When I took the amp into work Friday, I wanted to run a test on the bias circuit with an oscilloscope in order to confirm what I measured with a DVM on the bias voltage. Back story: a screen resistor failed some time ago and after the repair the problem was still present. Mesa 6L6 STR440 tubes would red plate when using the 45Wmode using the variac power setting as it was preferred over full power and 90W operation due to the ice pick. The way I was running the amp may have added to the issue (playing loud). Since I ran through three sets of tubes I changed over to Tung Sol 7581A tubes which held up and lasted for 6 months of use. Original tubes failed within the first two months of use. Any other tubes following lasted at least an hour. The last set I was using full power at 90W with no issue but when I tried my favorite arrangement 45W on Variac the center tubes redplated within 20 minutes. So when I changed to the Tung Sol I kept the amp at full power at 90W. 6 months is not long enough and that is when the tubes took a dramatic change in tone. Observing the plates when in use the tubes were operating at the onset of red plating since the plate seams were red hot but did not expand farther than that. SED =C= 6L6GC were the savior tubes, I got a NOS set that were sold by GT in the 90's and those lasted almost a year before I changed to a new set. I still have them and they are still good to use as is the other set. However, since SED (Svetlana Electron Devices) stopped selling consumer grade products since the glass supplier either closed shop or the quality of glass just was not suitable, SED shut down production of audio tubes.... what a bummer. It was time I needed to figure out what the problem was with the amp. It is related to the bias circuit. The other issue was the chassis was getting so hot I could not touch the back side of the amp, speaker cable insulation was getting really soft due to excess heat as well as the power transformer. The thermal issue changed when I started using the SED tubes. It got better when I converted the head to a combo as the speaker provided additional air movement.... the point of the story.... I took the liberty to change the bias voltage that should have raised the -51V (calculated value should be -51.096V) to -51.300V. I have seen some videos regarding the Mark V on diode replacement and one of the suspect didoes was the one associated with the bias circuit. (may have been an early model as there was issue with some diodes at one point). I swapped the 82.5K resistor with a 91k resistor and my woes with red plating Mesa 6L6GC tubes were over. So why the long story..... the amp is now running too cold due to the change in bass response. I had assumed there is still an issue that has been left unresolved since the screen resistor repair. Now to the point... I measured the bias voltage with an oscilloscope to ensure the diode was operating properly and that there was no notable ripple in the bias circuit. No ripple at all and was very stable but not at the correct value. Bias voltage was confirmed to be -59.01 Vdc. When the amp was taken out of stand by the bias amplitude would drop to -57.39 Vdc. Not the expected value of -51.3Vdc. This would indicate the 33k resistor is either open or there has been trace damage to the PCB, or it may be something else.
Perhaps it is time to work backwards and measure voltage drops on the complete bias circuit (at the PI tube and on the grids of the power tubes) before I replace any components. The 1.62V change when the amp is taken out of stand-by may be a concern if I am able to correct the bias voltage to where it should be, that change would definitely run the tubes too hot (-49.47Vdc) unless this is typical when the amp is out of standby. What is causing the 1.62V droop in voltage is something I need to figure out. At this point getting a bias probe kit may be a good idea just to verify that once the amp has been fixed the tubes will not red plate shortly after running the amp.
I do have another amp I can use to verify if bias shifts when out of standby. It may not be a Simul-class amp but should provide a clue to what I am looking for. Could be just a waste of time but I have plenty of that.
Perhaps it is time to work backwards and measure voltage drops on the complete bias circuit (at the PI tube and on the grids of the power tubes) before I replace any components. The 1.62V change when the amp is taken out of stand-by may be a concern if I am able to correct the bias voltage to where it should be, that change would definitely run the tubes too hot (-49.47Vdc) unless this is typical when the amp is out of standby. What is causing the 1.62V droop in voltage is something I need to figure out. At this point getting a bias probe kit may be a good idea just to verify that once the amp has been fixed the tubes will not red plate shortly after running the amp.
I do have another amp I can use to verify if bias shifts when out of standby. It may not be a Simul-class amp but should provide a clue to what I am looking for. Could be just a waste of time but I have plenty of that.