Help! A red plating mystery. New Cayin mt 12-n

[jbones]

First off here is the set up - Project carbon pro, audio note q1, Fosi X2, into a Cayin MT-12N and out to monitor Audio bronze 2s.

I’m a new audio file, just starting out, and hence why the setup is all pretty budget. But honestly, compared to what I have been running for the last 10 years, this is a huge sonic upgrade for me. So. To the fun part.

I connected up this new Cayin amp, with stock tubes, and within 50 hours of listening, first one of the the right power tubes (electro harmonic El84s) began red plating. I knew something was up as the sound was thin and breaking up/crackling. I turned it off, not knowing much about red plating. I took the tube out and put the tube back in, turned it on, and no red plating for 5 or 6 hours of play but at one point the amp started making a slight wine sound at all volume levels, I tuned volume down completely, and boom wine was gone. Weird. Guessing it’s a harmonic tube.

I turned the system off, then ran it for another few hours no issues, then one of the left power tubes started red plating. I turned it off, and have not turned it back on since.

I have ordered new tubes, but before I try them, only to find it’s an amp problem, I am wondering if anyone has encountered the same problem, or if there is any chance it’s just two bad tubes from the factory.

Another thought ( probably makes no sense) but is there a special way to connect 8ohm speakers to these posts that have 4-8ohm on them? I have them connected as usual, but just trying to figure out if I could have been messing something up.

I have contacted Cayin customer service.. and wow… just wow. Terrible. Regretting the purchase but really hoping a new set of tubes will do it, but from everything I’ve read, sounds like bias issues ( at minimum ).

Anyways, thought I would give posting a shot to see if the same had happened to anyone, and second so I can also update if and when I get it resolved for future users.

 

[Brice]

Looks like a classic cathode bias amp. Nothing to adjust here.

Have you tried to change the red plating tube to another socket to see if the red plating event follow the tube or stay in that socket?

 

[jbones]

Looks like a classic cathode bias amp. Nothing to adjust here.

Have you tried to change the red plating tube to another socket to see if the red plating event follow the tube or stay in that socket?

No, since two red plated. I was sort of confused on what to do. Maybe move those two to the other sockets?

 

[mcgjohn]

two areas you may want to check.
1. A leaking coupling cap going to the suspect tube or tubes (look for DC on the outboard side of the cap).
2. check for lose pins on the tube socket. Some Chinese sockets do not hold their tension well and will relax their grip on the pin over time. Could also cause your redplating issue.,

 

[gadget73]

^^ besides this, if the grid to ground resistor is a stupid value or making bad connection it will do the same thing. If the socket is not making contact its the same as not having a grid to ground resistor so far as the tube is concerned. In cathode bias mode it allows up to 1 megohm, so it would really have to be a stupid value to cause problems.

 

[Mr Boochie]

^^ besides this, if the grid to ground resistor is a stupid value or making bad connection it will do the same thing. If the socket is not making contact its the same as not having a grid to ground resistor so far as the tube is concerned. In cathode bias mode it allows up to 1 megohm, so it would really have to be a stupid value to cause problems.

Schematics online are poor for this unit. I noticed that the amp has multiple parallel cathode-to-ground resistors. One or two maybe defective or the wrong value.

I also highlighted the grid-to-ground resistors. It's impossible to make out the values of each component. Sorry.

There is also a possibility you have two defective output tubes. It's happened before.

 

[jbones]

thanks everyone! So much great info.
Update: I switched it on, and had now the left front tube red plating, this had not done before. I then moved that tube to the right side ( front) the one port that had never red plated and boom. Red plating. So it seems that tube is off, or just the whole system... so if anyone can follow that, could this possibly be that I had a bad tube match/bias? And that was causing issues? Now the left side is not red plating at all, and just this one single tube that is now front right.

 

[Mr Boochie]

thanks everyone! So much great info.
Update: I switched it on, and had now the left front tube red plating, this had not done before. I then moved that tube to the right side ( front) the one port that had never red plated and boom. Red plating. So it seems that tube is off, or just the whole system... so if anyone can follow that, could this possibly be that I had a bad tube match/bias? And that was causing issues? Now the left side is not red plating at all, and just this one single tube that is now front right.

Sounds like tubes. You have another set of known-good EL84 to try out?

 

[jbones]

Another update: I moved that tube ( problematic one I’m thinking) to the back right, and again, red plating.. so that explains this bad tube, but strange that this tube was not red tubing before. Can a miss matched tube cause others to red plate? Yes! @Mr Boochie i have some arriving in the mail any day now. I’ll pop them in, and report back. Hope I don’t just blow another set. Thank you all again for this awesome info

 

[Mr Boochie]

Another update: I moved that tube ( problematic one I’m thinking) to the back right, and again, red plating.. so that explains this bad tube, but strange that this tube was not red tubing before. Can a miss matched tube cause others to red plate? Yes! @Mr Boochie i have some arriving in the mail any day now. I’ll pop them in, and report back. Hope I don’t just blow another set. Thank you all again for this awesome info

No. Whether the tubes are matched or not shouldn't be the cause of red plating.
Hopefully, the new set solves the problem.

