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Old 20th March 2012, 11:31 PM   #1
WILD1 is offline WILD1  United States
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Default M100 Output Tube Problem!

Hi everyone,
I have been working on a set of M100 monoblocks from Audio Research with a multitude of problems. First was the voltage from the transformer due to a bad resistor, Second was another problem in the power supply with the regulator tubes which was originally due to possibly the opamps that regulate the regulator tubes and then compounded by a tech who added a zener to the opamp circuit to try and cure the problem. While doing the diagnostics on the second problem one of the output tubes V10 got really hot and very bright. Before I could turn the amp off it blew the fuse. I pulled the output tubes and then proceeded with the problem in the power supply thinking that if I fixed that problem it might solve this newest problem. I fixed the second problem today and so I re installed the output tubes. When I went to bias them V10 would not bias. I pulled that tube and put another tube in its place. Before I could bias it the same thing happened. This is getting costly. I am hoping I didn't ruin the tubes. What is going on and how should I proceed? A schematic of this amp is at ARCDB - The Audio Research Database - The unofficial & unauthorized source for ARC information Any input would be greatly appreciated.

Thank you,
WILD1

Last edited by WILD1; 21st March 2012 at 01:12 AM.
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Old 21st March 2012, 01:31 AM   #2
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It sounds like V10 is not getting bias.(assuming the amps are fixed bias)
Before inserting the output tubes,check for (negative)bias voltage on the grid pin FIRST. That will prevent you from cooking output tubes. There must be a -Voltage there,or the tube will be 'wide open' and go into runaway,like you've seen..taking fuses and other things with it.
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Old 21st March 2012, 01:54 AM   #3
WILD1 is offline WILD1  United States
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Hi
Yes the amps are fixed bias. I checked the grids and you are right. The grid I am having a problem with is pos.162 volts. The other three grids range from neg. 33 volts to 40 volts. What is causing this? How should I proceed?

Thanks for the help,
WILD1
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Old 21st March 2012, 11:16 AM   #4
WILD1 is offline WILD1  United States
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Okay I checked out the circuit of the grid for the 6550 output tube I am having problems with and compared it to another and it seems that I am losing my bias voltage at the caps connected to the grid of what ARC calls the "Direct Coupled Cathode Follower and Bias Controller" tube. I have it before the 475 ohm resistor in the bias circuit but not after. Does this mean a failure of the caps and if so which one? There are two. One is a 10nF Wondercap and the other is a 1uF ARC labeled cap underneath. Any insight would be appreciated.

Thanks,
WILD1
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Old 21st March 2012, 01:13 PM   #5
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did you try replacing the 475ohm resistor?
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Old 21st March 2012, 01:47 PM   #6
WILD1 is offline WILD1  United States
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Thanks for the reply. As for the resistor, no I have not however I have leaned towards resistor failure on another problem with this amp only to be wrong. I felt with the age of this amp [27 years] that the capacitors might be a more logical choice but as mentioned I have been wrong before. If you don't mind me asking, what makes you suspect the resistor? Is it that a bad capacitor would not effect the circuit this way or is it the position of where I described the resistor to be? I could take the resistor out of the circuit and check it however if I could rule out one or the other it would save some work, time and needless tampering with the PCB board.

Thanks again,
WILD1
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Old 21st March 2012, 01:53 PM   #7
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an electrolytic could be very well shot. I just think replacing a 10cent part would be a nice way to isolate the problem.
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Old 21st March 2012, 01:54 PM   #8
WILD1 is offline WILD1  United States
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Just another thought. Couldn't I check the voltage drop across the resistor? If it drops the total neg voltage -40 V more or less then it's bad. Right? I'm getting a headache and it is only 9:00 AM here.
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Old 21st March 2012, 02:16 PM   #9
WILD1 is offline WILD1  United States
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Checked the voltage across the resistors. With the problematic circuit I am getting a reading of 235V . On the other circuit I am getting a reading of -.04V. With my limited expertise I really do not know what to assume from this or if the readings help to define the problem. Help

Thanks,
WILD1
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Old 21st March 2012, 03:15 PM   #10
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Have you tried replacing the driver cathode follower tube? Seems that the cathode follower driving the ouput tubes might be shot, conducting too much. They're directly connected, so the CF's cathode voltage is the bias for the outputs. Awfully complicated schematic this one has...
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