again, as that appeared to be the trouble tube. Still hearing hum, so swapped out the output tube. Hum now gone, but with the master turned up, engaging the Deep Bass switch emits explosive LF sounds that wasn’t there before. Switched back to S/B, disconnected the speaker and powered up the scope and Amber 3501a Audio Analyzer to watch at this point, and not listen thru my speaker.
Powered back up, and tapping on the middle tube yielded a lot of LF noise, as did the output tube, so powered down to see how to disassemble this. I found after the harness connectors were unplugged, front panel handles and screws removed, there were three more bottom panel screws that threaded into right angle bracket of the front panel Assembly, so disconnecting the AC mains wires from the power switch, I could remove this entire panel/PCB assy from the chassis.
.
I de-soldered the Right Angle header connector pins one by one on the back side of the Preamp PCB assy, re-soldering each as I went thru the two connectors for the middle and output tubes. Cleaned the flux residue and put it back together to see if I’ve made any progress. The solder joints to begin with basically looked fine, with a lot of normal wicking of solder on the component side around each pin. Nothing really looked bad, but it seemed like a place to start. I also de-soldered and re-soldered the Master Volume pot.
I put it back into the chassis to give a listen. That seemed to have improved matters, but I was now seeing substantial noise that could be induced by touching/wiggling the input preamp tube, so I had that to do as well. Powered back down, removed the front panel assy and de-soldered/re-soldered the input tube’s right angle header. Then de-soldered and re-soldered the actual tube socket as well as the mating connections from the right-angle header to the tube socket PCB, and cleaned up all the flux from that task.
.
Put it back together to give a listen. THAT now seems to have tamed the beast, no longer having the vibration-sensitivity on the input preamp tube. I had swapped out the input tube earlier, using a Mesa 12AX7A tube. I restored the Electro-Harmonix tube to the output stage, while using a J/J ECC83S in place of the EH tube for the middle tube, which was also misbehaving.
Listened to the results as well as watching them on the scope, and all that LF noise problems seems to have been cured by de-soldering/re-soldering the tube PCB headers and that of the Input tube Preamp tube socket. Amp is again quiet and well-behaved, while visually, I didn’t see anything obvious in the appearance of all those connections.
Lead-Free Solder. I did use Lead-Free solder in restoring the solder joints on the input tube, though used lead-tin solder on the three right angle headers to the main preamp tube PCB..
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