Marantz 8B red plate

[dr*audio]

I am so happy to announce that my Marantz 8B is back to normal with wonderful sounds. I thank you for all the members here, they try to help and guide me to trouble shoot the issue. I special thank member "thr33p4c" - you give me lots of direction to find out the problem causing the red plate. Thank you so much.

So was the problem a bad tube?

 

[Mike Chan]

I am so sorry to tell you that the replacement tubes work fine. I can adjust the bias to the right spot and they sound beautifully for almost two hours. However, a few days later, when I try to listen to the amp again, after I turn it on and listen to the music for around 30 minutes, I found that that the two tubes start to have red plate on it, so I turn off the amp immediately. Let me know what should I do now?

 

[Hifi Dave]

I just read the entire thread but you never disclosed what you found wrong with the 8b. Your last post said it ran fine fir a while and then after setting for a bit the red plate issues came back. Please tell us so we can help

 

[dr*audio]

I am so sorry to tell you that the replacement tubes work fine. I can adjust the bias to the right spot and they sound beautifully for almost two hours. However, a few days later, when I try to listen to the amp again, after I turn it on and listen to the music for around 30 minutes, I found that that the two tubes start to have red plate on it, so I turn off the amp immediately. Let me know what should I do now?

Swap the 2 that redplate with the other 2 tubes. Monitor the bias. If the problem stays with the same tubes, it's the tubes.

 

[Mike Chan]

the tubes are brand new that I just bought。 When the tubes are in red plate. I check the reading on the bias meter on the 8B. It is pointed at the white line, so the bias should be good. I also switch the two tubes with other tubes, the red plate remains the same!

 

[ramjet]

the tubes are brand new that I just bought。 When the tubes are in red plate. I check the reading on the bias meter on the 8B. It is pointed at the white line, so the bias should be good. I also switch the two tubes with other tubes, the red plate remains the same!

It is possible that the bias meter on your amplifier is reading low. Run your bias to the left of the white line on the meter about an eighth of an inch.

 

[dr*audio]

the tubes are brand new that I just bought。 When the tubes are in red plate. I check the reading on the bias meter on the 8B. It is pointed at the white line, so the bias should be good. I also switch the two tubes with other tubes, the red plate remains the same!

Do you mean the same tubes redplate in their new position? If so the tubes are defective. I have had this happen.

 

[Mike Chan]

Do you mean the same tubes redplate in their new position? If so the tubes are defective. I have had this happen.

I do not switch the red plate tubes to the other socket, because they are new when I insert them in there. I also can't try to insert other set of tubes to these two sockets, otherwise they will get burn too!

 

[Mike Chan]

It is possible that the bias meter on your amplifier is reading low. Run your bias to the left of the white line on the meter about an eighth of an inch.

I think the bias meter is fine as the other set of tubes runs well with the same setting.

 

[dr*audio]

I do not switch the red plate tubes to the other socket, because they are new when I insert them in there. I also can't try to insert other set of tubes to these two sockets, otherwise they will get burn too!

It doesn't matter that they are new, they can still be defective and if you don't have a tube tester this is the only way to test it. It's only for a short time to see if they redplate in the other sockets and to see if tubes that were ok redplate in those sockets.

 

[gadget73]

If they red plate, the bias meter should also be reading high.

I happen to have an 8b on the bench right now, and its got slightly sketchy bias pots. All the way down one of the tubes loses bias and goes nuts, one on the other side goes nuts if its turned all the way up. All 4 have jerky response when adjusting. I wouldn't be even a little surprised if your has bias pot issues.