Can someone ID the bias resistor on the Mark V?

[bandit2013]

Sorry if this has been asked over and over again.
I believe there is an issue with my amp bias. In 45W mode I have been able to get the onset of red plate on three sets of tubes. Not quite the full glow but the where plate halves meet will glow red on V8. V9 is not as bad but will take longer to begin to show. This does not occur when using Ch1 or CH2. Only effects CH3 (primarily mark IV and Extreme modes). It does not seem to make much of a difference with the pentode/triode switch position. Bias switch is in the 6L6 position. Seems that the center tubes are running a bit too hot. In 90W mode things balance out and the onset of red plating almost goes away. Svetlana tubes (Reflektor versions) Tung Sol 7581, and Mesa 6L6GC (these really get red).

I have been using 6CA7 as a preference with bias switch in EL34 and have not had any issues with hot spots. Same applies to EL34's. The Tung Sol 7581 has more hours on them than the Svetlana. The Mesa 6L6GC were two of the originals that survived (two that were in V8 and V9 went completely cherry on me). Have not noticed any issues with the SED's though. I will not put them back into the amp until I can figure out any issues. Looks like I need to take a trip to RadioShack and get some small clip leads for my meter to get a reading on current.

I did check screen/grid resistors and they measure proper resistance (just in case one was open). I will keep searching for the other posts on the bias resistor. Would be helpful if someone already has the answer to chime in on this for me.

thanks. :|

 

[bandit2013]

V8 is definitely running hotter than V9. The PI tube is a match triode too. Even in 90W mode the screen glow is almost twice as bright on the V8 and V10 tube than on V9 and V11. Swapping tubes does not seep to follow the tube. Definitely isolated to V8. 45W power mode is the worst. I have been able to glow old and new tubes (except for the TAD6L6GC-STR, not sure if they would even glow considering the coating on the plates). 6CA7 tubes are running overboard as well. I guess after going though many tubes that would indicate something not kosher about my amp. All of the large power resistors seem to be good, no cracks, nothing burned. Still sounds good though. Would rather keep the plates out of the cherry zone though. All tubes I have purchased (except for the EH EL34) were specified for Mesa Mark V. I would rather have to bias the amp than guess if it is working properly. The Mesa branded 6L6GC seem to glow the reddest.

I am considering pulling the power tube board to get a good look on the other side. I may have a bad solder joint or open trace. The solder work through out the amp is a bit sloppy in my opinion. Suck that I will have to unsolder some of the wires to pull the board. At least it is well labeled where the wires go. This may also explain why I am getting a 60Hz hum in CH3 and all is quiet CH1 and CH2. Does not seem to be preamp tube related. Too bad, this is a great amp to have this type of quality. Considering how hot the amp gets overall, all it takes is elevated temperatures to reactivate rosin flux residue. Compare this to my Mark IV which has more hand soldering, it is clean and well made. Almost looks like a trainee built my Mark V compared to the Mark IV. If I find anything abnormal I am sure to post it in this thread, just in case someone else has a Mark V that eats the V8 tube.

 

[bandit2013]

Mesa was not kidding that the signal is out of phase when using the loop. I placed it into hard bypass just to see what would happen. V8 no longer is overdriven and cools down. V9 on the other hand does exactly what V8 was doing previously, hot spot on one side of tube at area where plates meet. Also the corona on the grids is also mimicking the what occurred in V8.

Any clue on that small transformer next to the filament transformer is. Appears to be a choke since there are only two orange wires coming out, one ties to the red wire that follows down to the output transformer and the other (I assume) connects to the PI tube. Wonder if the choke is shorted. Sees that there is no balance between V8 and V9 in 45W or 90W modes when using CH3. Also replaced the PI tube with no change. Should I care that I am getting a reddish glow on the plates? I could always just keep it in 90W power mode which seems to prevent the plates from glowing. Also, when I had 6L6GC tubes (with bias switch set correctly) I could get all 4 to get hot spots. Is this normal?

 

[bandit2013]

Perhaps this is normal?

Actually the Mark IV will get a slight glow on both tubes in Class A mode when the volume is turned up. At least that amp seems to operate the tubes with more balance. The mark V tends to push only one tube in 45W mode and leave the other appearing cold. Same applies in 90W, the outer most tube next to the one running hot in 45W shows similar trait but without the red glow on the plate. The amp sounds great. I would have not noticed it if the amp was in its shell. Side of the tube facing the front of the amp was where tube was glowing, other side looked normal.

I traced as many of the parts as I could. Due to the complexity of the 10W switching scheme it was hard to verify if the relay contacts were any good. I could not find any component issues provided they were measured at room temperature and not elevated while the amp is operating. Oh well. As long as the amp sound good and does not eat tubes too quickly that is fine with me.