Hi
I am working on this amp. It had 3 of the pre amp tubes(ECC83) dead. Upon checking, the bridge rectifier that provides the filament voltage has open. I replaced it as well as R133(470K) and R33(10 ohms) which are blown. Now when I turn on the power switch, after a few seconds there is sound. Strange because the standby switch is still in OFF position. I have checked the switch and it is OK. The voltage at W9 is at 460V, but the voltage at W10 (which connects to W8, the center tap of output transformer), with the standby OFF is at 380V. This 380V going to the OT is what causing sound at the speaker with the standy OFF. When I turn ON the standby switch, then the voltage at W10 goes up to 460V. However the sound is much softer and distorted. I have yet to check the bias and tube dissipation, which I will do tomorrow.
Thanks
I am working on this amp. It had 3 of the pre amp tubes(ECC83) dead. Upon checking, the bridge rectifier that provides the filament voltage has open. I replaced it as well as R133(470K) and R33(10 ohms) which are blown. Now when I turn on the power switch, after a few seconds there is sound. Strange because the standby switch is still in OFF position. I have checked the switch and it is OK. The voltage at W9 is at 460V, but the voltage at W10 (which connects to W8, the center tap of output transformer), with the standby OFF is at 380V. This 380V going to the OT is what causing sound at the speaker with the standy OFF. When I turn ON the standby switch, then the voltage at W10 goes up to 460V. However the sound is much softer and distorted. I have yet to check the bias and tube dissipation, which I will do tomorrow.
Thanks
Check C9. It looks like it is leaky. This would solve the standby switch problem. Is W10 just a connector?
I'm not an amp tech, but I do have that amp. I had a similar BR heater failure issue a couple of years ago.
I don't know exactly what would cause as you describe. But the heat issues that affect the BR can cause cracked or failed joints anywhere. To get mine working again, in addition to a new BR (with longer legs and a bolted-on heat sink), I spent a careful evening reflowing every solder joint across the main board.
I don't know exactly what would cause as you describe. But the heat issues that affect the BR can cause cracked or failed joints anywhere. To get mine working again, in addition to a new BR (with longer legs and a bolted-on heat sink), I spent a careful evening reflowing every solder joint across the main board.
Greetings,
Depending on the year of mfg of that amp, there were differing problems with the PC board layout, component mounting and values. The filament traces were not wide enough on the early boards and the rectifier was mounted flat on the board with no air flow. There were issues with trace layout and bias instability in others. If you search that model you should be able to figure out what manufacturing issues your specific amp may have from.The optimum fix if your amp falls into that period is to replace the PC board. It is available to registered shops and is complete except for tubes and fuses.
Read a bit here to start:
Marshall DSL 401 ''overheating'' question... | The Gear Page
The last one we did, we also installed a higher current filament rectifier up in the air on a support. That and re-soldering all the filament traces may be enough depending on the specific amp. I think I have some photos that show this, I'll try to find them and post a link.
Keep in mind there could be several simultaneous problems confusing troubleshooting. Good luck!
Regards, Jim
Depending on the year of mfg of that amp, there were differing problems with the PC board layout, component mounting and values. The filament traces were not wide enough on the early boards and the rectifier was mounted flat on the board with no air flow. There were issues with trace layout and bias instability in others. If you search that model you should be able to figure out what manufacturing issues your specific amp may have from.The optimum fix if your amp falls into that period is to replace the PC board. It is available to registered shops and is complete except for tubes and fuses.
Read a bit here to start:
Marshall DSL 401 ''overheating'' question... | The Gear Page
The last one we did, we also installed a higher current filament rectifier up in the air on a support. That and re-soldering all the filament traces may be enough depending on the specific amp. I think I have some photos that show this, I'll try to find them and post a link.
Keep in mind there could be several simultaneous problems confusing troubleshooting. Good luck!
Regards, Jim
W10 and W9 are connector pins between the standy switch. I have replaced C19(0.22uf) but still the same. With the standby OFF, there is sound but distorted. With standby ON it sound normal.
According to the schematic bias between pin 2(gnd) and pin 1 or 3 should be 1.35V to 1.4V, but the maximum reading I can get is 0.975V with PR1 fully CCW. I dare not set to max, so I set it at approx 0.8V for both sides. . With this setting I get 40mA on one side and 29mA on the other at a Plate voltage of approx 422V. Is this safe. The EL84's are Chinese Chuguang.
Thanks
I resoldered the whole board, and now the standby problem is solved. I would like to know what is the bias setting for this amp. I have set the bias at 600mv but the amp gets very hot. Can setting the bias lower reduce the heating. What should be the best bias setting for this amp.
Thanks
Thanks
600mV in 10R R33 is 60mA for four tubes, 15mA per tube. Assuming <400V supply, that's 6 Watts per tube. COLD.
Do you know how hot a four-EL84 amp normally runs? HOT.
Do you know how hot a four-EL84 amp normally runs? HOT.
Marshall used to spec 1350mV for the bias, then after
2003 changed to 550mV. The changes to the circuit were only a couple of resistors to allow this value to be set, nothing that really changed whether or not either of these settings was a good setting.
This amp just does run hot, and it suffers from it. I built fans into my 2005 model, I bias at about 750 to 800 mV, it sounds good and stays cool enough.
2003 changed to 550mV. The changes to the circuit were only a couple of resistors to allow this value to be set, nothing that really changed whether or not either of these settings was a good setting.
This amp just does run hot, and it suffers from it. I built fans into my 2005 model, I bias at about 750 to 800 mV, it sounds good and stays cool enough.