Mark 5:35 Loud hum on both channels

[steveroderick]

I hadn't used my amp in several months and played it tonight. It worked fine at a low volume for an hour or so, and then I heard a sporadic crackling coming from the speaker, which suddenly became a loud hum. The sound is only affected by changing the wattage (10W-25W-35W), in which case it gets louder the higher the setting. If I turn up the master volume, I can faintly hear my guitar without the hum getting louder. The hum is present with or without the instrument plugged in. The hum is only present when the standby switch is set to "on". Please help! I have an Associate of science degree in electronics technology but zero practical experience in complex or niche circuits. I have posted a video on youtube of the amp being turned on, waiting 30 seconds, and then hitting the standby switch. Here is a link:

One-minute video of the hum

 

[bandit2013]

I would have to say that sounds like Power tubes taking a dump. Remove the metal grill and watch the power tubes when you take it out of standby. If you notice the plates starting to turn color at the seams or on any surface they are due for replacement. EL84 tubes are goofy but have similar effect when they reach EOL. Brings me back to the Mark V90 woes I had from the beginning.

When one or more power tubes goes into current overload, any musical signal or content will not be heard due to one tube is in overload condition.

It is possible for the phase inverter tube to cause issue but not as likely as it is with power tubes.
If the power tubes are the issue, one way to find out for sure, remove the phase inverter tube as that will basically disconnect the preamp from the power section since there would be nothing to drive it other than the power tube bias current since it will be at idle condition. If one or more power tubes are bad, you will have the same noise, or should have the same noise.

 

[steveroderick]

I would have to say that sounds like Power tubes taking a dump. Remove the metal grill and watch the power tubes when you take it out of standby. If you notice the plates starting to turn color at the seams or on any surface they are due for replacement. EL84 tubes are goofy but have similar effect when they reach EOL. Brings me back to the Mark V90 woes I had from the beginning.

When one or more power tubes goes into current overload, any musical signal or content will not be heard due to one tube is in overload condition.

It is possible for the phase inverter tube to cause issue but not as likely as it is with power tubes.
If the power tubes are the issue, one way to find out for sure, remove the phase inverter tube as that will basically disconnect the preamp from the power section since there would be nothing to drive it other than the power tube bias current since it will be at idle condition. If one or more power tubes are bad, you will have the same noise, or should have the same noise.

I received my replacement tubes yesterday. The problem was resurfacing intermittently for a few days, then on a whim, I changed to a different electrical outlet. The wiring for that outlet is old 12 gauge with no ground. I did add a ground to that outlet a couple years ago, but it’s somewhat makeshift. The outlet that I switched to has brand new 12 gauge wire, home run to the service box. It’s been fine for three full sessions. I spoke with a technician at Mesa boogie, and he gave me some tips to employ (similar to the ones you gave me) when the problem resurfaces. Maybe it was just the outlet *shrug*. If not, at least I have the tubes to swap out.

 

[bandit2013]

Just remember if the hum happens again, before you hit the standby or power switch, look at the power tubes to see if they are red-plating. The dark grey plates should still look dark in color if they are not. The fuse is supposed to fail if this occurs as it may be caused by a temporary short or tube overload. If you know how to recognize red-plating tubes, some people may never have experienced it before. Not sure what amp this is, I believe it is a Hi-Fi amp of sorts. There are two EL84 tubes in the image that are red-plating and the one in the middle is not (that may be a 6X4/EZ90/6202 rectifier tube as those are about the same size and look similar). If your power tubes look anything like the one's on the left or right when the noise occurs, they are at their EOL and should be replaced.

Sure, having a bad ground could result in noise, the amp will still work, and sound can be produced that will be louder. I have run my Mesa's at my friends house, no ground and had to use an adapter plug, not a safe thing to do. The amp was noisy in that electrical environment compared to my home which is more modern in the electrical aspect. However, when a power tube is overloaded or red-plating, the amp will not provide any guitar sound as one or both tubes are no longer responding to the control grid signal on the tube. Aging of the tube is normal, as the getter flash (silver looking metal on the glass) is made of pure barium and is there to maintain tube vacuum and to react and absorb any outgassing from the heated components. It is the nature of the beast. Tubes are not perfect, but they do provide amazing sound compared to the alternatives.

It is always good practice to have on hand replacements in case any issues arise that are tube related. Power tubes should be changed in pairs since when one begins to overload, it sometimes causes the other tube to suffer even if it looks normal.