Marantz 8B red plate

[thr33p4c]

The most important thing that you need for setting AC balance is a dummy load capable of dissipating over 35W of power. I recommend (2) 8 ohm 50W or higher resistors, one connected across the C and 8 ohm terminals of each channel. In a pinch you can use a smart phone app to generate a test signal as long as you have the ability to convert its headphone output to RCA and you have variable volume control, I just use 1Khz. The idea is to drive the amp close to rated power and balance the signal to the output tubes so that the bias needle (Current Measurement) reaches the balance dot in both positions. The manual says you can short the C and 16 ohm taps for this adjustment but I would not recommend that, the risk of damaging something is too high. An oscilloscope is not needed at all for this adjustment.

If you do not have a dummy load I would suggest that you set both pots to center (equal ohms from the wiper to each leg) and leave it like that.

When I make the AC balance adjustment I first set it with the service manual method and then tweak it for optimal performance with a distortion analyzer. It helps to "hang" the front of the amp over the bench a bit so that you can reach the AC balance adjustments while reading the meter and manipulating the bias switch.

 

[Mike Chan]

The most important thing that you need for setting AC balance is a dummy load capable of dissipating over 35W of power. I recommend (2) 8 ohm 50W or higher resistors, one connected across the C and 8 ohm terminals of each channel. In a pinch you can use a smart phone app to generate a test signal as long as you have the ability to convert its headphone output to RCA and you have variable volume control, I just use 1Khz. The idea is to drive the amp close to rated power and balance the signal to the output tubes so that the bias needle (Current Measurement) reaches the balance dot in both positions. The manual says you can short the C and 16 ohm taps for this adjustment but I would not recommend that, the risk of damaging something is too high. An oscilloscope is not needed at all for this adjustment.

If you do not have a dummy load I would suggest that you set both pots to center (equal ohms from the wiper to each leg) and leave it like that.

When I make the AC balance adjustment I first set it with the service manual method and then tweak it for optimal performance with a distortion analyzer. It helps to "hang" the front of the amp over the bench a bit so that you can reach the AC balance adjustments while reading the meter and manipulating the bias switch.

I replaced all the capacitors bought from the ebay kits. the bias voltages measure are all ok. I insert the EL34 tubes back. Hold up the amp to 110V AC outlet. Adjust all the bias, the amp looks good. However, after 4-5 min minutes. The EL34 V4A get red plate again???

 

[thr33p4c]

Can you clarify if you are still using the tubes that you started with? Sometimes if a tube red plates it can be permanently damaged. Do you have a tube tester?

 

[thr33p4c]

Also consider this from earlier in the thread:

Two most likely causes are either a leaking coupling cap, or a bad connection between the octal socket and the tube pins.
Either can cause the socket to lose neg bias which turns the tube full on.

You might have a dirty tube socket or dirty tube pins and the negative bias voltage cannot get to the grid.

Are you able to share some pictures of the work that you have completed so far?

 

[dr*audio]

I can set the bias of the four tubes to the correct needle point through the switch just after power ON. However, when the tube start to turn red, the bias needle will jump to the top range suddenly/immediately.

I've had this happen with new tubes that fail. The quality of new tubes is lousy, especially JJ. I've had them fail and draw lots of current. If you have a tube tester you can verify this. If not, replace the tube. The fact that you can't control the bias with the pot means either the tube is bad or you have a bad tube socket or pot.

 

[mcgjohn]

Also consider this from earlier in the thread:

You might have a dirty tube socket or dirty tube pins and the negative bias voltage cannot get to the grid.

Are you able to share some pictures of the work that you have completed so far?

not sure how photos would show what to do.

1. measure DC leakage on the coupling cap socket side with a DCV meter. Determine if you are getting DC leakage through the cap. If you are this can turn on a tube regardless of the bias setting.

2. take a broken or worn out octal tube. Cut one pin off the worn out tube. With the amp powered off and the caps discharged, use that single pin to test the snugness of each receptacle pin in the suspect octal socket. All should be a snug fit. If you find one that is loose, then you need to replace the octal socket.

 

[thr33p4c]

not sure how photos would show what to do.

I was referring to the OP. I am interested in seeing what changes have been made since the start of the thread and if the suspect screen grid resistors that I identified have been changed.

1. measure DC leakage on the coupling cap socket side with a DCV meter. Determine if you are getting DC leakage through the cap. If you are this can turn on a tube regardless of the bias setting.

2. take a broken or worn out octal tube. Cut one pin off the worn out tube. With the amp powered off and the caps discharged, use that single pin to test the snugness of each receptacle pin in the suspect octal socket. All should be a snug fit. If you find one that is loose, then you need to replace the octal socket.

1. OP showed earlier that there were no signs of leakage with the original caps. #33 - Since then, I believe they have been changed but waiting on pics.

2. If there is indeed a poor connection at the socket it can be retensioned and cleaned. I have used a sharp pic tool for this with great success. Of course, replacing the socket would be more fool proof but personally I do not like drilling out rivets in classic Marantz gear if I can help it.

