Strange Mark V Issue

Hey guys,

So I recently put a whole new set of tubes (preamp and poweramp) in my Mark V. All the tubes were bought from the Mesa Boogie Store. I noticed something strange happens now that I put in the new tubes.

I will put my amp on to warm up the tubes and after about 3-4 minutes of warming up I will hear a click (the same sound that you hear when you change between ch1, ch2, ch3 with the floorboard). The power cuts out for a second or two and then it comes right back. This has even happened a few times after I started playing. Same exact thing where you hear the click, power cuts out, wait a second or two and then it comes back on.

I cannot figure out what is going on with my amp. Any ideas?

That is odd, Try it with the footswitch cable removed and use the back control knob to switch channels or leave it in the footswitch position (default should be ch1). Is the speaker plugged in? Is the power cord plugged in firmly? The power supply used to operate the 5 volt switch logic as well as the 12Vdc fan is a 14VAC tap off of the power transformer. If something is loading the main supply (the 345VAC feed) the low voltage circuits will drop out. It could be the 5U4G rectifier tube but your issue sounds more like a power tube problem.

Did you replace the 5U4G Rectifier tube? Verify that is in its proper location (large bottle tube closest to the power cord). If the sudden drop in voltage is occurring or the amp is drawing too much current when the stand by switch is turned off, you may have a bad tube. The 5U4G rectifier is more prone to shorts when compared to the 6L6 power tube. However, if it is only running on half wave vs full wave (has one bad rectifier circuit) it will power up the caps but will take longer and it will not sustain enough power to feed the amp when the tubes are active. Note that the Rectifier tube is before the stand by switch. If it has a bad rectifier it may be dropping the supply down even though there are silicone diodes that parallel the rectifier tube.

Change the power setting on each channel to 90W and use full power. Watch the 5U4G when you turn off the stand -by switch. If you see arc flash or lightning in the tube you may have an issue with the Rectifier tube (note you may see this when the stand by is switched on which may be normal as I get that all the time). If it shuts down as you mentioned, try again with the 5U4G removed. If this happens without the 5U4G, you can put it back in and look at the power tubes. Note it is safe to remove one power tube and turn on the amp for a short period. If they symptom persists, install the removed tube and take out the next one. Easiest way to confirm that any of the power tubes contribute to the issue.

Are all of the heater elements glowing bright orange? If one looks extremely dark relative to the others (would have a faint red color but not as bright as the others) the tube may have a crack in the glass. (this should not cause the supply to be pulled down hard which is what may be occurring but it could if there is other damage inside the tube).

Once you rule out the power section, you can turn your attention to the preamp tubes. Probable but not impossible that a preamp tube would load down the power supply but usually this does not happen.

Not sure if any of this will help.

Okay here is an update. I have been trying to figure out what is going on. So far, from what I can tell this only happens when I have the power on "Variac Power" This does not happen when I have the amp on "Full Power"

I am not sure if this information can help narrow down the problem.