Marshall Superbass Mk2 100W
& Matching 4 x 12 Cab Service & Repair
Supercool Superbass & 4 x 12 Cabinet
This is a 1992 model from 1979 which was having a few problems. It came with a matching 4 x 12 slanted cab equipped with a quad of Celestion G12-80s. The speaker socket was broken and the wiring botched so it wasn't reading the correct Ohms.
The Amp
The amp needed a new quad of EL34 to replace the odd set of different brands, and a deep service. Al the components are original and not one is out of spec! The owner didn't want a recap as the amp didn't hum and they all looked and measured good. He was going to buy a new power switch because it didn't light up. I persuaded him not to because I can fix them.
All the valve sockets were re-tensioned and cleaned. Out of interest I measured the valves emissions on my tester. Three were a bit weak and the Mullard was very strong but unfortunately microphonic. They all pulled different currents.
I usually set the bias with my home built probes and two multimeters. Rob Robinets Tube Bias Calculater is an excellent resource here. I like it because you can recalculate as the wall voltage fluctuates and set it to peak voltage.
The pre-amp valves all tested good but were replaced in the end because they squeaked, spat and sizzled a bit too often.
Anyway, Barrington was about to come over for his gear when I discovered that it sounded way too quiet when playing through the 4 x 12 downstairs. I was baffled because it scoped out really well on the bench and sounded great through my lab speaker, a Marshall 2 x 12. So it was carried back up 2 flights and the chassis taken out for another look.
I was worried the OT might be damaged. However, it was the impedance selector that was at fault. I really should have checked this earlier but was working to meat a deadline and no problem was indicated. With a speaker lead plugged in, I couldn't get a steady reading, even when pressing on the selector pins. They thing was dismantled, cleaned and squashed back a bit and now it's as good as new. Many techs just hard wire them but then you can't change cabs easily.
In view of this problem, I serviced the mains voltage selector in the same way. A final set of voltage measurements, a bias check, and an output power test into a 16 Ohm dummy load to be absolutely sure.
The Cab
I rewired the speakers, fixed the broken socket and turned the back upside down so it wouldn't get kicked and break again. I was about to plug in an amp to test it when I remembered to check the resistance with a multimeter. It measured about 1.3 Ohm! For my stupidity I was rewarded with wrestling the cab back onto the table, unscrewing the back and removing the jack socket again.
This time I took readings from all speakers until I got to one that read ~45 Ohm. Scratching my head, I unsoldered the connecting wires and found it to be open coil. Luckily Barrington managed to find one and it was with me in a couple of days. Meanwhile I fought to remove the speaker bolts which were totally seized. I managed to get three out with mole grips and cut one of their heads off with a Dremill. The penetrating oil hadn't worked. Finally, with the new driver installed, all the baffle screws were tightened, the sound post glued back in. I like to do the best job I can, so I found a few extra screws and a set of cup washers to put the back on properly. The cab is back to 15 Ohms and sounding great again.
Barrington is very pleased with the pair and the quick turn around for his new riff-based recording project!
The amp needed a new quad of EL34 to replace the odd set of different brands, and a deep service. Al the components are original and not one is out of spec! The owner didn't want a recap as the amp didn't hum and they all looked and measured good. He was going to buy a new power switch because it didn't light up. I persuaded him not to because I can fix them.
All the valve sockets were re-tensioned and cleaned. Out of interest I measured the valves emissions on my tester. Three were a bit weak and the Mullard was very strong but unfortunately microphonic. They all pulled different currents.
I usually set the bias with my home built probes and two multimeters. Rob Robinets Tube Bias Calculater is an excellent resource here. I like it because you can recalculate as the wall voltage fluctuates and set it to peak voltage.
The pre-amp valves all tested good but were replaced in the end because they squeaked, spat and sizzled a bit too often.
Anyway, Barrington was about to come over for his gear when I discovered that it sounded way too quiet when playing through the 4 x 12 downstairs. I was baffled because it scoped out really well on the bench and sounded great through my lab speaker, a Marshall 2 x 12. So it was carried back up 2 flights and the chassis taken out for another look.
I was worried the OT might be damaged. However, it was the impedance selector that was at fault. I really should have checked this earlier but was working to meat a deadline and no problem was indicated. With a speaker lead plugged in, I couldn't get a steady reading, even when pressing on the selector pins. They thing was dismantled, cleaned and squashed back a bit and now it's as good as new. Many techs just hard wire them but then you can't change cabs easily.
In view of this problem, I serviced the mains voltage selector in the same way. A final set of voltage measurements, a bias check, and an output power test into a 16 Ohm dummy load to be absolutely sure.
The Cab
I rewired the speakers, fixed the broken socket and turned the back upside down so it wouldn't get kicked and break again. I was about to plug in an amp to test it when I remembered to check the resistance with a multimeter. It measured about 1.3 Ohm! For my stupidity I was rewarded with wrestling the cab back onto the table, unscrewing the back and removing the jack socket again.
This time I took readings from all speakers until I got to one that read ~45 Ohm. Scratching my head, I unsoldered the connecting wires and found it to be open coil. Luckily Barrington managed to find one and it was with me in a couple of days. Meanwhile I fought to remove the speaker bolts which were totally seized. I managed to get three out with mole grips and cut one of their heads off with a Dremill. The penetrating oil hadn't worked. Finally, with the new driver installed, all the baffle screws were tightened, the sound post glued back in. I like to do the best job I can, so I found a few extra screws and a set of cup washers to put the back on properly. The cab is back to 15 Ohms and sounding great again.
Barrington is very pleased with the pair and the quick turn around for his new riff-based recording project!
