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Old 21st March 2012, 03:38 PM   #11
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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.
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Old 21st March 2012, 04:49 PM   #12
WILD1 is offline WILD1  United States
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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
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Old 21st March 2012, 08:42 PM   #13
WILD1 is offline WILD1  United States
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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.

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WILD1
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Old 23rd March 2012, 03:56 AM   #14
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Default 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.
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Old 23rd March 2012, 04:46 AM   #15
WILD1 is offline WILD1  United States
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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
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Old 23rd March 2012, 04:55 AM   #16
WILD1 is offline WILD1  United States
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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.
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Old 23rd March 2012, 11:49 PM   #17
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Default 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
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Old 24th March 2012, 03:16 AM   #18
WILD1 is offline WILD1  United States
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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
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Old 20th April 2012, 04:02 AM   #19
WILD1 is offline WILD1  United States
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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
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