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    Thermal event observed, need help

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    j beede

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    Re: Thermal event observed, need help

    Post by j beede on Sat May 13, 2017 6:38 pm

    Three that you can see, and perhaps several that you can't. Reflow.
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    deepee99

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    Re: Thermal event observed, need help

    Post by deepee99 on Sat May 13, 2017 7:18 pm

    j beede wrote:Three that you can see, and perhaps several that you can't. Reflow.
    Yes, and that task should be done again after a year's operating time and the amps have heated and cooled enough to have fully settled in. Then you should be down to monthly bias checks and replacing the odd worn-out tube. Again, a second, younger pair of eyes . . .
    I'd still look into a chat-up with Audio Bill.
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    LeGrace

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    Re: Thermal event observed, need help

    Post by LeGrace on Sun May 14, 2017 11:10 am

    Slowly going through the amp. Magnifying glass helps a lot. One discrepancy I've uncovered from the wiring schematic are the 4 resistors connecting to pin 3 of each power socket. According to the documentation these should be 100 ohm. Except the ones I have are 50 ohms according to my meter. Other resistor values match up OK to the schematic. I assume this was intentional and is OK but just want to confirm.

    nmchiefsfan

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    Re: Thermal event observed, need help

    Post by nmchiefsfan on Sun May 14, 2017 4:33 pm

    Is there a color code on the resisters? 50 ohms means you would be biasing them at a current twice as high as you think you are.

    nmchiefsfan

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    Re: Thermal event observed, need help

    Post by nmchiefsfan on Sun May 14, 2017 4:37 pm

    ^^^ Woops...ignore that post. I don't know why I had 100 ohm bias resisters on the brain. 50 ohms would mean you are biasing them cooler, not hotter. Sorry. I would delete the post but I don't know how. Still, see if the color code matches up to the measured resistance.
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    LeGrace

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    Re: Thermal event observed, need help

    Post by LeGrace on Sun May 14, 2017 5:20 pm

    I'm confident in my meter. All other resistors measurements match up perfectly with the schematic. I was worried a lower resistance meant hotter for the rectifier, but you're saying it works the opposite.

    Q for the experts. What causes red plating? Definitely different from the flash overs that took out my rectifiers previously.
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    LeGrace

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    Re: Thermal event observed, need help

    Post by LeGrace on Sun May 14, 2017 6:03 pm

    I've been googling red plating. Should those of us with 50 ohm M125's be running a higher bias voltage setting?
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    Bob Latino
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    Re: Thermal event observed, need help

    Post by Bob Latino on Sun May 14, 2017 6:45 pm

    LeGrace wrote:Slowly going through the amp. Magnifying glass helps a lot. One discrepancy I've uncovered from the wiring schematic are the 4 resistors connecting to pin 3 of each power socket. According to the documentation these should be 100 ohm. Except the ones I have are 50 ohms according to my meter. Other resistor values match up OK to the schematic. I assume this was intentional and is OK but just want to confirm.

    Two 100 ohm resistors on each side are in parallel with each other so they measure at 50 ohms - normal

    Bob
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    Bob Latino
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    Re: Thermal event observed, need help

    Post by Bob Latino on Sun May 14, 2017 6:50 pm

    LeGrace wrote:

    Q for the experts. What causes red plating? Definitely different from the flash overs that took out my rectifiers previously.

    "Red plating" is when the plate of an output tube turns red because that tube is drawing too much current. Possible causes are ...

    1. A short inside the tube

    2. The 10 ohm bias resistor blew

    3. The bias system loses control of that tube (or tubes in case of the M-125's)

    Bob
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    LeGrace

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    Re: Thermal event observed, need help

    Post by LeGrace on Sun May 14, 2017 6:54 pm

    I'm going with #1. I've eliminated #2 and #3 in that all readings read normal. Thanks for responding Bob!
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    Tubes4ever

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    Re: Thermal event observed, need help

    Post by Tubes4ever on Mon May 15, 2017 12:04 am

    LeGrace wrote:I'm going with #1. I've eliminated #2 and #3 in that all readings read normal. Thanks for responding Bob!

    LeGrace,

    Bob is referring to the causes of red plating in OUTPUT tubes, not rectifier tubes.
    Red plating in the GZ37 is caused by excessive current flow through the tube which is caused by a defect in the amp itself. A short in the rectifier tube will blow the fuse not cause it to red plate.

