M100 Output Tube Problem!

[WILD1]

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

 

[DigitalJunkie]

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.

 

[WILD1]

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

 

[WILD1]

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

 

[Alexontherocks]

did you try replacing the 475ohm resistor?

 

[WILD1]

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

 

[Alexontherocks]

an electrolytic could be very well shot. I just think replacing a 10cent part would be a nice way to isolate the problem.

 

[WILD1]

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.

 

[WILD1]

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

 

[Adolf Corkscrew]

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...

 

[speakerfritz]

your getting a lot of good info. I want to point out that sometimes when you have tubes that won't bias, often the tube in question is drawing too much bias current and the supporting circut can't adjust to bring it into range. If you have a bias probe and a tube tester, you can test all your power tubes and at the same time, use the bias probe during the tube test to base line the tubes Ip while in the tube testers. so if you have 4 tubes, and they have a gm of 11k, 11k, 8k, 14k and the bias probe shows 28.1, 28.1, 22.1, 31.1 the last tube could be too hot for your amp. case in point, I have 5 manley amps and if an Ip test while on my tube tester comes back at more than 31ma's the tube will not be useable since my bias pots will not be able to bring it down below .35ma in the actual amp circut. I replace the tube. Someone else may be able to use the tube if their amps require hotter running tubes.

 

[WILD1]

Hey Guys,
Yes once I narrowed it down to a problem with the cathode follower [7044] that was the first thing I did was swap tubes but to no avail. As far as it being a problem with the output tube, with the original tube that was in there [ KT88] I thought possibly the same thing that it was a poor match but with the second tube [6550] they were a closely matched set. Plus this is not something that slowly happens. No sooner than I put that second tube in and turned the amp on it got red hot and blew the fuse. Also this positive voltage at the cathode follower is happening irregardless of the output tube being in. I am still leaning towards the grid on the cathode follower however I don't want to mess with the PCB board needlessly. The board is very thick and it is hard to clear the holes of solder without heating things up pretty good. I have a finer tip for my solder station ordered and I will probably wait for any installations until then. I am just hoping that with the diagnostics I've performed it will alleviate unnecessary tampering. If anyone has a thought of whether it would be the resistor or capacitors please call.

Thanks,
WILD1

 

[WILD1]

Okay,I have been reading about coupling capacitors and I think I am right about the problem being due to a faulty capacitor. "In analog circuits, a coupling capacitor is used to connect two circuits such that only the AC signal from the first circuit can pass through to the next while DC is blocked. This technique helps to isolate the DC bias settings of the two coupled circuits." This is from Wikipedia. So if the capacitor failed it would allow the +DC current across it overrunning the smaller -DC current and you would show a + voltage at the cathode followers grid which is what I have. If it was the resistor that failed you still wouldn't see a + voltage at the grid. Maybe a minute amount due to leakage. Would one of you electronic gurus tell me whether I am right or explain to me why my reasoning is wrong so I can quit thinking about it.

Thanks,
WILD1

 

[peterpullar]

27K Driver tube cathode resistors

R64, R65, R66 and R67 can go open circuit and stop the negative voltage from biasing the output tubes. I am looking at an M100 where all four have failed.

 

[WILD1]

Yes I think I can see that, but would you get a positive voltage at the grid of the cathode follower in that instance? It will be interesting to see what the culprit is but it is going to have to wait. Spring has sprung and I am busier than a one armed paper hanger. Appreciate the input and I will definitely keep you posted on what I find.

Thanks,
WILD1

 

[WILD1]

Just another thought I guess I could pull the cathode follower and see if the -voltage you are talking about is at the cathode or for that matter turn the amp off pull the tube which would take those resistors out of circuit and check them. Maybe tomorrow.

 

[peterpullar]

You don't have to pull the resistors out to check them.

After the amplifier has cooled down and the electrolytics are discharged, the resistors can easily be checked without removing them. Interestingly 1 tube had + volts and 2 tubes had a low negative bias with the resistors open circuit. After replacing the resistors, all were stable consistent negative voltage adjustable with the bias pots

 

[WILD1]

Thanks for the info Peter. I would love to talk more but it is 10:00 , I just came in from work and I am Dog tired. I am definitely going to check those resistors out. Not this weekend. Too busy. Seems I have 48 people wondering who the heck is this Yahoo. I finally got around to filling out the profile.

Hast la manana,
Tom Wild

 

[WILD1]

Got it! Yes it was the capacitors. But due to my excessive personality [my self proclaimed nickname is "Captain Excess"] I wasn't happy just changing out the bad one, I had to change all the coupling caps. Not in just the one amp but both. The good one is the one I am working on now and as well I am replacing the Op amps as I did in the sick one to cure a problem in the power supply. Nice thing about them being Mono Blocks is that I at least have one to listen to while I am working on the other. Can't wait to here them in stereo! Oh yes it is the trials and tribulations of life that make it so damn interesting.

Hasta un otro ves y gracias,
Captain Excess