If not --- you are going to need to either try and get Cayin to ship you a new one. Or find an amp tech. BUMMER.

 

[jbones]

No. Whether the tubes are matched or not shouldn't be the cause of red plating.
Hopefully, the new set solves the problem. View attachment 3027094

If not --- you are going to need to either try and get Cayin to ship you a new one. Or find an amp tech. BUMMER.

Cayin seems like they will not do anything. 4 emails over a week, and they gave me a “sorry we don’t have a service in your area” cool. Thanks.

I just now moved the bad tube to another spot ( back left) and now the right front is red plating. But not the “problem” tube. So… seems like this is more of a complex issue, or maybe a few issues all at once.

 

[Mr Boochie]

Cayin seems like they will not do anything. 4 emails over a week, and they gave me a “sorry we don’t have a service in your area” cool. Thanks.

I just now moved the bad tube to another spot ( back left) and now the right front is red plating. But not the “problem” tube. So… seems like this is more of a complex issue, or maybe a few issues all at once.

Yeah. Unfortunately, looks like you have a very unstable amp.

You familiar with fixing these things? Have the equipment and tools?

 

[jbones]

No. Looks like I’ll be taking it in to be serviced by an expert. Fun. Once’s it out of the shop, I’m sure I’m going to be wishing I did not go with a “cheap” entry tube amp. There were really good reviews on these things, so hey, at least maybe this thread helps some others make a better choice.

 

[jbones]

Update just in case: I moved the problem tube back to back right, and now it’s the only one red plating again. Strange. But can’t help but think/wish that this tube is the source of the issues

 

[Mr Boochie]

Update just in case: I moved the problem tube back to back right, and now it’s the only one red plating again. Strange. But can’t help but think/wish that this tube is the source of the issues

Well, it could be contact issues with the socket. Especially, those crummy ceramic ones. You have any electronic contact cleaner to use on the socket and tube pins?

 

[jbones]

Again, can’t begin to thank you for the time and info you are giving me @Mr Boochie. Thank you. Gave them a clean. Problem persists, looks like I’ll probably just take it in; no use blowing a few hundred dollars worth of EL84s to check the wiring, when I could have an expert eye take a look. assuming that won’t take me months.. any recommendations for Vancouver BC?

 

[Mr Boochie]

Again, can’t begin to thank you for the time and info you are giving me @Mr Boochie. Thank you. Gave them a clean. Problem persists, looks like I’ll probably just take it in; no use blowing a few hundred dollars worth of EL84s to check the wiring, when I could have an expert eye take a look. assuming that won’t take me months.. any recommendations for Vancouver BC?

Sounds like a plan.

Sorry, I don't know of any good techs in the Vancouver area.

 

[gadget73]

In extreme cases a mismatch can cause this with cathode bias, but it has to be really bad. This needs both tubes flowing current to generate bias voltage. If one is dead or weak, the other tube will draw too much current and do this. I've also just had tubes that failed in some way that caused them to draw loads of current.

So basically unless you can confirm the other tube in that channel behaves itself, its hard to say for sure which is the problem.

A tube tester should turn this up even though many dont do a great job at testing power tubes. If it pegs as stupid strong or super weak, it probably junk.

 

[jbones]

In extreme cases a mismatch can cause this with cathode bias, but it has to be really bad. This needs both tubes flowing current to generate bias voltage. If one is dead or weak, the other tube will draw too much current and do this. I've also just had tubes that failed in some way that caused them to draw loads of current.

So basically unless you can confirm the other tube in that channel behaves itself, its hard to say for sure which is the problem.

A tube tester should turn this up even though many dont do a great job at testing power tubes. If it pegs as stupid strong or super weak, it probably junk.

Interesting. This may explain what I’m seeing.

Basically I move one problematic tube ( that makes a “ting” sound with a chopstick test) and when I move it to all 3/4 spots,it’s the one that red plates, but in certain spots it will not, but will cause the tube on the other side ( just looking at the unit, I don’t know if each channel is left/right front back) to red plate. It seems like it’s causing issues. I’m really hoping this is the case, but can’t take it into the city for another few weeks.

 

[jbones]

update: the mystery goes deeper. I found that actually it’s 2 tubes that are issues, and one just red plates faster than the other. So, I put them both on the same side of the unit, to test that they both glow ( as they each do in other positions), and lo and behold - no red plating. I’ve literally had the amp running 30mins and I could not go more than 2min without red plating. Would this be something to do with them being more bias? The amp sounds fine( not amazing as 2 obviously damaged tubes now I’ll bet) but can’t tell.

Does this help? Does this mean it was potentially just two garbage tubes from electro harmonics/cayin?