    GZ37s are not cheap. Please refer to my earlier post before deeming it to be bad.
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    LeGrace

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    Re: Thermal event observed, need help

    Post by LeGrace on Mon May 15, 2017 7:12 am

    Thanks for clarifying! Bob's note I see does clearly reference output tubes, doh. I checked the readings on pins 5/6 of the output tube sockets and they were -50v on V2 and V4 and -55v on V1 and V3. The bias resistors also check out good. I spent several hours going over every inch of the amp with a magnifying glass, wiggling wires etc. No loose connections uncovered. I touched up a couple solder joints but somehow doubt these were the issue. Next I replaced the rectifier with one of the Mullards that I had been using previously. Kept all other tubes the same, respecting the change only one thing at a time principle.  Rebiased and ran the amp for an hour keeping a close eye on all tubes, ready to switch off immediately on any sign of tube stress. Normal warmup and absolutely no signs of plating anywhere. Sounded perfect. Promising but I'm far from convinced I'm out of the weeds yet. Wish I was more knowledgeable.

    Edit 1)
    Day #2 Update:
    Problem child continues to behave perfectly. Fairly confident now that the red plate event didn't harm anything else. Some of the Google hits describing side damage that can result from red plating were truly frightening. Fortunately looks like I caught it in time.

    During my visual inspection I did notice something with the rectifier socket. Pin position #8 the metal receiving tube I noticed was not seated properly. In fact it was sticking out beyond the top surface of the socket, whereas all the other ones were down inside the socket. I used a metal pin to hammer it back down level with the other positions. Any chance this could be related?

    Anyway, so nice to have the tubes back! Basketball

    Did you check the quad cap? If it has high ESR or is out of tolerance that could also cause a rectifer to redplate I believe
    Thanks, I do have a cap meter on order. In the mean time I'm relying on a keep a close eye on the amp approach...


    Last edited by LeGrace on Mon May 15, 2017 2:44 pm; edited 1 time in total

    mcgyver74

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    Re: Thermal event observed, need help

    Post by mcgyver74 on Mon May 15, 2017 2:19 pm

    Did you check the quad cap? If it has high ESR or is out of tolerance that could also cause a rectifer to redplate I believe
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    LeGrace

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    Re: Thermal event observed, need help

    Post by LeGrace on Tue May 16, 2017 8:38 am

    Have accumulated a bunch more hours and no further red plate episodes. Have been checking the bias setting at regular intervals, holding nice and steady. Another question. If bias voltage was inadvertently set too low could this stress a rectifier?
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    LeGrace

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    Re: Thermal event observed, need help

    Post by LeGrace on Tue May 16, 2017 12:01 pm

    Maybe it was the one funky connector on the rectifier socket leading to a poor connection. Although a runaway stemming from operator error also fits the facts. Thinking back I'm wondering could I have misread my DVM when I installed the Haltrons?

    jmczaja

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    Re: Thermal event observed, need help

    Post by jmczaja on Wed May 17, 2017 9:19 am

    I feel your pain.. I have an M125 that was causing me intermittent grief. Bob helped me through my problem. My initial problem caused the 100ohm resistors to burn up and also burn one of the tube sockets pretty bad. I disassembled the problem tube sockets and cleaned all the carbon off of it and put it back together. It all seemed fine until i started to get intermittent pops and weirdness in the same channel. Opened the amp up, turned it on it's side and played some music very LOUD through it and noticed the tube socket was arc'ing in the very same spot the damage originally occurred. It looked clean but i assume some of the carbon just couldn't be cleaned off. I replaced the bad tube socket with a new one and all has been fine since.

    Good luck in getting your issue resolved.
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    LeGrace

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    Re: Thermal event observed, need help

    Post by LeGrace on Wed May 17, 2017 11:07 am

    Thanks for sharing! Reading your story I feel I got off easy by comparison. Although I have a sneaking suspicion my story may not be over yet. Anyway while for now things are calm rest assured that into the following days/weeks/months I'll be keeping a close watch on my tubes. Whenever I leave the room the amps go off, unless I'm just leaving to refill my glass!! drunken
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    deepee99

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    Re: Thermal event observed, need help

    Post by deepee99 on Wed May 17, 2017 11:17 am

    So, LeGrace, you threw enough guesses at it, think you've solved it, can you say with any confidence what the problem actually was? If not, I'll wager a KT-88 it acts up again at some point.
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    corndog71

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    Re: Thermal event observed, need help

    Post by corndog71 on Wed May 17, 2017 11:39 am

    Yep, I know that uneasy feeling well. And I don't miss it.
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    LeGrace

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    Re: Thermal event observed, need help

    Post by LeGrace on Wed May 17, 2017 12:31 pm

    deepee99 wrote:So, LeGrace, you threw enough guesses at it, think you've solved it, can you say with any confidence what the problem actually was? If not, I'll wager a KT-88 it acts up again at some point.

