Three that you can see, and perhaps several that you can't. Reflow.
+15
Peter W.
jmczaja
nmchiefsfan
Dogstar
cci1492
j beede
corndog71
wildiowa
Bob Latino
peterh
deepee99
vtshopdog
Tubes4ever
mcgyver74
LeGrace
19 posters
Thermal event observed, need help
j beede- Posts : 473
Join date : 2011-02-07
Location : California
deepee99- Posts : 2244
Join date : 2012-05-23
Location : Wallace, Idaho
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 . . .j beede wrote:Three that you can see, and perhaps several that you can't. Reflow.
I'd still look into a chat-up with Audio Bill.
LeGrace- Posts : 388
Join date : 2016-08-07
Location : Ontario, Canada
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- Posts : 77
Join date : 2012-03-21
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- Posts : 77
Join date : 2012-03-21
^^^ 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.
LeGrace- Posts : 388
Join date : 2016-08-07
Location : Ontario, Canada
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.
Q for the experts. What causes red plating? Definitely different from the flash overs that took out my rectifiers previously.
LeGrace- Posts : 388
Join date : 2016-08-07
Location : Ontario, Canada
I've been googling red plating. Should those of us with 50 ohm M125's be running a higher bias voltage setting?
Bob Latino- Admin
- Posts : 3262
Join date : 2008-11-26
Location : Massachusetts
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
Bob Latino- Admin
- Posts : 3262
Join date : 2008-11-26
Location : Massachusetts
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
LeGrace- Posts : 388
Join date : 2016-08-07
Location : Ontario, Canada
I'm going with #1. I've eliminated #2 and #3 in that all readings read normal. Thanks for responding Bob!
Tubes4ever- Posts : 167
Join date : 2015-07-14
Location : Star, Idaho
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.
LeGrace- Posts : 388
Join date : 2016-08-07
Location : Ontario, Canada
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!
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!
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...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
Last edited by LeGrace on Mon May 15, 2017 2:44 pm; edited 1 time in total
mcgyver74- Posts : 73
Join date : 2017-03-24
Location : Jersey City, NJ
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
LeGrace- Posts : 388
Join date : 2016-08-07
Location : Ontario, Canada
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?
LeGrace- Posts : 388
Join date : 2016-08-07
Location : Ontario, Canada
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- Posts : 21
Join date : 2017-02-09
Location : New Jersey
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.
Good luck in getting your issue resolved.
LeGrace- Posts : 388
Join date : 2016-08-07
Location : Ontario, Canada
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!! 
deepee99- Posts : 2244
Join date : 2012-05-23
Location : Wallace, Idaho
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.
corndog71- Posts : 840
Join date : 2013-03-19
Location : It can get windy here
Yep, I know that uneasy feeling well. And I don't miss it.
LeGrace- Posts : 388
Join date : 2016-08-07
Location : Ontario, Canada
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)
deepee99- Posts : 2244
Join date : 2012-05-23
Location : Wallace, Idaho
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.
LeGrace- Posts : 388
Join date : 2016-08-07
Location : Ontario, Canada
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...
Tubes4ever- Posts : 167
Join date : 2015-07-14
Location : Star, Idaho
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!
Peter W.- Posts : 1351
Join date : 2016-08-07
Location : Melrose Park, PA
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- Posts : 237
Join date : 2011-10-19
Location : South Suburban Chicago
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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