 

[Mike Chan]

I have two set of El34, I already insert the tubes which was not burned red from last time, so I don't think it is causes by the damaged tube! Attached please find the picture of the new cap and diodes.

 

[thr33p4c]

Thank you for adding the picture. The work looks nice and clean so far. Good job.

Some things to try next. Last you reported V4A red plated. Swap the tubes plugged into V4A and V4B and let us know if the red plating follows the tube or stays at the V4A socket.

If the alternate tube still red plates in the V4A socket then pin inspection is next.

Did you change or clean the bias control for V4A?

Also please thoroughly inspect all of your screen grid resistors. There is a 1W carbon comp installed in one of the positions were the rest are the stock 1/2w parts and it is very thermally stressed. These should be 100 ohm on pin 4 of each output tube.

 

[thr33p4c]

Sorry, my mistake the resistor that is stressed is one of the 6.8 ohm cathode resistors. 6.8 ohm 1W.

 

[mcgjohn]

I was referring to the OP. I am interested in seeing what changes have been made since the start of the thread and if the suspect screen grid resistors that I identified have been changed.


1. OP showed earlier that there were no signs of leakage with the original caps. #33 - Since then, I believe they have been changed but waiting on pics.

2. If there is indeed a poor connection at the socket it can be retensioned and cleaned. I have used a sharp pic tool for this with great success. Of course, replacing the socket would be more fool proof but personally I do not like drilling out rivets in classic Marantz gear if I can help it.

to really test the leakage at rated voltage, it should be removed from circuit and tested on a high voltage cap tester.

 

[thr33p4c]

to really test the leakage at rated voltage, it should be removed from circuit and tested on a high voltage cap tester.

OP has already replaced them with new IC MKP caps.

 

[Mike Chan]

Sorry, my mistake the resistor that is stressed is one of the 6.8 ohm cathode resistors. 6.8 ohm 1W.

On my Amp it is a 10 Ohm 1 W resistor instead of 6.8 Ohm, both of them are ok. The 100 Ohm resistor are ok too. I have checked the tube socket by measure the resistance from bottom leg of the socket to the top hole of the socket. All the pins are ok (no poor contact) with 0 Ohm on the readings. I also change the position of the tubes, the same position with red plate tube remains the same!

 

[Mike Chan]

Thank you for adding the picture. The work looks nice and clean so far. Good job.

Some things to try next. Last you reported V4A red plated. Swap the tubes plugged into V4A and V4B and let us know if the red plating follows the tube or stays at the V4A socket.

If the alternate tube still red plates in the V4A socket then pin inspection is next.

Did you change or clean the bias control for V4A?

Also please thoroughly inspect all of your screen grid resistors. There is a 1W carbon comp installed in one of the positions were the rest are the stock 1/2w parts and it is very thermally stressed. These should be 100 ohm on pin 4 of each output tube.

All the bias control are already change to new 5 K Ohm potentiometers

 

[ramjet]

On my Amp it is a 10 Ohm 1 W resistor instead of 6.8 Ohm, both of them are ok. The 100 Ohm resistor are ok too. I have checked the tube socket by measure the resistance from bottom leg of the socket to the top hole of the socket. All the pins are ok (no poor contact) with 0 Ohm on the readings. I also change the position of the tubes, the same position with red plate tube remains the same!

Perhaps a cold solder joint at that tube socket?

 

[thr33p4c]

If you have swapped tubes and the issue remains at V4A then we have it isolated to that socket. We know that the negative power supply is good because the other 3 sockets do not have the issue. we can assume that R14A is not at fault since it was just replaced.
Any of these components could be at fault:
R12 100K on the bias wiper
R17A 1K on pin 5 control grid
R20A 100 ohm screen grid resistor
V4A Tube Socket

C15A and R19A are common on both tubes so unlikely an issue.
I have never seen a bad output transformer on an 8B

My suggestion is to complete the test recommended by mcgjohn

2. take a broken or worn out octal tube. Cut one pin off the worn out tube. With the amp powered off and the caps discharged, use that single pin to test the snugness of each receptacle pin in the suspect octal socket. All should be a snug fit. If you find one that is loose, then you need to replace the octal socket.

Even if you have continuity at the socket there is no guarantee that it can actually make good contact through a tube pin. You also might have an intermittent carbon comp resistor that works when its cold but fails once warmed up. You could replace the 3 resistors above and retest. Another good test would be to keep a meter clipped to pin 5 of V4A at your next power up and see if you are losing grid bias when the tube starts to red plate.

 

[thr33p4c]

Perhaps a cold solder joint at that tube socket?

This makes sense as a possible cause. Reflowing all connections at the tube socket couldn't hurt.

 

[prww]

Just a note, I learned the hard way years ago do not solder on tube sockets with the tube plugged in. Can ruin the tube. Paul.

 

[Mike Chan]

I follow all the above comments and double check all the socket pins again to make sure it is connected. The related resistors are ok, too. Then I insert the tubes back. It looks good at the first two minutes. No red plate. However at around three minutes, the V4A tube has a "pop" from the inside of the tube (I think it is a flashlight). It gets burn out!

 

[Mike Chan]

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.