    No rose colored glasses here. The socket flaw (should have taken a pic to share) seems the most likely culprit, only time will tell whether that is truly the assignable cause. Speaking of red plating, I see you're no stranger to this phenomenon, interestingly sockets similarly implicated. (ref search function)
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    deepee99

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    Re: Thermal event observed, need help

    Post by deepee99 on Wed May 17, 2017 1:28 pm

    LeGrace wrote:
    deepee99 wrote:So, LeGrace, you threw enough guesses at it, think you've solved it, can you say with any confidence what the problem actually was? If not, I'll wager a KT-88 it acts up again at some point.

    No rose colored glasses here. The socket flaw (should have taken a pic to share) seems the most likely culprit, only time will tell whether that is truly the assignable cause. Speaking of red plating, I see you're no stranger to this phenomenon, interestingly sockets similarly implicated. (ref search function)

    Chief cause of socket-failure is tube-rolling. I finally gave that bad habit up and took up golf and a heroin addiction instead. Much cheaper and more productive.
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    LeGrace

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    Re: Thermal event observed, need help

    Post by LeGrace on Wed May 17, 2017 8:19 pm

    Well one thing it seems I can rule out is an issue with the GZ37. As a test I put it into my other M125 c/w KT120 power tubes. Operating fine for several hours now. How is that possible? I mean during the red plate it was like cherry red top to bottom and the heat output was incredible. With previous rectifiers after only a momentary flashover the result was toast. Those GZ37 must be tough old birds...
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    Tubes4ever

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    Re: Thermal event observed, need help

    Post by Tubes4ever on Wed May 17, 2017 8:24 pm

    deepee99 wrote:
    LeGrace wrote:
    deepee99 wrote:So, LeGrace, you threw enough guesses at it, think you've solved it, can you say with any confidence what the problem actually was? If not, I'll wager a KT-88 it acts up again at some point.

    No rose colored glasses here. The socket flaw (should have taken a pic to share) seems the most likely culprit, only time will tell whether that is truly the assignable cause. Speaking of red plating, I see you're no stranger to this phenomenon, interestingly sockets similarly implicated. (ref search function)

    Chief cause of socket-failure is tube-rolling. I finally gave that bad habit up and took up golf and a heroin addiction instead. Much cheaper and more productive.

    Deepee,

    Thanks for a good chuckle! You are not far from the truth! Shocked
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    Peter W.

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    Re: Thermal event observed, need help

    Post by Peter W. on Thu May 18, 2017 7:49 am

    LeGrace wrote:Well one thing it seems I can rule out is an issue with the GZ37.  As a test I put it into my other M125 c/w KT120 power tubes. Operating fine for several hours now.  How is that possible? I mean during the red plate it was like cherry red top to bottom and the heat output was incredible. With previous rectifiers after only a momentary flashover the result was toast. Those GZ37 must be tough old birds...

    If I followed this correctly, your GZ37 is a vintage tube made by Mullard 'back in the day', and not some bit of overpriced garbage made by a current producer. It should be no surprise, then that it should survive a certain amount of red-plate time. These were industrial tubes - meaning that they had to operate reliably and under a wide range of conditions and demands as unplanned downtime was unacceptable.

    Audio tubes produced today would be laughed right into the trash if foisted on the public 50 years ago.

    A good reason to pay the freight and purchase such tubes as that GZ37 where possible, practical, and available.

    Jim McShane

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    Re: Thermal event observed, need help

    Post by Jim McShane on Thu May 18, 2017 4:51 pm

    Bob Latino wrote:
    LeGrace wrote:

    Q for the experts. What causes red plating? Definitely different from the flash overs that took out my rectifiers previously.

    "Red plating" is when the plate of an output tube turns red because that tube is drawing too much current. Possible causes are ...

    1. A short inside the tube

    2. The 10 ohm bias resistor blew

    3. The bias system loses control of that tube (or tubes in case of the M-125's)

    Bob

    Let me just add that the plate turns red because it is overheating. The excess current creates heat load that the plate can't dissipate and its temperature increases until it glows red.

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